[Assessment of acute respiratory failure: shunt versus alveolar arterial oxygen difference]

Information provided by computation of shunt and alveolar arterial oxygen differences have been compared in 58 episodes of acute respiratory failure. In order to demonstrate the role of hemodynamic factors on pulmonary gas exchange, we compared blood gas measurements made in 29 patients with a high cardiac output and a reduced arteriovenous oxygen difference [C(a--v-)O2 less than 3.5 ml], with 33 measurements corresponding to a low cardiac output and a widened C(a--v-)O2 (greater than 6.5 ml). When the data was pooled together, the same P(A--a)O2 corresponded to many different shunt values, depending on the level of mixed venous oxygenation (PV-O2). QS/Qtot and P(A--a)O2 were quasi linearly correlated only when they corresponded to patients with the same C(a--v-)O2. For the same value of shunt, the PaO2 was always raised when the PV-O2 was raised. As far as pulmonary gas exchange is concerned, shunt calculation in acute respiratory failure is preferable to P(A--a)O2, especially when some hemodynamic disturbance is present.