Components of Mapleson Circuits
Corrugated tubes—made of rubber (reusable) or plastic (disposable)—connect the components of the Mapleson circuit to the patient ( Figure 3–5). 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 require-ments, 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. (Compli-ance is defined as the change of volume produced by a change in pressure.) Long breathing tubes with high compliance increase the differencebetween the volume of gas delivered to a circuit by a reservoir bag or ventilator and the volume actually delivered to the patient. For example, if a breathing circuit with a compliance of 8 mL gas/cm H2O is pressurized during delivery of a tidal volume to 20 cm H2O, 160 mL of the tidal volume will be lost to the circuit. The 160 mL represent a combination of gas compression and breathing-tube expansion. This is an important consideration in any circuit deliver-ing positive-pressure ventilation through breathing tubes (eg, circle systems).
Gases (anesthetics mixed with oxygen or air) from the anesthesia machine continuously enter the circuit through the fresh gas inlet. As discussed below, the relative position of the fresh gas inlet is a key differentiating factor in Mapleson circuit performance.
As anesthetic gases enter the breathing circuit, pres-sure will rise if the gas inflow is greater than the combined uptake of the patient and the circuit. Gases may exit the circuit through an APL valve, controlling this pressure buildup. Exiting gases enter the operating room atmosphere or, preferably, a waste-gas scavenging system. All APL valves allow a variable pressure threshold for venting. The APL valve should be fully open duringspontaneous ventilation so that circuit pressure remains negligible throughout inspiration and expi-ration. Assisted and controlled ventilation require positive pressure during inspiration to expand the lungs. Partial closure of the APL valve limits gas exit, permitting positive circuit pressures during reser-voir bag compressions.
Reservoir bags function as a reservoir of anesthetic gas and a method of generating positive-pressure ventilation. They are designed to increase in compli-ance as their volume increases. Three distinct phases of reservoir bag filling are recognizable ( Figure 3–6). After the nominal 3-L capacity of an adult reservoir bag is achieved (phase I), pressure rises rapidly to a peak (phase II). Further increases in volume result in a plateau or even a slight decrease in pressure (phase III). This ceiling effect provides some mini-mal protection of the patient’s lungs against high airway pressures, if the APL valve is unintentionally left in the closed position while fresh gas continues to flow into the circuit.