Nondiverting (Mainstream) Systems

In nondiverting systems, gas flows past the analyzer interface placed in the main gas stream. Until recently, mainstream analysis at the patient’s airway was possible only for carbon dioxide using infrared (IR) technology (Mainstream, Philips Respironics) and by the inspiratory unidirectional valve for oxygen using a fuel cell. As a result of advances in miniaturization, mainstream multigas analysis at the patient’s airway is now available (Phasein, Danderyd, Sweden); this permits rapid acquisition of breath-by-breath data for carbon dioxide, nitrous oxide, oxygen, and the five potent inhaled anesthetics. Although mainstream analyzers overcome the gas sampling problem, they require a special airway adapter and analysis module to be placed in the breathing system near the patient’s airway. From this location, the analyzers produce a sharp concentration-versus-time waveform in real time, but they may be vulnerable to damage with repeated drops onto hard surfaces. New designs are lightweight, such as the Phasein IRMA sensor head, which uses a miniaturized, spinning filter wheel that weighs only 1 oz. Such designs add only a small amount of dead space, and some, such as the Capnostat 5, use solid-state technology. In addition, waste gas scavenging is not necessary with nondiverting systems.

Mainstream analyzer modules are subject to interference by water vapor, secretions, and blood. Because condensed water blocks all IR wavelengths, leaving too low a source intensity to make a measurement, spurious carbon dioxide readings might result. The cuvette’s window may be heated (usually to 41° C), or it may be coated with water-repellant material to prevent such condensation and interference. Adding the mainstream analyzer to the breathing system creates two additional interfaces for a potential breathing circuit disconnection. Disposable breathing system adapters are now available, whereas previously, the nondisposable adapters required cleaning between patient uses. In addition, a mainstream carbon dioxide analyzer is now available for patients receiving oxygen via nasal cannula (Cap-ONE, Nihon Kohden America, Foothill Ranch, CA).

Sidestream (Diverting) Systems

Compared with mainstream analyzers, the advantages of diverting analyzers are that, because they are remote from the patient, they can be of any size and therefore offer more versatility in terms of monitoring capabilities. They can be used when the monitor must be remote from the patient, such as in magnetic resonance imaging (MRI) or radiation therapy. The sampled gas is continually drawn from the breathing circuit via an adapter placed between the circuit and the patient’s airway (the Y-piece in a circle breathing system); it passes through a filter or water trap (Fig. 8-1, arrows) before entering the analyzer. The gas sampling flow rate is usually about 200 mL/min, with a range of 50 to 250 mL/min. Disadvantages include problems with the catheter sampling system, such as clogging with secretions or water, kinking, failure of the sampling pump, slower total system response time (although usually <3 seconds), and artifacts when the gas sampling rate is poorly matched to the patient’s inspiratory and expiratory gas flow rates.

Rapid respiratory rates and long sampling catheter lines may decrease the accuracy of the readings and the fidelity of the tracings. This is because there would be samples from many breaths stored in the catheter, and the breaths could “smudge” into one another, thereby “dampening” the tracing with loss of clear peaks and troughs. If a diverting system is used with a very small patient, such as a neonate, and the gas sampling rate exceeds the patient’s expiratory gas flow rate, spurious readings may result because the expired gas will be contaminated by fresh gas. Similarly, if an uncuffed tracheal tube is used and a leak develops between the tube and the trachea, the gas sampling pump may draw room air into the tracheal tube and into the analyzer.

Ideally, the gas sampling flow rate should be appropriate for the patient and for the breathing circuit used. Thus the sampling flow rate may limit the use of low-flow or closed-circuit anesthesia techniques. If the gas sampling rate exceeds the fresh gas inflow rate, the potential exists for negative pressures to be created in the breathing system.² Once the sampled gas has been analyzed, it should be directed to the waste gas scavenging system or returned to the patient’s breathing system.

Leaks in the gas sampling line, both inside the monitor and between the patient’s airway and the monitor inlet, will result in erroneous readings that may or may not be obvious. These monitors require calibration using a certified standard gas mixture that is directed into the monitor’s gas inlet connection. A leak inside the monitor that allows room air to contaminate the gas sample would result in miscalibration.³

In multigas analyzers that incorporate a paramagnetic oxygen sensor, simultaneous room air sampling (10 mL/min) is required to provide a reference. This air is therefore added to the waste gas exiting the monitor at a rate of 10 mL/min; it may then be returned to the patient circuit. This might create a problem during closed-circuit anesthesia because nitrogen, albeit at a rate of about 8 mL/min, would be added to the breathing circuit (see Paramagnetic Oxygen Analyzers later in this chapter).

