Hemodynamic Benefit of Positive End-expiratory Pressure during Acute Descending Aortic Occlusion

Background: Acute aortic occlusion in vascular surgery patients abruptly increases arterial resistance and blood pressure, which, in turn, makes subsequent volume expansion during cross-clamp application difficult. The use of vasodilatory drugs or volatile anesthetic agents to attenuate this response may have persistent detrimental effects after clamp removal. Another potential therapy that produces rapid effects on myocardial loading conditions is positive end-expiratory pressure (PEEP). In a porcine model of acute aortic clamping, the hemodynamic consequences of 15 cm H2O PEEP with and without plasma volume expansion were studied.

Methods: Forty anesthetized pigs underwent 30-min occlusion of the abdominal aorta 1 cm above the origin of the celiac artery. Animals were randomly divided into four treatment groups (n = 10 each) to receive 15 cm H2O PEEP or zero end-expiratory pressure (ZEEP) with or without plasma volume expansion using 6% hetastarch (10 ml/kg) during cross-clamp application. Mean aortic pressure was measured with a transducer-tipped catheter placed in the ascending aorta; stroke volume was calculated using thermodilution cardiac output. End-expiratory pressure was discontinued upon aortic declamping, and animals were studied over the ensuing 30-min period.

Results: Aortic occlusion doubled systemic vascular resistance in all groups. Mean aortic blood pressure increased significantly in both ZEEP groups at 1 and 5 min but not in animals treated with 15 cm H2O PEEP. The application of PEEP with aortic cross-clamping reduced cardiac output and stroke volume by nearly 50%. Cardiac output and stroke volume increased after volume expansion regardless of end-expiratory pressure. After aortic declamping, aortic blood pressure decreased in all groups but was significantly greater in the PEEP + volume group than in either ZEEP group. Similarly, 5 min after declamping, stroke volume was greatest in the PEEP + volume animals.