Breathing systems provide the final conduit for the delivery of anesthetic gases to the patient. Breathing circuits link a patient to an anesthesia machine. Many different circuit designs have been developed, each with varying degrees of efficiency, convenience, and complexity. This chapter reviews the most important breathing systems: insufflation, draw-over, Mapleson circuits, the circle system, and resuscitation systems.
The term insufflation usually denotes the blowing of anesthetic gases across a patient’s face. Although insufflation is categorized as a breathing system, it is perhaps better considered a technique that avoids direct connection between a breathing circuit and a patient’s airway. Because children often resist the placement of a face mask (or an intravenous line), insufflation is particularly valuable during inductions with inhalation anesthetics in children. Carbon dioxide (CO2) accumulation under head and neck draping is a hazard of ophthalmic surgery performed with local anesthesia. Insufflation of air across the patient’s face at a high flow rate (>10 L/min) avoids this problem while not increasing the risk of fire from the accumulation of oxygen. Because insufflation avoids any direct patient contact, there is no rebreathing of exhaled gases if the flow is high enough.
Draw-over devices have nonrebreathing circuits that use ambient air as the carrier gas, though supplemental oxygen can be used if available. The devices can be fitted with connections and equipment that allow intermittent positive-pressure ventilation (IPPV) and passive scavenging, as well as continuous positive airway pressure (CPAP) and positive end-expiratory pressure (PEEP). The greatest advantage of draw-over systems is their simplicity and portability, making them useful in locations where compressed gases or ventilators are not available.
The insufflation and draw-over systems have several disadvantages: poor control of inspired gas concentration (and, therefore, poor control of depth of anesthesia), mechanical drawbacks during head and neck surgery, and pollution of the operating room with large volumes of waste gas. The Mapleson systems solve some of these problems by incorporating additional components (breathing tubes, fresh gas inlets, adjustable pressure-limiting [APL] valves, reservoir bags) into the breathing circuit.
Components of Mapleson Circuits
Corrugated tubes connect the components of the Mapleson circuit to the patient. The large diameter of the tubes (22 mm) creates a low-resistance pathway and a potential reservoir for anesthetic gases. To minimize fresh gas flow requirements, the volume of gas within the breathing tubes in most Mapleson circuits should be at least as great as the patient’s tidal volume. The compliance of the breathing tubes largely determines the compliance of the circuit. (Compliance is defined as the change of volume produced by a change in pressure.) Long breathing tubes with high compliance increase the difference between the volume of gas delivered ...