Role of the renin-angiotensin system in liver blood flow reduction produced by positive end-expiratory pressure ventilation.

Abstract

Positive end-expiratory pressure (PEEP), often used in critically ill patients, reduces cardiac output, and its adverse effects on splanchnic circulation imply a risk of regional secondary organ failure. To investigate if the renin-angiotensin system (RAS) mediates PEEP-induced circulatory changes, hemodynamic effects of PEEP were measured in four groups of pigs: controls (C), nephrectomized (N), or given enalaprilate, an angiotensin-converting enzyme inhibitor (E), or saralasin, a competitive inhibitor of angiotensin II (S). Groups N, E and S represented interference with RAS effects at different sites. With PEEP at 10 cmH2O, mean arterial pressure, cardiac index, portal venous and hepatic arterial blood flow decreased in all groups, while portal and central venous pressures rose, without significant intergroup difference. Systemic and preportal bloodflow resistance increased in groups S and N, and hepatic arterial resistance in group C. Accentuation of the flow and pressure changes occurred with 20 cm PEEP in all groups, with increase of systemic and hepatic resistance in S and N, or preportal resistance in group N and protal resistance in group C. The study suggested that RAS is not a major mediator of PEEP-induced circulatory changes. Differing responses within groups N, S and E may have been due to interference with the action of RAS and of other vasoactive substances.