Effect of Rebreathing

Patients breathe two gases from the anesthetic circuit: those delivered from the anesthetic machine and those previously exhaled by the patient and subsequently rebreathed. Because the patient removes (takes up) anesthetic from the rebreathed gas, the amount taken up and the amount rebreathed influence the inspired anesthetic concentration. An increase in uptake or rebreathing lowers the inspired concentration of a highly soluble gas more than the inspired concentration of a poorly soluble gas. Decreasing rebreathing by increasing the inflow rate diminishes this effect of uptake. With a ventilation of 5 L/min, use of a 5-L/min inflow rate essentially abolishes rebreathing.^[74]

High inflow rates (≥5 L/min) have the advantage of increasing the predictability of the inspired anesthetic concentration, but they have the disadvantages of being wasteful and of increasing atmospheric pollution. High inflow rates may be unacceptably costly because of the attendant greater consumption of expensive volatile anesthetics. High inflow rates also may result in drier inspired gas and greater difficulty in estimating ventilation from excursions of the rebreathing bag. These several disadvantages promote the use of low-flow techniques.