Number of Molecules (Partial Pressure)

An analytic method based on quantifying a specific property of a gas molecule determines in absolute terms—that is, in millimeters of mercury or kilopascals—the number of molecules of that gas that are present. Gas molecules composed of two or more dissimilar atoms—such as carbon dioxide, nitrous oxide, and the potent inhaled anesthetics—have bonds between their component atoms. Certain wavelengths of IR radiation excite these molecules, stretching or distorting the bonds, and the molecules also absorb the radiation. Carbon dioxide molecules absorb IR radiation at a wavelength of approximately 4.3 μm. The greater the number of molecules of carbon dioxide present, the more radiation at 4.3 μm absorbed. This property of the carbon dioxide molecule is applied in the IR carbon dioxide analyzer. Because the total amount of IR radiation absorbed at a specific wavelength is determined by the number of molecules present, and the motion of each molecule contributes to the total pressure, the amount of radiation absorbed is a function of partial pressure; thus, an IR analyzer measures partial pressure.

In the analysis of gases by Raman spectroscopy, as was used in the Datex-Ohmeda Rascal II analyzer (GE Healthcare), a helium-neon laser emits monochromatic light at a wavelength of 633 nm. When this light interacts with the intramolecular bonds of specific gas molecules, it is scattered and reemitted at wavelengths different from that of the incident monochromatic light. Each reemission wavelength is characteristic of a specific gas molecule present in the gas mixture and therefore is a function of its partial pressure. Thus Raman spectroscopy also measures partial pressures.

A sufficient number of molecules of any gas to be analyzed—that is, adequate partial pressures—must be present to facilitate gas analysis by the IR and Raman technologies. These systems also must be pressure compensated if analyses are being made at ambient pressures other than those used for the original calibration of the systems.⁴

Measurement of Proportion (Volumes Percent)

Another approach to gas analysis is to separate the molecular component species of a gas mixture and determine what proportion (percentage) each gas contributes to the total (100%). This approach is applied in mass spectrometry. Thus, if 21 molecules of oxygen were in a sample of gas containing 100 molecules, oxygen would represent 21% of the gas sample and therefore might reasonably be assumed to represent 21% of the original gas mixture. The result is expressed as 21 vol% or as a fractional concentration (0.21). This monitor does not measure partial pressures; it measures only proportions. If the system is provided with an absolute pressure reading that is equivalent to 100%, the basic measured proportions can be converted to readings in millimeters of mercury. In the above example, if 100% were made equivalent to 760 mm Hg, oxygen would have a calculated partial pressure of 159 mm Hg (760 × 21%).

These fundamental differences in the approaches to gas analysis and their basic units of measurement are important, particularly when the data presented by these monitors are interpreted in a clinical setting and may affect patient management.

Mass Spectrometry

For many anesthesia caregivers, the term mass spec is used as if it were synonymous with respiratory gas analysis. Indeed, many anesthetic record forms still incorrectly include this term, but this technology is no longer in routine clinical use. It was, however, the first multigas monitoring system in widespread clinical use, following a description by Ozanne and colleagues in 1981.⁵

The mass spectrometer is an instrument that allows the identification and quantification, on a breath-by-breath basis, of up to eight of the gases commonly encountered during the administration of an inhalational anesthetic. These gases include oxygen, nitrogen, nitrous oxide, halothane, enflurane, and isoflurane. Other agents—such as helium, sevoflurane, argon, and desflurane—could sometimes be added or substituted if desired. Although the technology of mass spectrometry had been available for many years, analyzer units dedicated to a single patient were too expensive for routine use in each operating room (OR). In 1981, the concept of a shared, or multiplexed, system was introduced.⁵ This arrangement allowed one centrally located analyzer to function as part of a computerized, multiplexed system that could serve up to 31 patient sampling locations (ORs and recovery room or intensive care unit [ICU] beds) on a time-share basis.⁶ The two multiplexed systems that became widely used were the Perkin Elmer system, which later became the Marquette Advantage system, and the System for Anesthetic and Respiratory Analysis (SARA).

Shared Mass Spectrometry Systems

Multiplexing permitted the sharing of one (rather expensive) mass spectrometer analyzer unit among up to 31 sampling locations, or stations. In a shared system, the gas from each sampling location is directed in sequence by the multiplexing valve system to the mass spectrometer for analysis. In a multiplexed system, the time between analyses at any particular location therefore depended on 1) the number of breaths analyzed from each sampling location (i.e., dwell time), 2) the number of sampling locations in use, 3) the priority settings, and 4) in the case of “stat” samples, the distance between sampling locations and the analyzer, which was up to 300 feet in some installations.

