Ventilation-perfusion distributions during mechanical ventilation with superimposed spontaneous breathing in canine lung injury.

Biphasic positive airway pressure (BIPAP) allows unrestricted spontaneous breathing throughout mechanical ventilation. Effects of spontaneous breathing during BIPAP on pulmonary gas exchange were studied on a randomized basis in 12 dogs with oleic acid-induced lung injury using the multiple inert gas elimination technique. Spontaneous breathing during BIPAP, accounting for 10% of minute ventilation (VE), increased PaO2 from 61 +/- 2 to 78 +/- 3 mm Hg (mean +/- SE) (p < 0.01), cardiac output from 4.2 +/- 0.3 to 4.6 +/- 0.3 L/min (p < 0.05), and oxygen delivery from 537 +/- 51 to 716 +/- 58 ml/kg/min (p < 0.05), whereas oxygen consumption and total VE remained unchanged. Improved pulmonary gas exchange caused by better ventilation/perfusion (VA/Q) matching was indicated by a 17 +/- 3% decrease (p < 0.01) in blood flow to shunt units (VA/Q < 0.005), a 15 +/- 3% increase (p < 0.05) in perfusion of normal VA/Q units (0.1 < VA/Q < 10), and a 6 +/- 3% reduction in ventilation of dead space (VA/Q > 100) areas (p < 0.05). Spontaneous breaths superimposed on mechanical ventilation may convert shunt VA/Q units to normal by increased ventilation of poorly or nonventilated units and/or increase blood flow to previously minimal or nonperfused areas.