RECOVERY FROM ANESTHESIA

General Principles

Many factors that govern the rate at which the alveolar anesthetic concentration rises on induction apply to recovery. The immediate decline in the alveolar anesthetic concentration is extremely rapid because the washout of the functional residual capacity by ventilation is as rapid as the washin. Only 2 minutes is required to eliminate 95% to 98% of nitrogen from the lungs when pure oxygen is breathed or to restore the nitrogen concentration when room air is breathed on recovery.

As ventilation sweeps anesthetic from the alveoli during recovery, an anesthetic partial pressure gradient develops between the partial pressure in the returning venous blood and the partial pressure in the alveoli. This gradient drives anesthetic into the alveoli, thereby opposing the tendency of ventilation to lower the alveolar concentration. The solubility of the anesthetic determines part of the capacity of the venous to alveolar gradient to oppose the tendency of ventilation to decrease the alveolar anesthetic partial pressure. A more soluble agent, such as isoflurane ( Fig. 5-18C ), opposes the elimination produced by ventilation more effectively than does a poorly soluble agent, such as desflurane (see Fig. 5-18A ), because a greater reserve exists in blood for the more soluble agent. For a given duration of anesthesia, more isoflurane is available at a given partial pressure for transfer to the alveoli. The fall in the alveolar partial pressure of isoflurane is slower than the fall with sevoflurane, which is less rapid than the fall with desflurane. The rate at which recovery occurs is similarly affected. It is most rapid with desflurane, less rapid with sevoflurane, and slowest with isoflurane. The rapidity of recovery largely depends on the solubility of the anesthetic. ^[84]