Chemical Absorption of Carbon Dioxide

Semiclosed and closed systems (i.e., circle and to-and-fro) rely on chemical absorption of carbon dioxide. Exhaled carbon dioxide is absorbed, and all other exhaled gases are rebreathed. The quantities of fresh oxygen and anesthetics equal those lost as a result of uptake, metabolism, and circuit leaks.^3,17,18

Dilution with Fresh Gas

Because of the intermittent nature of carbon dioxide excretion (during exhalation only) and the continuous inflow of fresh gas, the choice of inflow rate—as well as the locations of the inflow site, reservoir bag, and pop-off valves—contributes to the efficiency of carbon dioxide removal. When fresh gas flows are 1 to 1.5 times the minute volume (approximately 10 L/min in an adult), dilution alone is sufficient to remove carbon dioxide.^17,19-23 Such systems then behave the same as a nonrebreathing system.

Use of Valves to Separate Exhaled Gases from Inhaled Gases

Systems that use nonrebreathing valves are examples of this method of carbon dioxide removal.^6,9,11,24,25 A circuit that by virtue of high flows behaves as if it were nonrebreathing is not considered a nonrebreathing circuit in this analysis.

Use of Open-Drop Ether or a T-Piece Without a Reservoir to Release Exhaled Carbon Dioxide into the Atmosphere

Although similar to the second method above, systems that used open-drop ether or a T-piece without a reservoir were not truly breathing circuits. The T-pieces with an expiratory reservoir rely on dilution of carbon dioxide by both fresh gas and room air for its removal; these have been included in semiclosed circuits below.²⁶

Connection of the Patient to the Breathing Circuit

Either an anesthesia mask, supraglottic device, or a tracheal tube connects the circuit to the patient. Masks are made from rubber or clear plastic to make secretions or vomitus visible (Fig. 4-1). Most have an inflatable or inflated cuff, a pneumatic cushion that seals to the face. Masks are available in a variety of sizes and styles to accommodate the wide variety of facial contours. For example, a prominent nasal bridge may prevent a tight fit if the mask’s cuff is flat at that point. A prominent chin (mentum) with sunken alveolar ridge causes a leak at the corner of the mouth, and the volume of the mask contributes to apparatus dead space. The mask should fit between the interpupillary line over the nose and in the groove between the mental process and the alveolar ridge (Fig. 4-2). The average length of this area is 85 to 90 mm in adults. The newest disposable plastic masks are available in a wide range of sizes, intended to fit the faces of small children and large adults equally well. Choosing from a selection of mask sizes and styles is more rational than a “one size fits all” approach because a poorly fitting mask can result in trauma to the patient. This is especially true when the mask must be positioned above the eyebrows because it can cause pressure on, and possibly damage to, the optic and supraorbital nerves. Masks often have a set of prongs for attachment to a rubber mask holder or head strap; however, if pulled too tight, this mask holder may obstruct the airway. Masks connect to the Y-piece or elbow via a 22-mm (⅞-inch) female connection.

Breathing Tubing

The tubing used in breathing circuits typically is approximately 1 meter in length, has a large bore (22 mm) to minimize resistance to gas flow, and has corrugations or spiral reinforcement to permit flexibility without kinking. The internal volume is 400 to 500 mL/m of length. Although these tubes were formerly made of conductive rubber, disposable plastic tubing has almost completely replaced rubber. Electrical conductivity is no longer necessary when breathing tubing is used with nonflammable agents. The advantage of plastic is that it is lightweight; however, it is not biodegradable and thus is disposable by design although not by use. Plastic tubing for a breathing circuit is supplied sterile despite the lack of convincing epidemiologic data to support the necessity of sterile tubing.^27,28 On occasion, it is necessary to pass a breathing circuit on to the sterile surgical field (e.g., during an ex utero intrapartum treatment procedure). By convention, the ends of the tubing are 22 mm in internal diameter (ID) and are identical in design. Tubing should be inspected before use because manufacturing errors can result in obstruction of the lumen.^29,30 Compliance of the tubing varies from nearly 0 to more than 5 mL/m/mm Hg of applied pressure, and plastic tubing has lower values than rubber (Table 4-1). Apparent distensibility is even greater because compression of gas under pressure, to the order of 3% of the volume, occurs at typical inflation pressures. Inflation of a patient’s lungs to 20 cm H₂O peak inspiratory pressure compresses 30 to 150 mL of gas in the tubing.³¹ This volume is not delivered to the patient’s lungs, but some fraction of it may be measured by a spirometer within the circuit, adding a form of apparatus dead space to the system. The exact fraction depends on where the spirometer is placed in the circuit with respect to the unidirectional valves.

Resistance to gas flow in standard, corrugated breathing tubes is exceedingly small—less than 1 cm H₂O/L/min of flow.³² When it is desirable to have the anesthesia machine at some distance from the patient’s head, several tubes may be connected in series with connectors 22 mm (⅞ inch) in outside diameter (OD). Alternatively, extra-long tubing is available, including tubing that can be compressed to 200 mL of volume in approximately 50 cm of length or that can be stretched to nearly 2 m with an 800-mL volume. These “concertina” extensions do not increase the resistance of the system by any appreciable amount and affect the apparatus dead space only by their compliant volume (Fig. 4-3).

The pattern of gas flow through the circuit is almost always turbulent because of the corrugations in the tubing, which promote both radial mixing and longitudinal mixing. In documenting performance of one circuit, Spoerel³³ demonstrated complete mixing of dead space and alveolar gas after gas had passed through 1 m of such tubing. A change in gas composition at one end, such as when the delivered gas is altered at the anesthesia machine, completes a change in the inspired concentration at the patient connection within two to three breaths. The change in inspired concentration is nearly exactly the change in delivered concentration when high fresh gas flows are used (≥10 L/min). The change decreases to nearly imperceptible as inflow is decreased toward that of closed systems.

Lengths of breathing tubing are sometimes used to connect ventilators to the bag mount and to connect to scavenging devices. Optimally, either a 19- or 30-mm diameter ends on the scavenger mounts prevent inappropriate connections. Tubing of smaller diameter is made for use in circle systems designed specifically for infants and children, and their resistance to gas flow is insignificantly increased. With less compression volume, measured ventilation is more accurate.

Reusable rubber tubing is connected to the mask or tube by a separate Y-piece. Disposable sets often incorporate a Y that may or may not be detachable. Such a Y may be rigid, and it may incorporate an angle elbow or a pair of swivel joints. Although the swivel joints are convenient, they offer a greater chance of leaking; most connectors have negligible leakage, but those with swivels are twice as likely to leak.³⁴ Any circuit should be tested before use by determining the oxygen inflow required to maintain 30cm H₂O of pressure in the circuit (see also Chapter 32).

Unidirectional Valves

Unidirectional valves are incorporated into a breathing circuit to direct respiratory gas flow. They are commonly disks on knife edges or rubber flaps or sleeves. The essential characteristics of respiratory valves in breathing circuits are low resistance and high competence.^35,36 The valves must open widely with little pressure and must close rapidly and completely with essentially no backflow.

Circle and nonrebreathing systems use two nearly identical valves: the inspiratory valve opens on inspiration and closes on expiration, preventing backflow of exhaled gas in the inspiratory limb. The expiratory valve works in a reciprocal fashion to prevent rebreathing. These valves can be mounted anywhere within the inspiratory and expiratory limbs of the circuit. The only critical feature of their location is that one must be placed between the patient and the reservoir bag in each limb. Properly positioned and functioning, they prevent any part of the circle system from contributing to apparatus dead space.³⁷ Thus the only apparatus dead space in such a circuit is the distal limb of the Y-connector and any tube or mask between it and the patient’s airway. The respiratory valves on most modern anesthesia machines are located near, or incorporated into, the carbon dioxide absorber canister casing along with a fresh gas inflow site and excess gas (pop-off) valve. In the past, unidirectional valves have been incorporated into the housing of the Y-piece to decrease the apparatus dead space effect of compliance volume, but they have fallen into disfavor because of the weight they add to the mask. More importantly, they cause an obstruction to respiration if they are accidentally incorporated backward to the conventional valves in the circle.³⁸ When valved Y-pieces were used, it was recommended that circle system valves be removed. Failure to reinsert the circle system valves when a normal nonvalved Y-piece was used has caused needless complications.