Use of multiplexed mass spectrometry systems led anesthesia caregivers to appreciate the value of continual (i.e., frequently repeated), although not continuous (without interruption), respiratory gas monitoring and the limitations—the main one being that the systems were shared, which meant that data were updated only intermittently.

In October 1986, the American Society of Anesthesiologists (ASA) first approved standards for basic intraoperative monitoring. As the standards evolved, the requirement for continuous capnometry was not met by a shared mass spectrometry system. The systems were expensive to install and maintain, and they were not always easily upgradable when the new agents desflurane and sevoflurane were introduced. In addition, the long sampling catheters combined with rapid respiratory rates led to significant artifact; the sampling pump could cause considerable negative pressure in the breathing system, and when the system failed, all the monitored sites were affected.^7,8 Practitioners demanded continuous gas monitoring. The most important functions of a multiplexed mass spectrometry system could be served by other dedicated gas monitors, and the multiplexed, mass spectrometer–based systems became extinct.

Dedicated (Stand-Alone) Mass Spectrometry Systems

A number of smaller stand-alone mass spectrometers were developed for dedicated single-patient use. One model, the Ohmeda 6000 Multigas Analyzer, was a quadrupole-filter mass spectrometer.⁹ It worked on the principle that, with regard to the m/z ratio of ions, a controlled electrostatic field can prevent all but a narrow range of charged particles from reaching a target. In the Ohmeda 6000 unit, the gas sampling rate was fixed at 30 mL/min. One advantage of the quadrupole system was that it could be adapted to measure new or additional agents by changes in software only. A number of other stand-alone, quadrupole-filter mass spectrometers were marketed, but their production was discontinued as a result of cost and reliability issues.

1. A source of IR radiation, typically a heated black body. A black body radiator is a theoretical object that is totally absorbent to all thermal energy that falls on it; it does not reflect any light and therefore appears black. As it absorbs energy, it heats up and reradiates the energy as electromagnetic radiation. Heating the black body causes emission of IR radiation.

2. A sample cell, or cuvette. The gas to be analyzed is drawn through the cuvette by a sampling pump.

3. A detector that generates an output signal. The signal is related to the intensity of the IR radiation that falls on it.

4. A narrow-band pass filter. This filter allows only radiation at the wavelength bands of interest to pass through; it is interposed between the IR source and the cuvette (see Fig. 8-8) or between the cuvette and the detector (see Fig. 8-9). The intensity of radiation reaching the detector is inversely related to the concentration of the specific gas being measured.

Single-Beam Positive Filter

In single-beam positive-filter designs, the IR beam may be divided in time or in space. If the IR beam is divided in time, precision optical band-pass filters mounted on a chopper wheel spinning at 40 to 250 rpm sequentially interrupt a single IR beam. The beam retains energy at a narrow wavelength band during each interruption. If the IR beam is divided in space, after passing through the cuvette, the beam is divided into separate paths using beam splitters and mirrors before passing through optical filters. For each gas of interest, a pair of band-pass filters is selected at an absorption peak and at a reference wavelength at which relatively little absorption occurs. The chopped IR beam then passes through a cuvette containing the sample gas. The ratio of intensity of the IR beam for each pair of filters is proportional to the partial pressure of the gas and is insensitive to changes in the intensity of the IR source.

Single-Beam Negative Filter

In the single-beam negative-filter design, the filters usually are gas-filled cells mounted in a spinning wheel. During each interruption, the IR beam retains energy at all wavelengths except those absorbed by the gas. The chopped IR beam then passes through a cuvette containing the sample gas. Analogous to the positive-filter design previously described, the ratio of IR beam intensity for each pair of filter cells is proportional to the partial pressure of the gas and is insensitive to changes in intensity of the IR source.

Dual-Beam Positive Filter

In the dual-beam positive-filter design, the IR energy from the source is split into two parallel beams: one passes through the sample gas, the other passes through a reference gas. A spinning blade passes through the beams and sequentially interrupts one, the other, then both. The two beams are optically focused on a single point, where a band pass optical filter selected at the absorption peak of the gas of interest is mounted over a single detector. As before, the ratios of the intensities of the sample and reference beams are proportional to the partial pressure of the gas.

