POSITIVE END-EXPIRATORY PRESSURE REDUCES BRONCHIAL BLOOD-FLOW AFTER ASPIRATION INJURY

We hypothesized that since added airway pressure compresses bronchial vessels, the airway hyperemia found following airway injury would be r educed by positive end-expiratory pressure (PEEP). Accordingly, we mea sured the effect of 15 cm H2O PEEP on bronchial and pulmonary blood fl ows by the radioactive microsphere reference flow technique in closed chested goats (n = 7) before and after aspiration injury to the left l ung with 0.1 N HCl. Thirty minutes after aspiration, the pulmonary blo od flow to the injured left lung was reduced by one third, whereas the total bronchial blood flow to the left lung (normalized to mean syste mic pressure of 100 torr) doubled (11.3 +/- 2.2 to 20.6 +/- 1.0 ml/min 100 torr; p < 0.01). Increasing PEEP from 5 to 15 cm H2O decreased to tal bronchial blood flow by about half both before (11.3 +/- 2.2 falli ng to 5.7 +/- 1.4 ml/min/100 torr) and after injury (20.6 +/- 1.0 fall ing to 10.3 +/- 2.7 ml/min/100 torr). The airway portion (down to 2-3 mm airways) of the total bronchial blood flow of the injured lung incr eased more than three fold (1.4 +/- 0.5 rising to 5.5 +/- 1.3 ml/min/1 00 torr; p < 0.01). This increased flow after aspiration was less affe cted by PEEP of 15 cm H2O (5.5 +/- 1.3 to 2.8 +/- 0.7 ml/min/100 torr, p = 0.09) than before injury (1.4 +/- 0.5 falling to 0.5 +/- 0.1 ml/m in/100 torr; p < 0.05). The increase of the parenchymal portion of the bronchial blood flow after injury, although apparent (9.9 +/- 1.8 inc reasing to 15.1 +/- 1.2 ml/min/100 torr), was not significant (p = 0.0 8). However, 15 cm H2O PEEP halved this parenchymal flow (15.1 +/- 1.2 to 7.5 +/- 2.4 ml/min/100 torr; p < 0.05). We conclude that within ha lf an hour after acid aspiration of one lung its pulmonary blood flow falls by one third but its total bronchial blood flow doubles. The bro nchial blood flow to the airway, the tissue most injured by the acid, increases more than the bronchial flow to the lung parenchyma. A PEEP level of 15 cm H2O essentially eliminates the compensatory increase in bronchial flow.