The common valves in anesthetic circuits are dome valves consisting of a circular knife edge occluded by a very light disk of slightly larger diameter (Fig. 4-4). The disk lifts off the knife edge when flow is initiated by the patient’s inspiratory effort, when positive pressure is applied to the reservoir bag, or when the ventilator bellows empties. The disk is contained either by a small cage or by the dome itself. It must be hydrophobic so that water condensation does not cause it to stick to the knife edge and thereby increase the resistance to opening. Most modern disks are made of hydrophobic plastic and are light and thin. When properly functioning, the disk in a unidirectional valve can be lifted with a circuit pressure of 0.31 cm H₂O or less. Most unidirectional valves are mounted vertically, with the disk oriented horizontally, so that it will fall properly into the closed position and seal the circuit from backflow. The valve disks also can be oriented vertically, as on the absorber block of the Datex-Ohmeda ADU workstation (GE Healthcare, Waukesha, WI; see Fig. 4-4, D). Failure to seal converts a large volume of the circuit into apparatus dead space, resulting in rebreathing. The top of the valve is covered by a removable clear plastic dome so that the disk can be easily seen and periodically cleaned or replaced.

A nonrebreathing system requires two appropriately placed one-way respiratory valves (Fig. 4-5). Nonrebreathing valves permit the patient to inspire fresh gas from a reservoir and exhale alveolar gas into the room or into a scavenger. Such valves usually consist of a pair of leaflets in the same housing: one opens during inspiration, the other opens during expiration. The early nonrebreathing valve designs, such as the Digby-Leigh or Steven-Slater, required the anesthesiologist to occlude the expiratory valve with a finger if assisted or controlled ventilation was needed (Fig. 4-5, A).^9,24 Modern designs that use springs, magnets, or flaps automatically close the expiratory valve when respiration is controlled.^39-44 Other designs use the pressure difference across the inspiratory valve to inflate a mushroom-shaped balloon (Frumin) valve (Fig. 4-5, B),¹¹ or to depress a dome-shaped cover on the expiratory (Fink) valve.⁶ Resistance is negligible in both designs, but the Frumin valve has the marked advantage of collapsing if the inspiratory supply is inadequate, permitting inspiration of room air. The Frumin valve also is lighter and more compact than the others. Some nonrebreathing valves are position sensitive and must be vertically oriented to function properly.⁴⁵ Those that use flexible rubber leaflets or collapsible rubber tubing to provide the sealing function are not positional. Most nonrebreathing valves connect to masks and/or tracheal tubes, but a valve can be built into a mask.⁴⁴

Self-inflating resuscitators for air or air-oxygen mixtures use similar pairs of valves to control gas flow.^10,46,47 The Ruben valve has an expiratory bobbin-shaped structure that, when open, occludes the inspiratory limb (Fig. 4-6). Anesthetic vapors and secretions tend to expand this bobbin slightly, causing it to jam.⁴⁷ Such resuscitator valves should not be used in anesthesia, nor should they be used for transporting patients who are still exhaling anesthetic agents.

Breathing Bags

Breathing bags, also known as reservoir bags or counterlungs, have three principal functions: 1) they serve as a reservoir for anesthetic gases or oxygen, from which the patient can inspire; 2) they provide the means for a visual assessment of the existence and rough estimate of the volume of ventilation; and 3) they serve as a means for manual ventilation. A reservoir function is necessary because anesthesia machines cannot provide the peak inspiratory gas flow needed during normal spontaneous inspiration. Although the respiratory minute volume of an anesthetized adult is rarely more than 12 L/min, the peak inspiratory flow rate may reach 50 L/min, with 20 L/min not uncommon. For example, assume a patient is breathing at a rate of 20 breaths/min with a tidal volume of 500 mL and a minute volume of 10 L/min. If the inspiratory to expiratory ratio (I:E) is 1:2, each breath takes 1 second for inspiration and 2 seconds for exhalation. The tidal volume of 500 mL inspired in 1 second is an average inspiratory flow (volume per unit time) of 500 mL/sec or 30 L/min. This is many times greater than the commonly used fresh gas flows. The peak flow in mid-inspiration may be 30% to 40% higher.

Assessment of the presence and volume of spontaneous ventilation is affected by the fresh gas flow. In low-flow techniques, virtually all the gas inhaled by the patient comes from the reservoir bag, and its excursion thus reflects tidal volume. If the fresh gas inflow rate from the machine exceeds 10 L/min, most of the gas inhaled by the patient comes from the fresh gas supply, and the reservoir bag shows little excursion. In a spontaneously breathing patient with a circuit gas inflow rate of 6 L/min, nearly half the tidal volume comes from the fresh gas inflow, halving the apparent tidal volume as indicated by movement of the bag.

Reservoir bags for anesthesia machines usually are ellipsoid so they can be easily grasped with one hand. They are made of nonslippery plastic or latex in sizes from 0.5 to 6 L. To improve grip, some have an hourglass shape or a textured surface; nonlatex bags are available for use with patients who have a latex sensitivity. The optimally sized bag can hold a volume that exceeds the patient’s inspiratory capacity; that is, a spontaneous deep breath should not empty the bag. For most adults, a 3-L bag meets these requirements and is easy to grasp. Bags with a nipple at the bottom for use as an alternate pop-off site are available but are rarely used.

In circle systems, the breathing bag usually is mounted at or near the carbon dioxide absorbent canister via a T-shaped fitting, usually near the pop-off valve. The bag also may be placed at the end of a length of corrugated tubing leading from the T-connector to provide some freedom of movement for the anesthesiologist. The pressure-volume characteristics of overinflated bags become important if the pop-off valve is accidentally left in the closed position and gas inflow continues (Fig. 4-7). Rubber bags become pressure limiting with maximum pressures of 40 to 50 cm H₂O, although prestretching may favorably lower the maximum distending pressure.^48-50 Disposable bags may reach twice the pressure of rubber bags and then rupture abruptly.

Gas Inflow and Pop-off Valves

Gases are delivered from the anesthesia machine common gas outlet to the circuit via thick-walled tubing connected to a nipple incorporated into the circuit. In circle systems this gas inflow nipple is incorporated with the inspiratory unidirectional valve or the carbon dioxide–absorbent canister housing. The preferred fresh gas inflow site is between the carbon dioxide absorber and the inspiratory valve. The location for other circuits depends on whether breathing is spontaneous, assisted, or controlled because the type of breathing influences the efficiency of carbon dioxide elimination.

Pop-off valves—also known as overflow, outflow, relief, spill, and adjustable pressure-limiting (APL) valves—permit gas to leave the circuit, matching the excess to the inflow of fresh gas. The efficiency of an APL valve is related in part to the placement of the fresh gas inflow. There are many different designs, but most are constructed like a dome valve loaded by a spring and screw cap (Fig. 4-8). The valve should open at a pressure of less than 1 cm H₂O. As the screw cap is tightened down, more and more gas pressure in the circuit is required to open it, permitting PEEP during spontaneous ventilation or pressure-limited controlled respiration. The number of clockwise turns from fully open to fully closed should be one or two: fewer turns make it difficult to set a desired circuit pressure accurately, whereas more make it tedious to use. The exhaust from any of the commonly used pop-off valves can be collected by scavenging system transfer tubing connected at this point.⁵¹

The Datex-Ohmeda GMS absorber uses an APL valve similar in design to that shown in Figure 4-8, which basically is a spring-loaded disk. When the spring is fully extended, it exerts a pressure of approximately 1 cm H₂O on the disk to hold the valve closed. This is necessary because the waste gas scavenging interface is connected downstream of the APL valve and transfer tubing. If an active scavenging system is used—that is, if suction is applied to the interface—the negative pressure could potentially be applied to the patient circuit (see Chapter 5). To prevent this, the Ohmeda scavenger interface uses a negative-pressure relief (“pop-in”) valve that opens at a pressure of −0.25 cm H₂O to allow room air to enter the interface. Thus the greatest negative pressure needed to open the APL valve (−0.25 cm) is less than the least spring tension needed to keep the valve closed (~1 cm H₂O). This arrangement, with the use of an active scavenging system, protects against application of excess negative pressure to the breathing circuit. In the fully closed position, the maximum spring pressure applied to the Datex-Ohmeda APL valve disk is 75 cm H₂O. Thus, in the manual/bag mode, the circuit pressure in an Datex-Ohmeda breathing system is limited to 75 cm H₂O. Note that in the ventilator mode, the circuit pressure is limited by high pressure-limit settings on the Datex-Ohmeda ventilator (up to 100 cm H₂O with the Datex-Ohmeda 7800 and 7900 ventilators; see Chapter 6).

