PULMONARY CLEARANCE OF ANAESTHETIC AGENTS A Theoretical Study

The pulmonary clearance of anaesthetic agents (Cl) representsa useful index of their elimination from the lungs. Its valuedepends on pulmonary blood flow ("), alveolar ventilation (VA),and anaesthetic blood solubility () according to equation: Cl= (VAQ)/(VA+Q). When the alveolar ventilation and pulmonaryblood flow are constant the pulmonary clearance of the anaestheticdepends on its blood solubility. In this case the lower theblood solubility is, the higher the value of pulmonary clearancebecomes. For each anaesthetic agent a change in alveolar ventilationor in pulmonary blood flow will produce a change in pulmonaryclearance which in its turn depends on anaesthetic blood solubility.When = (VA/Q)² a change in alveolar ventilation or in pulmonaryblood flow will affect the pulmonary clearance to the same extent.When the anaesthetic blood solubility value is higher than (VA/Q)²,its pulmonary clearance is predominantly affected by alveolarventilation, and when anaesthetic blood solubility is lowerthan (VA/Q)² its pulmonary clearance is predominantly dependenton pulmonary blood flow. Changes in pulmonary clearance producedby changes in alveolar ventilation (/VA) are larger with halothane,nitrous oxide and cyclopropane than with agents of greater blood-gassolubility at all VA/Q ratios in the range 0.5—2.0.