Detectors of Infrared Radiation

To measure carbon dioxide, nitrous oxide, and sometimes anesthetic agents, a radiation-sensitive solid-state material, lead selenide, is commonly used as a detector. Lead selenide is quite sensitive to changes in temperature; it therefore is usually thermostatically regulated (cooled or heated) or temperature compensated.

Anesthetic agents, carbon dioxide, and nitrous oxide are sometimes measured with another detector called a Luft cell. This detector uses a chamber filled with gas that expands as IR radiation enters the chamber and is absorbed. A flexible wall of the chamber acts as a diaphragm that moves as the gas expands, and a microphone converts the motion to an electrical signal.

The signal processor converts the measured electrical currents to display gas partial pressure. First, the ratios of detector currents at various points in the spinning wheel’s progress, or from multiple detectors, are computed. Next, electronic scaling and filtering are applied. Finally, linearization according to a reference table for the point-by-point conversion from electrical voltage to gas partial pressure is accomplished by a microprocessor. Compensation for cross-sensitivity or interference between gases can be accomplished by the microprocessor after linearization.

Infrared Wavelength and Anesthetic Agent Specificity

IR analyzers must use a specific wavelength of radiation according to the absorbance peak of each gas to be measured. Early agent analyzers, such as the Datex Puritan-Bennett Anesthetic Agent Monitor (Fig. 8-11),¹⁴ used a wavelength of 3.3 μm to measure the potent inhaled anesthetics. However, use of a single wavelength did not permit differentiation among these agents (see Fig. 8-7). When this system was used, the analyzer had to be programmed by the user for the particular agent being administered. This set the appropriate gain in the software program, and the displayed reading was then accurate for the one agent in use. Obviously, programming such an analyzer for the wrong agent, or the use of mixed agents, would lead to erroneous readings.^15,16

Modern IR analyzers are agent specific; that is, by measuring each agent with a unique set of wavelengths, they have the capability to both identify and quantify mixed agents in the presence of one another. Contemporary analyzers that can identify and quantify anesthetic agents incorporate individual wavelength filters in the range of 8 to 12 μm. An example is the Datex-Ohmeda Compact Airway Module (GE Healthcare) (see Figs. 8-2 and 8-9), which measures the absorption of the gas sample at seven different wavelengths selected using optical narrow band filters. In this analyzer module, the IR radiation detectors are thermopiles. Carbon dioxide and nitrous oxide are calculated from absorption measured at 3 to 5 μm (Fig. 8-12). Identification and calculation of the concentrations of anesthetic agents are done by measuring absorption at five wavelengths in the 8- to 9-μm band and solving for the concentrations from a set of five equations, one for each agent (Fig. 8-13).¹⁶ A schematic of a multiwavelength analyzer in which the beam of radiation is interrupted mechanically is shown in Figure 8-14.

Sampling Systems and Infrared Analysis

Sidestream-sampling analyzers continuously withdraw between 50 and 250 mL/min from the breathing circuit through narrow-gauge sample tubing to the optical system, where the measurement is made. One of the disadvantages of sidestream monitors is the need to deal with liquid water and water vapor. Water vapor from the breathing circuit condenses on its way to the sample cuvette and can interfere with optical transmission. NAFION tubing, a semipermeable polymer that selectively allows water vapor to pass from its interior to the relatively dry exterior, is commonly used to eliminate water vapor. Also, a water trap often is interposed between the patient sampling catheter and the analyzer to protect the optical system from liquid water and body fluids (see Fig. 8-1). Filters integrated with the sampling tubing have replaced water traps in many of the currently available systems. Three different methods are available; these include 1) blocking the water, such as with a hydrophobic filter with large surface area (Oridion Medical, Jersusalem, IL), 2) absorbing and blocking, such as with a hydrophilic fibrous element followed by a hydrophobic plug (Philips Respironics, Wallingford, CT), and 3) active water removal with a hydrophilic wick, such as an elastomer, described as being able to “sweat” water collected from the gas sample flow to the outer surface of the cover (Nomoline, PhaseIn).

