Increase of alveolar pressure reduces systemic-to-pulmonary bronchial blood flow in humans

We studied the effect of positive alveolar pressure (PA) on systemic to pulmonary bronchial blood flow, Q?? in humans. The Q?? was measured during total cardiopulmonary bypass as the volume of blood accumulating in the left heart. This blood was vented by gravity from the left heart via a cannula introduced in the right upper pulmonary vein and advanced to the lowest portion of the left heart. In group A (n = 10) the Qbr(s-p) was measured for 25 to 95 min with constant PA (4.0 +/- 0.2 cm H2O, mean +/- SE). In group B (n = 10) Qbr(s-p) was measured for 20 min with PA = 4.1 +/- 0.2 cm H2O and for a further 20 min with PA = 14.1 +/- 0.4 cm H2O. The Qbr(s-p) ranged between 0.32 and 2.76 percent of cardiac output (pump flow) and remained constant with time (group A). The increase of PA from 4.1 +/- 0.2 to 14.1 +/- 0.4 cm H2O reduced Qbr(s-p) by approximately 40 percent (p less than 0.01, group B). We conclude that positive PA reduces Qbr(s-p) during total cardiopulmonary bypass. Therefore, we advise using low PA during assisted ventilation to preserve bronchial blood flow.