In Dräger Medical (Telford, PA) anesthesia delivery systems, the design of the APL valve differs from those described above (see Fig. 4-8, B). This design uses a needle valve instead of a spring-loaded disk, and adjusting the knob varies the size of the opening between the needle valve and its seat, which in turn adjusts the amount of gas permitted to flow to the scavenger system. A check valve prevents gas from the scavenging system from entering the breathing system. With this design, the needle valve can be totally closed; it therefore does not function as a true pressure limiter.

Special types of pop-off valves permit spontaneous or assisted respiration without tedious adjustment.^40,52-54 The simplest is the Steen valve (Fig. 4-9), which essentially is two knife-edge valves of the dome type, one inverted over the other, that share a common disk.²⁴ A relatively slow flow of gas during the latter part of exhalation, up to 10 L/min, lifts the valve disk at one side only so that the exhaled gas escapes around the disk. An abrupt increase in pressure lifts the valve vertically, seals it against the upper knife edge, and closes the circuit so that no gas is lost. The Georgia valve adds a light spring loading to the same design, which increases the range of gas flows it can exhaust; this is necessary for use with mechanical ventilators.⁵⁵ Most current anesthesia ventilators have such an automatic pop-off valve built in so that gas is exhausted only at end exhalation (see also Chapter 6).

Indicator Dyes

Organic dyes are added to soda lime and barium hydroxide lime to provide a visual indication of its state. As carbonate is formed from the hydroxide, the pH becomes less alkaline and the granules change color: ethyl violet changes from white to blue violet with exhaustion, ethyl orange from orange to yellow, and cresyl yellow from red to yellow. Ethyl violet is the dye most commonly used because the color change is vivid and of high contrast at a pH intermediate between NaOH and CaCO₃. It can be bleached by intense light, but in the usual OR setting this is not a problem. A slight fading of color can be seen in the zone of active absorption when use stops. This so-called regeneration occurs where the lime is nearly exhausted of calcium hydroxide but has all alkaline hydroxides neutralized.

The color changes only because of the regeneration of a small amount of sodium and potassium hydroxide. At the next use, the expended nature of the soda lime rapidly becomes evident. There is no true regeneration of activity, and the color change of indicator lime is not to be relied on. The anesthesiologist must know what color change is expected of the absorbent being used, allow for the effects of regeneration, be cognizant of the effects of preferential gas flow at the surface between the smooth plastic canister and the irregular granules (channeling), and understand the effects of the fresh gas flows chosen based on how long a given charge of soda lime can be expected to last. No indicator or rules offer absolute predictions, but the use of capnometry to detect increasing inspired carbon dioxide remains the gold standard for assessing adequate carbon dioxide removal.

Mesh Size and Channeling

Soda lime is precisely manufactured to maximize its absorptive qualities and to minimize resistance to gas flow.⁵ The granules are sized 4 to 8 mesh (i.e., they will pass through a strainer having 4 to 8 wires per inch) and have a rough, irregular surface that maximizes the surface/volume ratio that facilitates the rapid diffusion of carbon dioxide through the pores to the voids within the granules.^56,59-62 Approximately half the volume of a packed canister is gas. The gas volume of the voids is inversely proportional to the water content of the granules and is 1 to 2 times that of the volume between granules. Soda lime is supplied either in quart cartons that fill a canister, disposable canisters, or bulk containers ranging from 5 pounds to 5 gallons. The volume between granules can be reduced by overzealous packing at the risk of creating fine particles and dust that are irritating. Channeling, or flow moving preferentially along the sides of the canister and within the absorbent itself, was a problem with to-and-fro canisters that were often horizontal and improperly packed. This problem can be minimized by the use of baffles, placement so that gas flow is vertical, permanent mounting to avoid frequent canister movement, use of prepackaged cylinders, and avoidance of overly tight packing. Modern carbon dioxide–absorbent canisters (Fig. 4-10) follow the design of the Roswell Park absorber with double chambers to promote efficient use, circular baffles to minimize channeling, and mixing space at the top and bottom.^19,56 Although most have clear plastic walls, some are tinged blue for the purpose of enhancing the appearance of color change in the indicator dye.

Other Reactions with Absorbents

Sevoflurane did not gain U.S. Food and Drug Administration (FDA) approval for use until 1995 despite its use in Japan and elsewhere since the 1970s. When sevoflurane was first described, early testing revealed the production of fluoromethyl-2,2-difluoro-1-(trifluoromethyl) vinyl ether, better known as compound A, when sevoflurane is exposed to various alkalis, including soda lime.⁶³ This reaction occurs when hydrogen fluoride is eliminated from the isopropyl moiety of sevoflurane. Concern arose over evidence that compound A is nephrotoxic—and, at higher concentrations, lethal—in rats.^64-66 Although studies of the nephrotoxicity of compound A in humans have had conflicting results,^67-69 sevoflurane has been administered with apparent safety for several years.⁶⁸ What is clear is that certain factors related to the breathing system can contribute to the production of compound A during anesthesia with sevoflurane. These include increasing inspired sevoflurane concentration, increasing absorbent temperature, decreasing absorbent water content (desiccation), and a decreasing fresh gas flow rate.^70,71 Hypotheses to explain the effect of low flows on compound A production include increased contact of exhaled gas with carbon dioxide absorbent, increased rebreathing of compound A, and increased absorbent temperature. Current sevoflurane labeling indicates that whereas a fresh gas flow from 1 to 2 L/min may be used safely for fewer than 2 minimum alveolar concentration (MAC) hours, flows less than 2 L/min should not be used for more than 2 MAC hours, and flows below 1 L/min are not recommended for any duration with this agent. The choice of absorbent, whether barium hydroxide lime or soda lime, does not seem to significantly influence production of compound A (Fig. 4-11).^70,72

Desiccation of carbon dioxide absorbents is known to be related to another potential hazard, that of carbon monoxide production. The notion that carbon monoxide could be found in detectable amounts in anesthesia breathing circuits containing soda lime was first reported with the use of trichlorethylene and during closed-circuit anesthesia secondary to endogenous production.⁷³ More recently, it has been recognized that difluoromethyl ethers in common use today, including desflurane and isoflurane, can liberate carbon monoxide during destruction of these agents by carbon dioxide absorbents.⁷⁴ Difluoromethyl ethers are fluorinated volatile anesthetics that contain an –O–CHF₂ moiety. Early case reports of unexplained carboxyhemoglobinemia during enflurane anesthesia pointed to an association with “first case on Monday morning” anesthetics⁷⁵ or with older absorbents.⁷⁶ This has been explained by invitro studies that revealed a marked association between dryness of the absorbent and production of carbon monoxide.⁷⁴ It is theorized that high oxygen flows over the absorbent canister during the weekend, or after prolonged absorbent use, desiccate the absorbent. The type of anesthetic is important, with the order of carbon monoxide production being desflurane, which produces the most, followed by enflurane then isoflurane. Sevoflurane and halothane lack an –O–CHF₂ moiety and do not appreciably produce carbon monoxide in the presence of carbon dioxide absorbents. Barium hydroxide lime seems to cause greater carbon monoxide liberation than does soda lime, but both absorbents produce more carbon monoxide with increasing temperatures and with increasing concentrations of anesthetic.⁷⁴ Recognition of the presence of carbon monoxide in an anesthesia breathing circuit in which desflurane or isoflurane is being used may be facilitated by the use of a multiwavelength pulse oximeter (pulse CO-oximeter; see Chapter 11) that can continuously measure carboxyhemoglobin. Such devices are available and used in emergency departments but are not yet widely used in the operating room.^77-78 It is recommended that absorbent that is known or suspected to be desiccated not be used, especially with desflurane, isoflurane, or enflurane. The FDA recommends replacement of any absorbent suspected of contributing to the presence of carbon monoxide in the breathing circuit, although some investigators have suggested rehydration of existing absorbent as a practical and more cost-effective alternative.⁷⁹

