Positive End Expiratory Pressure Reduces Bronchial Blood Flow after Aspiration Injury

Abstract

We hypothesized that since added airway pressure compresses bronchial vessels, the airway hyperemia found following airway injury would be reduced by positive end-expiratory pressure (PEEP). Accordingly, we measured the effect of 15 cm H₂O PEEP on bronchial and pulmonary blood flows by the radioactive microsphere reference flow technique in closed chested goats (n = 7) before and after aspiration injury to the left lung with 0.1 N HCl. Thirty minutes after aspiration, the pulmonary blood flow to the injured left lung was reduced by one third, whereas the total bronchial blood flow to the left lung (normalized to mean systemic pressure of 100 torr) doubled (11.3 ± 2.2 to 20.6 ± 1.0 ml/mini00 torr; p < 0.01). Increasing PEEP from 5 to 15 cm H₂O decreased total bronchial blood flow by about half both before (11.3 ± 2.2 falling to 5.7 ± 1.4 ml/min/100 torr) and after injury (20.6 ± 1.0 falling 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 increased more than threefold (1.4 ± 0.5 rising to 5.5 ± 1.3 ml/min/100 torr; p < 0.01). This increased flow after aspiration was less affected 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/min/100 torr; p < 0.05). The increase of the parenchymal portion of the bronchial blood flow after injury, although apparent (9.9 ± 1.8 increasing to 15.1 ± 1.2 ml/min/100 torr), was not significant (p = 0.08). However, 15 cm H₂O PEEP halved this parenchymal flow (15.1 ± 1.2 to 7.5 ± 2.4ml/min/100 torr; p < 0.05). We conclude that within half an hour after acid aspiration of one lung its pulmonary blood flow falls by one third but its total bronchial blood flow doubles. The bronchial 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.

Pulmonary and bronchial circulatory responses to segmental lung injury

“…In dogs, we found that anastomotic bronchial systemicto-pulmonary blood flow increases acutely when an isolated left lower lobe is injured through either the vascular route or via the airways (9). Total bronchial blood flow also increases in sheep after smoke inhala-tion (13) and in goats after acid aspiration (1,14). Prior studies have reported either anastomotic bronchial blood flow (i.e., in the part that drains into the pulmonary circulation) (9) or the total flow through the bronchial branch of the bronchoesophageal artery when using flow probes (13).…”