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Gas exits the anesthesia machine at the common gas outlet and then enters an anesthetic circuit. The function of the anesthesia breathing circuit is to deliver oxygen and anesthetic gases to the patient and to eliminate carbon dioxide. Carbon dioxide can be removed by washout with adequate inflow of fresh gas or by the use of carbon dioxide absorbent media (e.g., soda lime absorption). This discussion is limited to semiclosed rebreathing circuits and the circle system.
In 1954, Mapleson described and analyzed five different semiclosed anesthetic systems, which are classically referred to as the Mapleson systems and designated A to E.[96] Willis and colleagues[97] added the F system to these five original. The Mapleson systems are shown in Figure 9-20 . System components can include a facemask, a spring-loaded pop-off valve, reservoir tubing, fresh gas inflow tubing, and a reservoir bag. Three distinct functional groups emerge, and they include the A, the BC, and DEF groups. The Mapleson A, also known as the Magill circuit, has a spring-loaded, pop-off valve located near the facemask, and the fresh gas flow enters the opposite end of the circuit near the reservoir bag. In the B and C systems, the spring-loaded, pop-off valve is located near the facemask, but the fresh gas inlet tubing is located near the patient. The reservoir tubing and breathing bag serve as a blind limb where fresh gas, dead space gas, and alveolar gas can collect. In the Mapleson DEF group, or the T-piece group, the fresh gas enters near the patient, and excess gas is popped off at the opposite end of the circuit.
Even though the component arrangement and components are simple, functional analysis of the Mapleson systems can be complex.[98] [99] The amount of carbon dioxide rebreathing associated with each system is multifactorial, and variables that dictate the ultimate carbon dioxide concentration include the following: (1) the fresh gas inflow rate, (2) the minute ventilation, (3) the mode of ventilation (spontaneous or controlled), (4) the tidal volume, (5) the respiratory rate, (6) the inspiratory to expiratory ratio, (7) the duration of the expiratory pause, (8) the peak inspiratory flow rate, (9) the volume of the reservoir tube, (10) the volume of the breathing bag, (11) ventilation by mask, (12) ventilation through an endotracheal tube, and (13) the carbon dioxide sampling site.
Performance of the Mapleson systems is best understood by studying the expiratory phase of the respiratory cycle (see Fig. 9-20 ).[100] During spontaneous ventilation, the Mapleson A has the best efficiency of the six systems, requiring a fresh gas inflow rate of only one times the minute ventilation to prevent rebreathing of carbon dioxide. However, it has the worst efficiency during controlled ventilation, requiring minute ventilation of as much as 20 L/min to prevent rebreathing. Systems D, E, and F are slightly more efficient than systems B and C. To prevent rebreathing carbon dioxide, systems D to F require a fresh gas inflow rate of approximately 2.5 times the minute ventilation, whereas the fresh gas inflow rates required for systems B and C are somewhat higher.[99]
The following summarizes the relative efficiency of different Mapleson systems with respect to prevention of rebreathing, during spontaneous ventilation: A > DFE > CB. During controlled ventilation, DFE > BC > A.[99] [101] The Mapleson A, B, and C systems are rarely used today, but the D, E, and F systems are commonly employed. In the United States, the most popular representative from the DEF group is the Bain circuit.
The Bain circuit is a modification of the Mapleson D system. It is a coaxial circuit in which the fresh gas flows through a narrow inner tube within the outer corrugated tubing.[102] The central tube originates near the reservoir bag, but the fresh gas enters the circuit at the patient's end ( Fig. 9-21 ). Exhaled gases enter the corrugated tubing and are vented through the expiratory valve near the reservoir bag. The Bain circuit may be used for spontaneous and controlled ventilation. The fresh gas inflow
Figure 9-20
Gas disposition at end expiration during spontaneous
(left) and controlled (right)
ventilation in circuits A through F. FGF, fresh gas flow. (Adapted from
Sykes MK: Rebreathing circuits: A review. Br J Anaesth 40:666, 1968.)
This circuit has many advantages. It is lightweight, convenient, easily sterilized, and reusable. Scavenging of the gases from the expiratory valve is facilitated because the valve is located away from the patient. Exhaled gases in the outer reservoir tubing add warmth to inspired fresh gases. Hazards of the Bain circuit include unrecognized
Figure 9-21
The Bain circuit. (Adapted from Bain JA, Spoerel
WE: A streamlined anaesthetic system. Can Anaesth Soc J 19:426, 1972.)
The outer tube should be transparent to allow inspection of the inner tube. The integrity of the inner tube can
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