Optimization
of Circle System Design
Although the major components of the
circle system (unidirectional valves, fresh gas inlet, APL valve, CO2 absorber, and a reservoir bag) can be placed in
several configurations, the following arrangement is preferred (Figure 3–8):
·
Unidirectional
valves are relatively close to the patient to prevent backflow into the
inspiratory limb if a circuit leak develops. However, unidirectional valves are
not placed in the Y-piece, as that makes it difficult to confirm proper
orientation and intraoperative function.
·
The
fresh gas inlet is placed between the absorber and the inspiratory valve.
Positioning it downstream from the inspiratory valve would allow fresh gas to
bypass the patient during exhalation and be wasted. Fresh gas introduced
between the expiratory valve and the absorber would be diluted by recirculating
gas. Furthermore, inhalation anesthetics may be absorbed or released by soda
lime granules, thus slowing induction and emergence.
·
The
APL valve is usually placed between the absorber and the expiratory valve and
close to the reservoir bag. Positioning of the APL valve in this location (ie,
before the absorber) helps to conserve absorption capacity and minimizes the
venting of fresh gas.
·
Resistance
to exhalation is decreased by locating the reservoir bag in the expiratory
limb. Bag compression during controlled ventilation will vent expired gas through
the APL valve, conserving absorbent.
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