Alternatives to soda lime are available. For example, lithium hydroxide offers a little more carbon dioxide absorption capacity per unit of volume but more than three times per unit of weight; it is therefore used in submarines. Barium hydroxide lime contains 20% Ba(OH)₂ and little alkali. Its dust is slightly less alkaline when dissolved in water, and it is a suitable alternative to soda lime.⁸⁰ The end products are barium carbonate and calcium carbonate. It is initially pink but turns blue-gray with exhaustion because of two indicator dyes, Mimosa Z and ethyl violet. Barium hydroxide is as efficient as soda lime per unit of volume, but because of its density, it is half as efficient per unit of mass.

Methods of carbon dioxide removal other than chemical reaction with metal hydroxides have also been investigated. Much of the research in this area has originated from aviation, space, and submarine technology. One method of particular interest is a molecular sieve that uses synthetic zeolites, crystalline hydrated aluminosilicate materials, arranged in a three-dimensional tetrahedral framework that contains entry pore sites and cavities.⁸¹ Gases that enter the sieve separate on the basis of size and polarity. Carbon dioxide is a polar molecule and is retained in certain zeolites by the action of Van der Waal forces. Because chemical bonding does not take place, the process can be reversed by slight changes in pressure and temperature, thus allowing regeneration of saturated zeolite. This technology has already been used extensively in industry for petroleum refining, water purification, and drying of gases and liquids. It has also been used for aviation and medical purposes in oxygen generators.^82,83 Advantages suggested for the use of molecular sieves in anesthesia breathing systems include lack of compound A in the presence of sevoflurane, avoidance of carbon monoxide production, removal of nitrogen dioxide in circuits that deliver nitric oxide, and cost savings as a regenerative process.^84,85

Mixing Devices

Resistance to gas flow in a modern circle system is less than 1 cm H₂O/L/min of gas flow, one half of a person’s normal airway resistance to gas flow.^35,36 Patients, including infants and children, may safely breathe spontaneously from a circle system for prolonged periods. However, before circuit designs permitted such low resistances, attempts were made to decrease resistance to breathing by providing a continuous flow of gas around the circle, thereby causing the inspiratory and expiratory valves to float open rather than open and close with each breath. Both pumps and Venturi devices driven by fresh gas flow were designed, the most prominent of which was the Revell circulator.^86,87 Although these devices can decrease dead space and resistance in the apparatus, the potential benefit is slight; in some circumstances these devices can backfire, actually increasing the work of breathing. If the low resistance of modern equipment is still a concern, it can be eliminated by controlled ventilation. Ways to reduce the mean airway pressure during controlled ventilation have been suggested but are not commonly used.⁸⁸

Mixing of the expirate is required to measure carbon dioxide production and physiologic dead space. Standard physiologic testing usually collects all the expired gas for several minutes to mix and measure the mixed expired gas concentrations needed in these calculations (see also Chapter 8). Simply averaging a continuous capnogram will not do; this yields a time-weighted average instead of the required volume-weighted average. To understand the difference, consider what happens toward the end of a respiratory cycle. While the carbon dioxide is increasing slightly, the flow is rapidly decreasing and may become zero at the end-expiratory pause. Carbon dioxide excretion should be the integral of concentration with respect to flow; when flow falls to zero, so does carbon dioxide excretion, but the increased end-tidal value continues to increase the time-weighted integral of the capnogram. Special volume mixing devices can be used, but suitable sites for such measurements may include the breathing bag, the ventilator bellows, or the expiratory port of the ventilator pressure relief valve at its connection with the scavenging system.

Circle Breathing Systems

The basic components of a circle breathing system are an inspiratory and expiratory limb, each with a unidirectional valve, and a reservoir bag or counterlung that moves reciprocally with the patient’s lungs. The system may be divided into quadrants (Fig. 4-12). The patient and counterlung, but not the absorber, separate the inspiratory and expiratory limbs of the system; the valves separate the patient from the bag side of the system. The position of the valves within the limbs is not necessarily fixed; they may be anywhere between the patient and the bag with little practical difference in function. They usually are incorporated with the bag mount, pop-off valve, and absorber for manufacturing ease and durability. Even if the valves are moved to other locations, it is convenient to think of four quadrants when analyzing circle systems.

For practical anesthesia, it is necessary to add three other components: a carbon dioxide absorber, a fresh gas inflow site, and a pop-off valve for venting excess gas. Each of the three may be placed in any of the four quadrants. There are hundreds of different ways to place three components in four quadrants, but only a few are used.⁹⁹ Different manufacturers have used different arrangements, and older designs allowed the user to change the configuration with slight change in function. The optimal configurations for spontaneous and controlled ventilation are different,¹⁸ and most current designs are optimal for controlled ventilation.

This analysis emphasizes a frequently overlooked or misunderstood point: The bag, not the absorber, is on the opposite side of the circle from the patient. Most schematics and diagrams of breathing circuits perpetuate this misconception by showing symmetrical circuit limbs on either side of the absorber. The position of the bag is at neither the inspiratory limb nor the expiratory limb; rather it separates the two.³⁷

Placement of the Carbon Dioxide Absorber, Fresh Gas Inflow, and Pop-off Valve

The carbon dioxide absorber may be placed in any of the four quadrants but is almost invariably placed in the inspiratory limb on the bag side so that apparatus resistance to inspiration may be overcome with assisted or controlled ventilation (Fig. 4-13). Placement on the expiratory side (Foregger circles) adds a mild degree of expiratory resistance that, like PEEP, may be beneficial for oxygen exchange but increases the risk of barotrauma. A bypass valve to permit carbon dioxide to build up was considered desirable by a majority of anesthesiologists but not by those who build the apparatus.¹⁰⁰ The carbon dioxide absorber is absolutely necessary in a low fresh gas flow technique because no other method is available to remove carbon dioxide; that is, it is removable neither by dilution nor exclusion from reinspiration. As fresh gas flow is increased above gas uptake rates, the other methods become increasingly effective. This is noted by the anesthesiologist as longer intervals before the indicator in the absorbent suggest that it be replaced. At a 5 L/min fresh gas flow, the absorbent lasts more than twice as long as at 500 mL/min. This is not cost effective because the extra anesthetic gases and vapors cost more than the absorbent saved. In addition to saving on agents, the absorbent has two other beneficial effects: both expired humidity and heat are partially conserved, and both are optimized as flow decreases.^101,102 The risks of absorbent (e.g., soda lime) include airway reaction to inspired alkaline dust, which can be minimized by good design and technique; alkaline “burns” from condensed water and/or dust spilled from the canister housing; and a breathing circuit with more connections and pieces, which provides more opportunity for user error.

Although the inflow site for fresh gas may be physically placed in any of the four quadrants, efficiency and manufacturing convenience also are factors in placement.^18,103 The inflow site usually is incorporated with the other components; that is, the absorber, bag mount, and valves. If the inflow site is located on the patient side of the inspiratory valve (quadrant B, Fig. 4-13), gas flows continuously around the circle throughout the respiratory cycle. Thus spirometry in the expiratory limb is inaccurate unless total gas flow is shut off.¹⁰⁴ If inflow is located on the patient side of the expiratory valve (quadrant C, Fig. 4-13), any carbon dioxide–containing alveolar gas between the Y-piece and the patient is washed into the patient’s lungs during inspiration at the rate of the fresh gas flow. This may be negligible during closed-system anesthesia, but it can produce rebreathing of up to half of the previously exhaled alveolar gas at total flows of 10 L/min. Placement in quadrant D is simply inefficient because some fresh gas will be lost to the pop-off valve in most circles.

