ANESTHETIC CIRCUITRY

The previous discussion generally considered that the alveolar anesthetic concentration (FA) was moving toward a constant inspired anesthetic concentration (FI). In practice, the inspired concentration is usually not constant because a non-rebreathing system is not used. The rebreathing that results from the use of an anesthetic circuit causes the inspired concentration to be less than that in the gas delivered from the anesthetic machine. The inspired concentration is influenced by the delivered concentration, by the need to "wash in" the circuit, and by the depletion of anesthetic in rebreathed gases produced by uptake of anesthetic.

Washin of the Circuit

To begin anesthesia, the anesthetic must be washed into the volume of the circuit. At inflow rates of 0.5 to 8 L/min and a circuit volume of 7 L (i.e., 3-L bag, 2-L carbon dioxide absorber, and 2 L of corrugated hoses and fittings), the washin of the circuit is 50% to 100% complete in 10 minutes ( Fig. 5-15 ). Higher inflow rates produce a more rapid rise in the inspired concentration, which suggests that induction can be accelerated and made more predictable by use of high inflow rates.