The Arterial-Alveolar Nitrogen Difference for the Assessment of Ventilation-Perfusion Mismatch

Abstract

Impaired pulmonary gas exchange leading to arterial hypoxaemia is one of the most common problems in critically ill patients. This arterial hypoxaemia and, hence, an increased alveolar-arterial oxygen partial pressure difference (D(A-a)O₂)—at a given fraction of inspired oxygen (FiO₂) and a stable cardiac output—are mainly attributed to ventilation-perfusion mismatch (low V_A/Q areas) and direct intrapulmonary right-to-left shunting. The discrimination of these two components has been of interest as their causes and treatment may be different (Bendixen 1964; Pesenti et al. 1983). Berggren (1942) tried to separate the two effects by calculating the venous admixture (Q_VA/Q_T) with and without nitrogen present in the inspired gas. In practice this method yielded contradictory results, both enhanced and decreased Q_VA/Q_T values being found (Quan et al. 1980; Shapiro et al. 1980; Suter et al. 1985), and evidence for absorption collapse of alveoli being observed (Dantzker et al. 1975; Wagner et al. 1974a). Moreover, increasing the oxygen tension in mixed venous blood by pure oxygen breathing may increase the right-to-left shunt (Bishop and Cheney 1983) by reversing hypoxic pulmonary vasoconstriction in unventilated lung areas (Domino et al. 1983).