Recommended placement for the fresh gas inflow is on the bag side of the inspiratory limb between the inspiratory valve and absorber (quadrant A, Fig. 4-13). If the absorber housing has appreciable head space, as is common, this inflow site stores the continuously delivered fresh gas during exhalation. Gas flows down the inspiratory limb only during inspiration. During exhalation, fresh gas flows backward toward the absorber, bag, and pop-off valve. Thus fresh gas provides most of or all the respired gas with high-flow techniques, or it enriches the oxygen and anesthetic-depleted expiratory gas with low-flow techniques.

The pop-off valve may be placed anywhere in the circle, but some locations are more rational than others. Locating the valve in the inspiratory limb (quadrants A and B, Fig. 4-13) tends to vent fresh anesthetic and carbon dioxide–free gas. Furthermore, locations in the inspiratory limb on the patient side (quadrant B) permit some carbon dioxide–containing exhaled gas to enter the inspiratory limb during the end of a spontaneous breath. This rebreathing is clearly undesirable. It is mechanically convenient to place the pop-off valve between the expiratory valve and the bag mount, or opposite the bag mount, before the absorber (i.e., in quadrants D or A, but close to the bag mount). Incorporating the pop-off valve into a one-piece absorber/bag mount/expiratory valve/pop-off valve assembly provides convenience and durability.

There is at least theoretic value to locating the pop-off valve on the patient side at the Y or next to it (quadrant C, Fig. 4-13). During spontaneous inspiration, the pressure at this site is below atmospheric pressure and the pop-off valve is closed. During exhalation, the pressure is just slightly above atmospheric pressure, and gas flows to the reservoir bag until it is distended to its nominal volume. Then the pressure in the entire circuit increases as fresh gas inflow and exhalation from the patient continue, opening the pop-off valve. The gas vented is primarily carbon dioxide–rich, oxygen-depleted, and anesthetic-depleted end-tidal gas. However, the situation is reversed during assisted or controlled ventilation. During inspiration, the pressure in the circuit at the Y-piece is positive with respect to atmospheric pressure. A pop-off valve located at the Y-piece would dump fresh gas, and one near the bag would dump a mixture of dead space gas and end-tidal gas. Two pop-off valves on the same circuit double the risk of hypoventilation when ventilation is changed to manual because the anesthesiologist may fail to close both valves. For this reason, pop-off valves at the Y-piece are no longer used.

Flow and Concentration

The gas mixture inspired by a patient breathing from a circle system is determined by the fresh gas inflow, the configuration of the circle, the respiratory pattern, and the uptake by the patient of oxygen and anesthetics. At fresh gas inflow rates that exceed minute ventilation, the circle behaves similar to a nonrebreathing system. The concentration of inspired gas closely approaches that being delivered from the gas flowmeters on the machine. As flow is progressively decreased, a disparity between fresh gas inflow and actual inspired concentration of anesthetics increases.

In a closed system, in which inflow matches loss from the system, the composition of reinspired gas is not predictable from the inflow concentration of gases.¹⁰⁵ Inspired and exhaled gases differ because of uptake of oxygen and anesthetic and excretion of carbon dioxide. Oxygen concentration in mixed exhaled gas usually is 4% or 5% lower than that in inspired gas. Although oxygen uptake remains relatively constant during anesthesia (approximately 250 mL/min standard temperature and pressure dry [STPD]) in the average adult, provided that body temperature does not change appreciably, anesthetic uptake varies; it is greatest at the start of anesthesia and decreases with time. If oxygen concentration is maintained constant during closed-system anesthesia, the flowmeter values reflect oxygen consumption and anesthetic uptake by the patient rather than inspired concentrations. When nitrous oxide is used in closed-system anesthesia, the oxygen tension or concentration in the circle must be continuously monitored because nitrous oxide uptake declines but oxygen uptake does not. The inspired gas mixture may become hypoxic if the nitrous oxide inflow is not gradually decreased. Whether it is necessary to measure the concentration of potent volatile anesthetics during closed-system anesthesia remains controversial. Some believe that it is necessary, and others prefer to monitor anesthetic depth and patient responses clinically (see also Chapter 26).

Semiclosed Systems: Mapleson Classification

Mapleson configurations of Magill circuits are characterized by a reservoir that can be filled by fresh gas, exhaled gas, or both. They may or may not have a pop-off valve, but if one is present it does not prevent rebreathing. Carbon dioxide, eliminated by both dilution with fresh gas and by efficient arrangement of the circuit components, is critically affected by total fresh gas flow and the pattern of respiration. The pattern of respiration includes the respiratory rate, tidal volume, dead space, I:E ratios, and inspiratory and expiratory flow patterns. The earliest semiclosed systems consisted of a bag attached to a mask by an elbow or a length of breathing tubing if it was desirable to move the large bag away from the face. The inlet for fresh gas was through a nipple on the elbow or through the tail of the bag, and a pop-off valve was placed either at the bag’s tail or at the elbow. The bag ideally contained a volume approximating the patient’s inspiratory capacity, about 3 L in an adult, to permit spontaneous deep breaths without the feeling of suffocation. Gas flows totaling 1 to 2 times the minute volume were used.^8,101 Mapleson organized the various configurations into five typical models and later added a sixth (Fig. 4-14).^7,26 Each of these breathing systems may be thought of as part of the continuum shown in Table 4-2.

In Table 4-2, the far right situation represents complete rebreathing and is of use only in the study of respiratory control. The original Mapleson B and C configurations lie midway on the continuum and are judged to have no particular merit, except perhaps for brief procedures or to transport patients while supplemental oxygen is administered and breathing is augmented.

None of the Mapleson systems meets the requirements on the far left of Table 4-2, but with optimal configuration and high fresh gas flow, they may approach the function of an open system. Efficiency of the systems in this context can be translated as the lowest fresh gas flow that will ensure normal removal of carbon dioxide, thereby minimizing rebreathing. There is normally a partial pressure difference for carbon dioxide such that alveolar CO₂ (P_ACO₂) is greater than mixed expired CO₂ (P_ECO₂), which is greater than inspired CO₂ (P_ICO₂).

The most efficient configurations place the pop-off valve where the highest concentration of carbon dioxide is found during the phase of breathing in which the circuit pressure is above atmospheric pressure. This occurs at end expiration during spontaneous breathing and during inspiration with manually assisted or controlled breathing.

A great deal of attention has been devoted to determining the lowest gas flow that can be safely used in clinical anesthesia. A variety of claims have been made for the various circuits and for proprietary modifications of the circuits, such as Bain, Lack, Humphrey ADE, and Mera F circuits. Unfortunately much of the published work has one or more of the following flaws:

However, consensus has been reached in one regard: systems classified as Mapleson A (Magill attachment, Lack, and Humphrey A) are most efficient for spontaneous, unassisted ventilation, and those classified as Mapleson D, E, or F (Jackson-Rees, Bain, Humphrey DE) are most efficient for assisted or controlled ventilation.