Infrared Photoacoustic Spectrometer

The photoacoustic spectrometer is similar to the basic IR spectrometer (Fig. 8-15). IR energy is passed through optical filters that select narrow-wavelength bands that correspond to the absorption characteristics of the respired gases. Carbon dioxide is measured at a wavelength of 4.3 μm, nitrous oxide at 3.9 μm, and the potent inhaled agents at a wavelength between 10.3 and 13.0 μm.¹⁷ Evenly spaced windows are located along the circumference of a rotating wheel. The optical components are located astride the wheel along one of its radii. A series of IR beams pulse on and off at particular frequencies, according to the rate of rotation of the wheel and the spacing of the windows. The gas flowing through the measurement cuvette is exposed to the pulsed IR beams. As each gas absorbs the pulsating IR energy in its absorption band, it expands and contracts at that frequency, and resulting sound waves are detected with a microphone. The partial pressure of each gas in the sample is then proportional to the amplitude, or “volume,” of the measured sound.

The photoacoustic technique has the distinct advantage over other IR methods in that a simple microphone detector can be used to measure all the IR-absorbing gases. However, this device is sensitive to interference from loud noises and vibration. Also, because only one wavelength is used to measure the potent inhaled anesthetics, this monitor is unable to distinguish among the agents, which requires that it be programmed for the agent in use; also, erroneous readings might arise in the presence of mixed anesthetic agents. This technology was used in the Brüel and Kjær Anesthetic Gas Monitor 1304, and it is used currently in atmospheric trace gas monitors, such as the Innova 1412 (LumaSense Technologies, Santa Clara, CA).

Recent Technologies

Philips Respironics introduced a flexible monitoring interface, now known as CO₂nnect & Go, which allows the user to choose, based on the patient and environment, which carbon dioxide–monitoring modality should be used, sidestream or mainstream. A complete measurement system is available for both mainstream (Capnostat 5) and sidestream (LoFlo, either external or internal format) modes of gas sampling.

Mainstream multigas IR analysis has recently been introduced by Phasein in their IRMA series of multigas analyzers. The IRMA mainstream probe measures IR light absorption at 10 different wavelengths to determine gas concentrations in the mixture (Fig. 8-16). Adult, pediatric, and infant disposable airway adapters are available. In a bench study, the monitor was found to have a response time for carbon dioxide (96 vs. 348 ms) and oxygen (108 vs. 432 ms) that was significantly less than a contemporary sidestream-sampling gas monitor.¹⁸ The same miniaturized technology is used in Phasein’s EMMA Emergency Capnometer, a device that displays respiratory rate and incorporates apnea, high carbon dioxide, and low carbon dioxide audible and visual alarms (Fig. 8-17).

Although Microstream (Oridion Medical) capnography is now used as a generic term to refer to a sampling flow rate of 50 mL/min, a rate now available from several vendors, it also encompasses the unique IR emission source. This approach is called molecular correlation spectroscopy (MCS),¹⁹ and it produces selective emission of a spectrum of discrete wavelengths, approximately 100 discrete lines in the 4.2- to 4.35-μm range, which match those for carbon dioxide absorption. This is compared with the broad IR emission spectrum of black body emitters used with conventional nondispersive IR technology and permits use of a smaller sample cell (15 μL). This technology has been adapted for use in portable carbon dioxide monitors.²⁰

Fuel Cell Oxygen Analyzer

The fuel cell (Fig. 8-23), or galvanic cell, is basically an oxygen battery that consists of a diffusion barrier; a noble metal cathode, either gold mesh or platinum; and a lead or zinc anode in a basic, usually potassium hydroxide, electrolyte bath (Fig. 8-24). The sensor is covered by an oxygen-permeable membrane and is exposed to the gas in the breathing circuit. Oxygen diffusing into the sensor is reduced to hydroxyl ions at the cathode in the following reaction:

The hydroxyl ions then oxidize the lead or zinc anode, and the following reaction occurs at the anode:

The flow of current depends on the uptake of oxygen at the cathode, according to Faraday’s first law of electrolysis, and the voltage developed is proportional to the oxygen partial pressure (PaO₂). No polarizing potential (battery) is needed because the cell produces its own. The fuel-cell sensor voltage is measured and is electronically scaled to units of partial pressure, or equivalent concentration in volumes percent, and is displayed as a readout. Like any battery, the fuel cell has a limited life span, usually several months, depending on its length of exposure to oxygen. For this reason, machine manufacturers have recommended that the cell be removed from the breathing system when not in use. The response time of standard fuel cell oxygen analyzers is slow (~30 seconds); therefore they are best used to monitor the average O₂ concentration in the inspiratory limb of the breathing system.

A faster galvanic oxygen sensor is in development and is designed to be used with the mainstream multigas analyzer (Phasein IRMA; see Fig. 8-16). This sensor claimed to have a lifetime exceeding 100,000 oxygen hours.

