Appendix D

• In a closed rebreathing system the oxygen flow rate is low, so any loss of oxygen or its dilution by room air may be detrimental. The pop-off valve is therefore nearly or completely closed, to prevent oxygen escape.

• Check the machine for leaks before use. If leaks are present, oxygen may escape from the circuit and room air may leak into and dilute the oxygen in the circuit.

• Induction is the most challenging period of anesthesia when low oxygen flows are used. The reservoir bag should be emptied and filled with oxygen 2 to 3 times during the first 15 minutes of anesthesia and every 30 minutes thereafter to help prevent patient hypoxema and to eliminate nitrogen (N₂) that is being exhaled by the patient (a process known as denitrogenization). Alternatively, the anesthetist may provide 5 to 10 minutes of high oxygen flow (200 mL/kg/min) at the start of anesthesia, until the patient reaches a surgical plane. The pop-off valve should be open when high flow rates are used. This flushes room air out of the system and replaces it with oxygen. Thereafter, much lower flow rates (5 to 10 mL/kg/min for small animals and 3-5 mL/kg/min for large animals) can be used, and the pop-off valve should be nearly or completely closed. This amount of oxygen will meet the metabolic oxygen requirements of the anesthetized patient.

• Closely monitor the reservoir bag. If there is a leak in the system, if the pop-off valve is too far open, or if the flow rate of oxygen is inadequate, the bag will not remain inflated. On the other hand, if the fresh gas flow exceeds the patient’s demand for oxygen and anesthetic (which can occur if the patient is too deeply anesthetized, the patient is too cold, or any other factor reduces patient demand for oxygen), the bag will become distended. In this case, after the patient has been checked and it is apparent that no other problems exist, either the oxygen flow rate should be reduced or the pop-off valve should be opened. In any case, it is wise to leave the pop-off valve slightly open during low-flow anesthesia, in case pressure in the circuit rises and excess gas must be vented.

• It may be difficult to change the patient’s anesthetic depth quickly. If a rapid change in anesthetic depth is required, the vaporizer setting should be changed and the breathing system converted to a semiclosed (partial) partial rebreathing system by increasing the oxygen flow rate and opening the pop-off valve. If low oxygen flow rates are maintained, changes in the vaporizer setting may not affect the concentration of anesthetic in the circuit for many minutes.

• If a precision out-of-circuit vaporizer is used for low-flow, closed-circuit anesthesia, the setting required during the maintenance period will often be well above the setting normally used to maintain surgical anesthesia with a semiclosed partial rebreathing system, at least until a state of equilibrium is reached (in which the concentration of anesthetic in the brain and blood is equal to the concentration of anesthetic in the circuit). In a closed system this may take a considerable period of time to occur, owing to the low gas flow rate (see Box 4-2).

• In contrast, if an in-circuit nonprecision vaporizer is used for low-flow, closed-circuit anesthesia, the required vaporizer setting will usually be lower than that used for a semiclosed rebreathing system. This is because the fresh gas flow is not adequate to dilute the anesthetic in the circuit.

• The low oxygen flow rates used in a closed rebreathing system may be inadequate for accurate delivery of anesthetic by some vaporizers. Consult the vaporizer manual for minimum recommended flow rates, and be aware that the anesthetic concentration indicated by the dial may be incorrect at lower flows. Closed rebreathing systems should not be used at all with certain vaporizers, including the Fluothane Tek-2, Copper Kettle, and Vernitrol.

• The low oxygen flow rates in a closed system are unsuitable for use with a Bain coaxial circuit or other non-rebreathing system.