A general criticism of the published analyses of breathing circuits is a failure to distinguish between the quantity of reinspired carbon dioxide and minimum inspired carbon dioxide tension. The most common tool, a capnograph, displays a signal of airway concentration as a function of time. Thus, if airway carbon dioxide falls slowly with inspiration, just reaching zero near end inspiration, it is interpreted as no rebreathing, even though a significant amount of carbon dioxide has been reinspired. Conversely, if airway carbon dioxide falls to a low but nonzero concentration for all of inspiration, carbon dioxide excretion may be adequate despite perceived rebreathing if total ventilation is increased. Inspired carbon dioxide is properly calculated as the integral of instantaneous flow multiplied by instantaneous carbon dioxide concentration, which is difficult or impossible to measure with simple instrumentation. (See also sections on volumetric capnography in Chapters 8 and 10.) A better analysis is based on the equation of defining alveolar ventilation (_A):

(4-6)

Equation 4-5 states that alveolar ventilation is the quotient of carbon dioxide production and alveolar fractional concentration of carbon dioxide. Because the fractional volume of alveolar carbon dioxide (F_ACO₂) is proportional to the partial pressure of alveolar carbon dioxide (P_ACO₂), a specific PCO₂ defines one and only one _A in a given patient. Equation 4-6 demonstrates that any increase in dead space ventilation (V_DS × f) can be accommodated by an equivalent increase in minute volume (_E), keeping alveolar ventilation, and hence carbon dioxide elimination, constant. Any amount of carbon dioxide may be rebreathed, at a concentration equal to or below alveolar carbon dioxide, if an increase in minute volume maintains alveolar ventilation.

Even if instantaneous PCO₂ reaches zero near end inspiration, a significant volume of carbon dioxide may be rebreathed, requiring an appropriate increase in minute volume. A numerical example may help clarify this. Consider a patient in need of 4 L/min of _A with F_ACO₂ of 0.05 (5%) and a current _E of 6 L/min with a respiratory rate (f) of 20 breaths/min. The patient could rebreathe 2.5% carbon dioxide and keep F_ACO₂ at 5% if the apparent alveolar ventilation doubled to 8 L/min. If the dead space ventilation did not change—it probably would, but not much, depending on whether tidal volume or f were increased—a total minute volume of 10 L would suffice. Clearly, an inspired carbon dioxide load can be compensated for. Note that the ventilatory response to carbon dioxide of an awake person (slope of 2 L/min/mm Hg) would require a rise of less than 2 mm Hg P_ACO₂. However, at a sensitivity to carbon dioxide frequently seen in an anesthetized person (e.g., 0.5 L/min/mm Hg), the carbon dioxide tension would have to rise nearly 8 mm Hg. Thus a significant difference is apparent between spontaneous breathing, for which ventilation is set by the patient’s P_ACO₂ and responsiveness, and controlled ventilation, for which minute volume is set by the anesthesiologist.

Any gas mixture that contains carbon dioxide can be considered to consist of a fraction of carbon dioxide–free gas and a fraction of alveolar gas. Any rebreathing of alveolar gas is simply added dead space and can be compensated for by increasing overall ventilation. For example, given 4 L/min of alveolar ventilation and 2 L/min of dead space ventilation, what will happen if a patient suddenly inspires 1% carbon dioxide? Each unit of alveolar ventilation now holds only four-fifths of the previous level of newly produced carbon dioxide, so increasing alveolar ventilation by 125% will result in the same degree of carbon dioxide elimination.

Proprietary Semiclosed Systems

Although a variety of pieces of anesthesia hardware can be used to assemble Mapleson circuits A through F, several specific circuits with eponymous identities have been introduced that offer specific advantages. These include the Jackson-Rees, Bain, Lack, Mera F, and Humphrey ADE. The last four are conveniently coaxial; they have a tube-within-a-tube arrangement that moves the physical location of the fresh gas inflow and/or the expiratory valve away from the patient connection elbow while preserving the advantages of the A, D, or F circuits. The Bain and Lack circuits are shown in Figure 4-17.

The Bain circuit is basically a coaxial Mapleson D design. Instead of a separate small-bore tube for delivery of fresh gas to the patient elbow, the delivery tube enters the corrugated expiratory tube near the bag mount and pop-off valve and runs coaxially to the patient end, where the end is secured by a plastic “spider” in the center of the tube. Thus fresh gas is delivered at the patient end, and the pop-off valve exhausts gas at the bag end of the corrugated tube, a Mapleson D arrangement. Various recommendations for fresh gas flow have been published. One commercial brand has a package insert that recommends a fresh gas flow of 100 mL/kg/min. Such recommendations often were based on an instantaneous inspired carbon dioxide concentration of zero for some portion of the cycle. However, with suitably augmented minute ventilation—150 mL/kg or more, instead of the 90 mL/kg for a normal person at rest—adequate carbon dioxide elimination results from a fresh gas flow of 70 mL/kg/min during assisted or controlled ventilation.^12,33,110 Although not recommended for prolonged periods, spontaneous ventilation requires a greater fresh gas flow, up to 150 mL/kg/min.^25,111,112

The Bain circuit may malfunction if the central tube (fresh gas delivery) becomes disconnected, either where it enters the corrugated outer tube or from its retaining spider at the patient end. Either disconnection effectively increases the apparatus dead space, and for any given minute volume, it reduces alveolar ventilation accordingly.¹¹³ Disconnection at the bag end is by far the more serious problem, and visual inspection alone may not identify the problem. Because of this, two tests have been proposed: one uses a very low oxygen flow (50 mL/min) and occlusion of the inner tube with a finger or plunger from a small disposable syringe¹¹⁴; the flowmeter bobbin should fall with occlusion of the inner tube. Alternatively, filling the reservoir bag with gas and operating the oxygen flush will normally create a Venturi effect that partially empties the bag.¹¹⁵

The Lack circuit (coaxial Mapleson A) appears similar to the Bain circuit externally. Near the bag are both a pop-off valve and a fresh gas inflow nipple. However, the central tube is larger in diameter and serves as an expiratory limb, leading from a spider that centers it coaxially to the pop-off valve.¹⁴ The circuit is long enough that this central tube has a volume of 500 mL. Fresh gas flows between the external corrugated tube and the central tube to the patient connection end. This is essentially a Mapleson A circuit and is optimal for spontaneous breathing. Figure 4-17 shows the Lack system at end expiration. The first part of exhalation has passed retrograde between the corrugated hose and inner tube toward the bag, which is simultaneously filling with fresh gas. Because this first part contains little or no carbon dioxide, little carbon dioxide is found in the outer channel. When the bag reaches its nominal volume, the pressure rises enough to open the pop-off valve; for the rest of exhalation, carbon dioxide–rich gas passes into the inner channel. If expiratory flow falls to nearly zero at the end-expiratory pause, fresh gas flows toward the patient through the outer channel and even into the inner tube, pushing alveolar gas out through the pop-off valve. Any carbon dioxide remaining in the inner tube is not rebreathed during the subsequent inspiration because the expiratory valve closes, and little gas flows backward in the inner (expiratory) tubing. Reports that the Lack system is more efficient, equally efficient, or less efficient than a Magill attachment may be found in the literature, but the differences are always slight.^108,116,117 Fresh gas flow at 70% of minute volume results in negligible carbon dioxide rebreathing. It is important to note that with spontaneous breathing, increasing fresh gas flow is of little value in lowering arterial PCO₂, which is set by the patient’s intrinsic respiratory control centers.

The original Humphrey ADE circuit was available as a coaxial volume or as a parallel circuit.¹¹⁸ A fixed, machine-mounted valve assembly with one control lever permitted selection of either a Mapleson A or D configuration. With modern anesthesia ventilators that exhaust excess gas at end expiration, the system in D mode becomes equivalent to a Mapleson E circuit, hence the suffix ADE.¹¹⁸ In the parallel setup, the Humphrey ADE can be used with or without a carbon dioxide absorber (Fig. 4-18). The disadvantage of forgetting to switch the lever and the ubiquitous circle systems in U.S. hospitals have led to greater interest outside than inside the United States. A notable exception is the work of Artru and Katz,¹¹⁹ who studied patients and used a rise in end-tidal rather than inspired carbon dioxide concentration as the criterion for rebreathing. They found that with an appropriate lever setting, an FGF of 66 mL/kg/min prevented an increase in end-tidal carbon dioxide during both spontaneous and controlled ventilation.

Since the original concept of combining the benefits of the Magill, Lack, Bain, and T-piece into one universal unit in 1984, there have been significant improvements to the design and function of the Humphrey ADE system. A new exhaust valve has significantly improved function for spontaneous respiration and eliminated the necessity for a D mode (Fig. 4-19). This new valve also has allowed the T-piece mode for children to be superseded by use of the system in the much more efficient Mapleson A system. A detachable soda lime canister has been added, turning the system into a multipurpose apparatus.