Paramagnetic Oxygen Analyzer

The oxygen molecule has two electrons in unpaired orbits, which makes it paramagnetic; that is, it is susceptible to attraction by a magnetic field. Most other gases are weakly diamagnetic and are repelled. The paramagnetic oxygen sensor uses the strong, positive magnetic susceptibility of oxygen in a pneumatic bridge configuration to determine oxygen concentration by measuring a pressure differential between a stream of reference gas (room air at about 10 mL/min) and one of the measured gas, as the two streams are exposed to a changing magnetic field (Fig. 8-25). An electromagnet is rapidly switched off and on (at a frequency of 165 Hz in the GE Healthcare Compact Airway Module¹), creating a rapidly changing magnetic field between its poles. The electromagnet is designed to have its poles in close proximity, forming a narrow gap. The streams of sample and reference gas have different oxygen partial pressures, and the pressure between the entrance and exit of the respective gas streams differs slightly because of the magnetic force on the oxygen molecules; this generates sound waves from each gas stream. A sensitive pressure transducer (i.e., a microphone) is used to convert the sound waves to an electrical signal. The output signal is proportional to the oxygen partial pressure difference between the two gas streams and should be displayed as the PO₂, but it is more typically displayed as the equivalent concentration in volumes percent. Paramagnetic oxygen analysis is used in most of the contemporary sidestream-sampling multigas analyzers.

The main advantage of paramagnetic analysis over the standard fuel cell is that it has a very rapid response that permits continuous breath-by-breath monitoring of the respired oxygen concentration. The graphic representation of this can be displayed as the oxygram, which is essentially a mirror image of the capnogram (Fig. 8-26).

Normally the gas exiting the multigas analyzer is directed to the waste gas scavenging system of the anesthesia delivery system. If a low-flow or closed-circuit anesthesia technique is being used, the gas exiting the analyzer usually is returned to the breathing system. In this case it must be remembered that nitrogen from the room air reference gas stream is being added also, albeit at a low rate (8 mL/min), and will accumulate in the breathing circuit.²⁷

Oxigraphy

Another technology used to measure oxygen available from Oxigraf (Palo Alto, CA) uses laser diode absorption spectroscopy in the visible spectrum, similar in principle to the IR absorption methodology used to measure carbon dioxide, nitrous oxide, and the potent inhaled anesthetic agents. The wavelength used to measure oxygen is 760 nm because there is no interference by other gases at this wavelength. The emission line width of the laser and the absorption line width of oxygen are both very narrow (<0.01 nm) compared with the absorption band for carbon dioxide (~100 nm). The laser is thermally tuned precisely to the oxygen line; as the oxygen concentration increases, the intensity of transmitted light is attenuated as energy is absorbed by the oxygen molecules. The response of the photodetector varies linearly with the concentration of oxygen. The analyzer can measure oxygen in the range of 5% to 100% with an accuracy of 0.1% and has an adjustable gas sampling rate of 50 to 250 mL/min. As with the IR analyzers, the Oxigraf analyzer measures the partial pressure of oxygen in the sample chamber. The pressure of the sample at the time of measurement also is needed to convert data from partial pressure to percent oxygen.

Calibration of Oxygen Analyzers

Oxygen analyzers require periodic calibration. Because all analyzers produce an electrical signal proportional to the oxygen partial pressure, the constant of proportionality (gain) must be determined. In general, the electrical signal in the presence of 0% oxygen is known to be near zero; therefore no offset correction is required. In the fuel cell, the gain changes over time because of changes in electrolyte, electrodes (the anode is sacrificial), and membrane; the anesthesia caregiver must therefore calibrate it to display 21% by removing it from the breathing system and allowing equilibration in room air. It must be remembered that the fuel cell is actually measuring the ambient pressure of oxygen (PO₂, normally 159 mm Hg in dry air at sea level), but for convenience, it displays 21 vol%. Therefore, if a fuel cell that has been calibrated to read 21% at sea level is used at a much higher altitude, the readout will show less than 21% because the ambient PO₂ is lower, even though the composition of the atmosphere is still 21% by volume.

The gain of the paramagnetic sensor changes with temperature, humidity, and pneumatic factors. The contemporary paramagnetic oxygen analyzers perform their own periodic computer-controlled automated calibration process.

Gases in the anesthesia delivery system can be analyzed by a number of modern technologies, each of which is based on application of some specific physical property of the gas molecule. The analysis methods and their applications are summarized in Table 8-1. In interpreting gas analysis data, it is important to understand the principles of how the data were obtained so that erroneous data can be identified and, if necessary, rejected.

