Baroreceptor-mediated compensation for hemodynamic effects of positive end-expiratory pressure

The roles of the carotid arterial baroreceptor reflex and of vagally mediated mechanisms during positive end-expiratory pressure (PEEP) were determined in pentobarbital-anesthetized dogs with isolated carotid sinuses. Spontaneously breathing dogs were placed on PEEP (5-10 cmH2O) with the carotid sinus pressure set to the systemic arterial pressure (with feedback) or to a constant pressure (no feedback). Right atrial volume was measured with a conductance catheter. With carotid baroreceptor feedback before bilateral cervical vagotomy, total peripheral resistance increased (P < 0.01) and mean arterial pressure decreased (-9.8 +/- 4.3 mmHg) in response to PEEP. With no feedback after vagotomy, mean arterial pressure decreased to a greater extent (-45 +/- 6 mmHg, P < 0.01), and total peripheral resistance decreased (P < 0.05) in response to PEEP. In contrast, cardiac index decreased similarly during PEEP (P < 0.01) for all baroreceptor and vagal inputs. This response comprised a decrease in the passive phase of right ventricular filling (P < 0. 01) that was not matched by the estimated increase in active right atrial output. Although the carotid baroreceptor reflex and vagally mediated mechanisms elicit vasoconstriction to compensate for the effects of PEEP on the arterial pressure, these processes fail to defend cardiac output because of the profound effect of PEEP on the passive filling of the right ventricle.