The efficiency of Mapleson A systems for spontaneous respiration depends on the preservation of unused dead space gas within the system during the early phase of exhalation and elimination of alveolar gas in the second half. Standard exhaust valves often open unpredictably, with significant loss of dead space gas, and there can be significant contamination by alveolar gas at the mixing interface within the tube. To prevent the loss of dead space by premature opening of the exhaust valve, a valve was designed that functions in four phases rather than two (i.e., simply open or closed).

The new valve is designed to always ensure that no exhaled gas is vented until the reservoir bag on the inspiratory limb is completely full, whether in the Mapleson A mode or with the soda lime canister. It has a 5-mm chimney encircling the knife edge on which the valve seat rests and a spring that exerts approximately 1 cm H₂O pressure on the seat. When the patient exhales, any pressure within the system lifts the valve seat into the chimney, but the spring keeps it closed (phase 1). In the Mapleson A mode, the expiratory limb remains closed in phase 1, and all dead space gas consequently is preserved along the inspiratory limb, as is fresh gas within the reservoir bag. In mid-exhalation, alveolar gas now flows into the inspiratory limb; at this point, the bag fills completely and the pressure rises within the system. The valve seat now lifts above the chimney to open the valve, allowing it to vent what is now alveolar gas (phase 2). Once open, the valve acts a variable orifice offering almost no resistance to flow. At this stage, mixing at the interface between alveolar gas and dead space is reduced by the use of 15-mm parallel smooth bore tubes (less turbulence and mixing) rather than coaxial or corrugated tubing. A “flip-flop” design Y-patient connector also helps direct the flow of alveolar gas into the expiratory limb. Exhalation continues with venting of alveolar gas through the exhaust valve until the falling pressure allows the valve seat to drop back into the chimney to close the valve (phase 3). At this point, there is a small backpressure (PEEP) within the system, preventing possible alveolar collapse that can occur under anesthesia, especially in children and the elderly. Exhalation is now complete. During the inspiratory phase (phase 4) the valve is completely closed, shutting off the expiratory limb. The patient can now breathe only from the inspiratory limb, the first part being warmed, humidified dead space gas. The fresh gas flow required is approximately equal to that needed to vent alveolar gas. Under anesthesia, this can be as low as 50% of respiratory minute volume.

The significant difference from standard valves is that the efficient function of the new valve is ensured and does not vary from patient to patient or with variable respiratory patterns. The valve cap is always fully open because it functions automatically. With almost no impedance to flow and being easy to breathe through, the design of this valve is such that it is appropriate for pediatric anesthesia. The PEEP effect also is beneficial for children. It was a logical step that such a valve could be used in the Mapleson A mode for spontaneous respiration in children.¹²⁰ In this more efficient mode, fresh gas flows can be reduced to approximately one fourth of that required for the T-piece,¹²⁰ and a scavenging tube is conveniently connected to the exhaust valve well away from the patient. The dead space gas adds beneficial heat and moisture to the system. The ADE system became a standard system for pediatrics in 1996 in the United Kingdom. Consequently, the Mapleson A mode is used for both adults and children, simplifying the concept of a multipurpose system.

To facilitate manual ventilation, the stem of the valve seat has been extended out of the top of the valve. Manual downward pressure on the stem closed off the valve such that the bag could be squeezed to assist ventilation manually without having to screw the valve cap down. Interestingly, the altered gas dynamics within the system proved to be more efficient than the Mapleson D mode.¹²¹ For automatic ventilation, the exhaust valve and reservoir bag are automatically excluded by the lever, which, at the same time, brings in the ventilator connection. This is a safe design because forgetting to switch it causes the ventilator to sound immediately (high pressure obstruction alarm signal).

In summary, in the semiclosed mode, the Humphrey ADE system is now used in the Mapleson A mode for spontaneous respiration and manual ventilation for adults and children, while configuratively it is simply two tubes for automatic ventilation (Mapleson E). (The term ADE system is retained to indicate both adult and pediatric use because there is no longer a D mode.) The simplicity of the system is that the lever automatically switches between these modes. The more popular version uses parallel rather than coaxial tubing.

Finally, the introduction of the detachable soda lime canister to the original ADE system in 1999 added a new dimension to allow recycling by the absorption of carbon dioxide and included a better design of the circle system. The four-phase exhaust valve (although fully open) stays closed during exhalation such that exhaled gas is automatically recycled back to the reservoir bag until the bag is fully distended. Only then does it open to vent any excess exhaled gas. By positioning the reservoir bag on the inspiratory limb, fresh gas is efficiently preserved into this bag during the expiratory phase. (In contrast, when the bag is positioned on the expiratory limb on alternative equipment, fresh gas is forced backward through the canister and is potentially lost from the system.) The lever operates in the same way, switching between spontaneous, manual, and controlled ventilation. The anesthesiologist thus has two basic decisions to make: to use the Humphrey ADE system in a semiclosed mode or with the recycling soda lime canister, and to set the system for spontaneous and manual ventilation (lever up) or for automatic ventilation (lever down).

The Mera F system, introduced in Japan in 1978, uses a modern circle canister-valve assembly as the mounting for a coaxial Bain-type circuit. Byrick and colleagues¹¹² found that at 100 mL/kg/min fresh gas flow, the Mera F functioned as well as the standard Bain circuit. During controlled respiration, this flow kept P_ACO₂ at 45 ± 9 mm Hg during light anesthesia, despite a lower minute volume, because of a slightly improved capture of dead space gas for reinspiration. The Mera F tubing is quite suitable for the Humphrey ADE single-lever hardware.

The Universal F system (King Systems, Noblesville, IN) is essentially equivalent in design and function to the Japanese Mera F and is based on the same patents, but it is manufactured and available in the United States (Fig. 4-20, A). It also uses a coaxial arrangement and mounts on standard circle system hardware. Differences from the Mera F include larger diameter inspiratory and expiratory tubing to decrease resistance, corrugation of the inspiratory tube to prevent kinking, and the ability to be used as a transport system. When used with circle system hardware, the Universal F system is functionally no different than a standard circle system circuit, with the exception that the inspiratory limb is enclosed by the expiratory limb for the purpose of uncluttering the tubing apparatus and providing improved humidification of inspired gas. For patient transport, the Universal F system can be converted to a Bain circuit by the attachment of an oxygen source to the inspiratory limb and a reservoir bag with a pop-off valve to the expiratory limb (Fig. 4-20, C). As with a standard Bain circuit, it is important that a fresh gas flow appropriate for the mode of ventilation being used be supplied when using this circuit for transport (70 to 100 mL/kg/min for controlled ventilation and ≤150 mL/kg/min for spontaneous ventilation). Following transport, this system can function as part of a nonrebreathing system on an intensive care unit ventilator.

