Factors Affecting Arterial Concentration

FACTORS AFFECTING ARTERIAL CONCENTRATION Fa

Ventilation/Perfusion Mismatch

Normally, alveolar and arterial anesthetic partial pressures are assumed to be equal, but in fact, the arterial partial pressure is consistently less than end-expiratory gas would predict. Reasons for this may include venous admixture, alveolar dead space, and nonuniform alveolar gas distribution. Furthermore, the existence of ventilation/perfusion mismatch-ing will increase the alveolar–arterial difference. Mismatch acts as a restriction to flow: It raises the pressure in front of the restriction, lowers the pres-sure beyond the restriction, and reduces the flow through the restriction. The overall effect is an increase in the alveolar partial pressure (particularly for highly soluble agents) and a decrease in the arte-rial partial pressure (particularly for poorly soluble agents). Thus, a bronchial intubation or a right-to-left intracardiac shunt will slow the rate of induction with nitrous oxide more than with halothane.