Oxygen

The qualitative and quantitative oxygen-specific analyzer in the anesthesia breathing system is probably the most important of all of the monitors on the anesthesia workstation. Before the general use of an oxygen analyzer in the anesthesia breathing system, a number of adverse outcomes from unrecognized delivery of a hypoxic gas had been reported.²⁹

To be used correctly, the oxygen analyzer must be calibrated, and appropriate low and high audible and visual concentration alarm limits must be set. Because it samples gas in the inspiratory limb of the breathing system, this analyzer provides the only means of ensuring that oxygen is being delivered to the patient. If a hypoxic gas or gas mixture is delivered, the alarm will go off. Such situations can occur if there is a pipeline crossover (e.g., O₂ for N₂O), an incorrectly filled oxygen storage tank or cylinder, or failure of a proportioning system intended to prevent delivery of a gas mixture that contains less than 25% oxygen. The high oxygen concentration alarm limit is important when caring for patients for whom a high oxygen concentration may be harmful, such as premature infants and patients treated with chemotherapeutic drugs (e.g., bleomycin), who are more susceptible to oxygen toxicity.

Continuous monitoring of inspired and end-expired oxygen is very helpful in ensuring completeness of preoxygenation of the lungs, before a rapid sequence induction of anesthesia, or in patients at increased risk for hypoxemia during induction of anesthesia, such as the morbidly obese.^30-32 During preoxygenation, nitrogen is washed out of the lungs and is replaced by oxygen.³³ Preoxygenation is ideal when the inspired oxygen concentration is 100% and the end-tidal oxygen is 95%, the difference of 5% being the exhaled carbon dioxide. In general, however, an end-tidal oxygen greater than 90% is considered acceptable.

The rapid-response oxygen analyzer makes possible the display of the oxygram, a continuous real-time display of oxygen concentration on the y-axis against time on the x-axis. Now that it is possible to accurately measure inspiratory and expiratory gas flows, and therefore volumes (i.e., flow = volume/time), via the airway, the oxygram can be combined with these flow signals to plot inspired and expired oxygen concentrations against volume. This is termed volumetric oxygraphy. In theory, the integral of simultaneous flow and oxygen concentration during inspiration and expiration is the inspired and expired volume of oxygen. From the difference between these two amounts, oxygen consumption can be estimated. Barnard and Sleigh³⁴ used this method (with a Datex Ultima monitor) to measure oxygen consumption in patients under general anesthesia and compared it with that obtained simultaneously using a metabolic monitor. These investigators concluded that the Datex Ultima may be used with moderate accuracy to measure oxygen uptake during anesthesia. The analogous plot for the capnogram, volumetric capnography, allows measurement of carbon dioxide production.

The application of this concept is termed indirect calorimetry, which is used in the GE Healthcare bedside metabolic module,³⁵ which has been validated in both the ICU and anesthesia environments.³⁶ By monitoring flow and measuring the gas concentrations, this module provides measurements of oxygen consumption (VO₂) and carbon dioxide elimination (VCO₂), and it calculates respiratory quotient and energy expenditure (Fig. 8-28).³⁷ Although the applications may be more pertinent to patients in the ICU, some have found it useful during liver transplantation surgery in predicting the viability of the organ once in the recipient. It might also be useful in the early detection of a hypermetabolic state in a patient under general anesthesia (e.g., malignant hyperthermia) and in distinguishing it from insufflation of carbon dioxide during a laparoscopic procedure.³⁸ One development worth noting is the trend toward complete in-line measurement of oxygen consumption and carbon dioxide elimination, evidenced by the testing of a prototype in-line system that uses a luminescence quenching sensor for O₂, IR sensing for carbon dioxide, and a fixed orifice for flow sensing.³⁹

The oxygen analyzer is one of the most important monitors in the breathing system because it is both qualitative and quantitative. In that location, it helps ensure that the patient does not receive a hypoxic gas mixture. In that regard, it should be noted that if an oxygen delivery device, such as a mask or nasal cannula, is connected to the auxiliary oxygen outlet of the anesthesia workstation—which derives gas from the machine’s high-pressure system for oxygen, or to a wall oxygen outlet flowmeter—there is no oxygen monitoring of the delivered gas. Thus if a hypoxic gas were delivered by the oxygen pipeline system to the auxiliary oxygen outlet, or an oxygen wall flowmeter were to somehow become connected to a nitrous oxide wall outlet, an adverse outcome could be expected.⁴⁰

