Influence of Abdominal Pressure and Sympathetic Vasoconstriction on the Cardiovascular Response to Positive End-Expiratory Pressure

The role of changes in abdominal pressure and sympathetically mediated vasoconstriction in the cardiovascular response to positive end-expiratory pressure was evaluated in 9 mongrel dogs. When the abdomen was widely opened, the decrease in cardiac output caused by positive end-expiratory pressure was the same as that found during control studies. When the abdomen was tightly bound, cardiac output was higher at any positive end-expiratory pressure than in control state (P < 0.01), but the per cent decrease produced by increasing positive end-expiratory pressure was the same. a-Adrenergic blockade with phenoxybenzamine produced a significantly greater decrease in cardiac output at any given positive end-expiratory pressure and thus appeared to inhibit the previously operative peripheral vascular adjustments to positive end-expiratory pressure. The major compensatory mechanism in the cardiovascular response to positive end-expiratory pressure thus appears to be mediated via a-adrenergic sympathetic factors.