The Enclosed Afferent Reservoir (EAR) breathing system (Fig. 4-21) reflects an attempt to create a semiclosed system that retains efficiency during both spontaneous and controlled ventilation without the use of switching valves. Use of the term afferent denotes a system in which the reservoir is located on the portion of the system closely associated with the fresh gas supply. Mapleson A systems, including the Lack system, are afferent reservoir systems; they preferentially exhaust alveolar gas during the expiratory phase of spontaneous respiration, but with controlled ventilation, the pop-off opens during inspiration as well, which limits efficiency. Efferent systems have the reservoir closely associated with the pop-off valve and include Mapleson systems D through F. As previously noted, they are most efficient during controlled ventilation. An enclosed afferent system prevents venting of gas from the expiratory valve during controlled inspiration by enclosing the reservoir in a chamber; as pressure in the chamber is increased to deliver a breath, the expiratory valve is forced to close. It has been demonstrated that efficiency during controlled ventilation using the EAR system is similar to that of the Bain.¹²² During spontaneous ventilation, the EAR system is essentially identical to the Mapleson A. In adults, it is suggested that a fresh gas flow of 70 mL/kg is sufficient to prevent rebreathing during controlled ventilation with the EAR system.¹²³ Appropriate fresh gas flow during spontaneous ventilation using the EAR in adults can vary and are best determined by clinical assessment, but in a mathematical model, a value greater than 0.86 times the minute ventilation has been shown to prevent rebreathing.¹²⁴ In children, a fresh gas flow equal to 0.6 times the weight of the child has been demonstrated to provide normocapnia to mild hypocapnia regardless of the mode of ventilation.¹²⁵

The Jackson-Rees breathing circuit is a modification of Ayre’s T-piece, which is used in pediatric anesthesia worldwide. It adds a corrugated tube, a bag, and sometimes a variable, spring-loaded valve to the expiratory limb of the T-piece. Mapleson added it to his classification system as “F” in 1975.²⁶ Nightingale and colleagues²³ have pointed out that if the volume of the expiratory limb exceeds the tidal volume, Mapleson D, E, and F systems function identically during spontaneous breathing and should be supplied with a fresh gas flow of 100 mL/kg/min or more. Further, if the Jackson-Rees system is assembled with a spring-loaded valve in the tail of the bag, it becomes a Mapleson D and can be used with controlled ventilation with a fresh gas flow of 70 mL/kg/min.

Semiopen Systems

Semiopen systems, also known as nonrebreathing systems, eliminate carbon dioxide by use of two valves that exclude rebreathing. The reservoir bag contains pure fresh gas, which must be supplied at rates at or above the current respiratory minute volume. Apparatus dead space is minimized if the pair of valves, one opening on inspiration and one on exhalation, are incorporated into one small-volume assembly near the mask or tracheal tube (see Fig. 4-5, B). The valves must be designed to open with little pressure and to close quickly with the change in gas flow between inspiration and expiration. Clever design permits the pressure gradient across the inspiratory valve to positively close the expiratory valve, making transition from spontaneous to controlled breathing automatic; all the anesthesiologist has to do is squeeze the bag. The Frumin and Fink valves were once commonly used in anesthesia, and Ruben valves and others still find application in self-inflating resuscitator bag valve assemblies. Most mechanical ventilators used postoperatively and in ICUs also use such circuits, actively closing the expiratory valve by the machine pressure generated to start inspiration. Gas masks and scuba gear are other examples of nonrebreathing circuits, as were the intermittent-flow anesthesia machines of earlier times. The major advantages of semiopen circuits were the relative accuracy of flowmeters at high gas flow, ensuring inspired gas concentrations, and the ability to measure respiratory minute volume by setting fresh gas flow to keep the breathing bag just partly distended at each end expiration. With instrumentation currently available, neither advantage is of much value. Semiopen systems could be heated, humidified, and scavenged as readily as could semiclosed systems,^44,126-129 but often they required a higher fresh gas flow, typically 80 to 100 mL/kg/min.

Positive End-Expiratory Pressure

To improve oxygenation, the application of PEEP to a breathing system may be required. PEEP may be achieved either by adding a freestanding mechanical PEEP valve, such as a Boehringer valve (Boehringer Laboratories, Wynnewood, PA) between the expiratory limb of a circle system and the expiratory unidirectional valve (Fig. 4-22) or, in the case of the Datex-Ohmeda 7900 ventilator, by electronically regulating pressure on the expiratory valve by means of precision-controlled gas flow from the bellows. A schematic of a mechanical add-on PEEP valve is shown in Figure 4-23. A weighted ball must be lifted off its seat by the gas flow. The weight of the ball determines the amount of PEEP. Valves are available for application of various levels of PEEP, including 2.5, 5, 10, 15, and 20 cm H₂O. Such valves can be used in series to create any desired level of PEEP. It is essential that they be placed correctly—that is, vertically—in the expiratory limb (Fig. 4-23, B). Placement in the inspiratory limb would completely obstruct the circuit (see Chapter 30). Also, if not mounted vertically, they can malfunction. The use of add-on mechanical PEEP valves with the 7900 ventilator is not recommended because this could cause errors in the electronic control of PEEP, pressure, and flow by the ventilator.

“Electronic PEEP,” as supplied by the 7900 ventilator, is available only during mechanical ventilation because it relies on the function of the bellows to operate. In theory, a mechanical PEEP valve could be added to the circuit during bag-mode ventilation only, but this could result in ventilator errors if the valve is left in place after switching to the ventilator mode. Because the PEEP is controlled precisely by a computer-operated flow control valve, an advantage of electronic PEEP is that any gas leak from the airway can be compensated for by increasing airway pressure from the bellows. The result is a stable, repeatable, calibrated PEEP level.

Dräger Medical offers mechanical PEEP valves as an optional part of their Narkomed anesthesia breathing systems, as does Datex-Ohmeda as part of the 7800 and earlier machines. These valves are purpose-designed and built into the system to prevent erroneous placement. The Ohmeda mechanical PEEP valve is essentially a spring-loaded, expiratory, unidirectional valve that is adjustable between 2 and 20 cm H₂O (Fig. 4-24). It is essential that the PEEP valve be installed only on the exhalation unidirectional valve, never on the inhalation valve. Thus, in Figure 4-4, if an adjustable spring were to be placed between the valve disk and the dome, it would tend to prevent the opening of the disk, thereby requiring a higher pressure upstream for gas to flow across the valve. Adjusting the spring tension with the calibrated knob thereby adjusts the level of PEEP applied to the circuit between the inspiratory valve and the expiratory, unidirectional PEEP valve, as was the case in the old Ohmeda GMS Absorber. In that system, the pressure in the circuit is sensed just downstream of the inspiratory unidirectional valve so that the circuit pressure gauge and any alarms will detect the presence of PEEP in the circuit (see also Chapters 9 and 30).

With an old Ohmeda GMS absorber in the APL or bag mode, and with the patient breathing spontaneously, the setting on the APL valve must be higher than the PEEP setting. If it is not, inadequate tidal volumes may be delivered to the patient during inspiration. Ohmeda also warns that the PEEP valve should be installed into the GMS absorber only when the use of PEEP is anticipated for that case. It should be removed when not in use; otherwise, deposits from the breathing circuit can collect in the valve mechanism and cause it to malfunction (see also Chapter 30).¹³⁰ The use of PEEP in the patient circuit may result in a significant decrease in delivered tidal volume. Factors that influence the tidal volume delivered to the patient include patient circuit pressure (affected by PEEP), compliance, and resistance. The ventilation must be appropriately adjusted to compensate for any decrease in tidal volume caused by the addition of PEEP.¹³¹

The Narkomed delivery systems use a PEEP valve based on magnetic principles. Instead of using the force of a weighted ball or a spring, the force of attraction between two magnets is used to adjust the pressure needed to open the valve (Fig. 4-25). Because this valve is located between the patient circuit and the selector block that houses the switch for manual (bag) or automatic (ventilator) mode, it must permit bidirectional gas flow so that during inspiration gas flows from the reservoir bag or ventilator bellows, through the valve, and to the patient circuit via a spring-loaded, one-way valve (Fig. 4-26). On exhalation, the one-way inspiratory valve closes, and gas pressure must now overcome the force of attraction between the two magnets to open the magnetic valve and flow to the bag or ventilator bellows (Figs. 4-27 and 4-28). This PEEP valve is not calibrated, but the knob is marked to show direction of rotation for increasing or decreasing PEEP. A slide switch is incorporated into the latest version of this valve to control and clearly indicate whether the PEEP valve is on or off (Fig. 4-29).

The location of the PEEP valve in relation to the expiratory unidirectional valve (freestanding valve or Datex-Ohmeda PEEP valve) or in the manual/automatic switch block (Dräger) permits application of PEEP to the patient circuit during all modes of ventilation: spontaneous, assisted, controlled, and automatic. In this location, the circuit pressure gauge and alarms sensing pressure at the absorber will detect the presence and level of PEEP applied (see also Chapter 30).

Acknowledgment

We thank David Humphrey, MB BS, DA, for his contributions to this chapter.

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