FACTORS AFFECTING ARTERIAL CONCENTRATION Fa
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.