Effect of low isoflurane concentrations on the ventilation-perfusion distribution in injured canine lungs

Rapid recovery and weaning from ventilatory support and cardiovascular stability are suggested advantages of isoflurane inhalation, in concentrations ranging from 0.1 to 0.6 vol%, for long-term sedation in mechanical ventilated patients. This study was designed to determine whether isoflurane in low concentrations impairs pulmonary gas exchange by increasing ventilation and perfusion (V(A)/Q) mismatch during lung injury.

Fourteen anesthetized dogs received in random order 0, 0.25, or 0.5 vol% end-tidal isoflurane before and after induction of lung injury with oleic acid. Gas exchange was assessed by blood gas analysis and by estimating the V(A)/Q distributions using the multiple inert gas elimination technique.

Administration of oleic acid produced a lung injury with severe V(A)/Q mismatch and 38 +/- 4% intrapulmonary shunting of blood. During lung injury, isoflurane accounted for a dose-related increase in blood flow to shunt units from 38 +/- 4 to 42 +/- 3 (0.25 vol%) and 48 +/- 4% (0.5 vol%) (P < 0.05), dispersion pulmonary blood flow distribution from 0.94 +/- 0.07 to 1.01 +/- 0.09 (0.25 vol%) and 1.11 +/- 0.11% (0.5 vol%) (P < 0.05), and a decrease in perfusion of normal V(A)/Q units from 58 +/- 5 to 55 +/- 4 (0.25 vol%) and 50 +/- 4% (0.5 vol%) (P < 0.05) (mean +/- SE). Isoflurane decreased arterial oxygen partial pressure from 72 +/- 4 to 62 +/- 4 mmHg (0.25 vol%) and 56 +/- 4 mmHg (0.5 vol%) (P < 0.05) and oxygen delivery from 573 +/- 21 to 529 +/- 19 ml. kg. min (0.25 vol%) and 505 +/- 22 ml. kg. min (0.5 vol%) (P < 0.05). Gas exchange, perfusion of shunt and normal V(A)/Q units, and pulmonary blood flow distribution was similar in absence of lung injury with and without isoflurane. Isoflurane 0.5 vol% lowered cardiac output during all conditions (P < 0.05). CONCLUSIONS Inhalation of low concentrations of isoflurane contributed to increased V(A)/Q mismatch and decreased systemic blood flow and oxygen delivery in mechanically ventilated animals with injured lungs.