Carbon Dioxide

The introduction of carbon dioxide monitoring into clinical practice is one of the major advances in patient safety. Before its introduction, many cases of esophageal intubation were unrecognized and led to adverse outcomes, not to mention increases in malpractice premiums. Detection of carbon dioxide on a breath-by-breath basis is considered to be the best method to confirm endotracheal intubation,⁴¹ and the applications of time and volumetric carbon dioxide monitoring (capnography) are numerous, such that entire textbooks have been devoted to this subject.^42,43

Nitrogen

Where available, measurement of nitrogen is useful in following preoxygenation (nitrogen washout), detecting venous air embolism, and detecting air leaks into the anesthesia breathing system. Fortunately, alternative means are now available to perform these functions.

Potent Inhaled Anesthetic Agents and Nitrous Oxide

Contemporary multigas analyzers measure the inspired and end-tidal concentrations of N₂O as well as the potent inhaled anesthetic agents desflurane, enflurane, halothane, isoflurane, and sevoflurane in the presence of one another. Although no study has established the value of this monitoring modality, the possible applications make the potential benefits of its use obvious; it makes possible the monitoring of anesthetic uptake and washout and allows setting high and low alarm limits for agent concentrations.

By adding the minimum alveolar concentration (MAC) values for N₂O and the potent agents to the analyzer software, and because MAC values are additive, once the composition of the gas mixture is known, the total MAC value of the inhaled anesthetic agents can be displayed. This is useful as an indication of anesthetic depth and a form of awareness monitor and has many potential applications.

Monitoring agent levels during uptake permits a safer and more intelligent use of the anesthesia vaporizer to reach target end-tidal concentrations in the patient. A high fresh gas flow and vaporizer concentration dial setting will ensure that the gas composition in the circuit changes rapidly, and the technique of “overpressure” can be used to speed anesthetic uptake. Once the desired end-tidal concentration has been attained, the vaporizer concentration dial setting can be decreased. When equilibrium has been reached—that is, when inspired and end-tidal agent concentrations are almost equal—the fresh gas flow can be decreased to maintain the equilibrium and conserve anesthetic agent. Monitoring of the anesthetic concentration during elimination provides information regarding the state of its washout. The washout rate can then be increased by increasing the fresh gas flow, hence increasing its removal into the waste gas scavenging system.

The anesthetic agent high-concentration alarm can be used to alert the anesthetist to potential anesthetic overdosing. Indeed, the applicable U.S. consensus standard for anesthetic agent monitors requires a high-concentration alarm; a low-concentration alarm is an option.⁵⁸ The ASA Closed Claims Project includes several adverse outcomes as a result of anesthetic overdose and other reports of vaporizer malfunction that have led to higher than intended output concentrations.^59,60 The low-concentration alarm can be used to help prevent awareness by maintaining an anesthetic agent concentration that exceeds MACawake, which is the average of the concentrations immediately above and below those that permit voluntary response to command.⁶¹ In general MACawake is approximately one third of MAC.⁶² The anesthetic agent low-concentration alarm also serves as a late sign that the vaporizer is becoming empty. An agent analyzer may also alert the anesthetist to an air leak into the breathing system, causing an unintended low-agent concentration.⁶³

Assuming that the analyzer has been calibrated according to the manufacturer’s instructions, it can be used to check the calibration of a vaporizer as well as detect mixed agents in a vaporizer. Monitoring of N₂O concentrations may be helpful when discontinuing or avoiding its administration; this may apply in patients who have closed air spaces that might expand with the use of N₂O.

The continuous analysis of all of the respired gases by a multigas monitor also facilitates recording of the data by an automated anesthesia information management system. Many anesthesia caregivers record end-tidal concentrations of the gases administered. Monitoring of agent concentration, fresh gas flow, and time facilitates calculation of the consumption of the potent agents, both in liters of vapor and in milliliters of liquid agent. Attention to these data can be used to promote a more economical use of the more expensive anesthetics.

Gas Flow Sensor

Flow measurements in anesthesia are most commonly made with fixed-orifice or variable-orifice flowmeters. The Datex-Ohmeda D-Lite and Pedi-Lite flow sensors allow the qualitative and quantitative monitoring of respired gases using the density of the gas mixture for the computation of flow (see Chapter 9). Thus:

Standard II

Standard IV

The patient’s condition shall be evaluated continually in the PACU.

In many postanesthesia care units, capnometry is used in patients who are tracheally intubated. If a patient requires reintubation, a means to confirm tracheal placement of the tube should be available, and its use must be documented.

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