Aortic Unclamping

The hemodynamic and metabolic effects of unclamping were listed in Table 52-10 . The hemodynamic response to unclamping depends on the level of aortic occlusion, the total occlusion time, the use of diverting support, and the intravascular volume. Hypotension, the most consistent hemodynamic response to aortic unclamping, can be profound after removal of a supraceliac cross-clamp ( Fig. 52-13 ). Reactive hyperemia distal to the clamp and the resultant relative central hypovolemia are the dominant mechanisms of the hypotension. Washout of vasoactive and cardiodepressant mediators from ischemic tissues, as well as humoral factors, may also contribute to the hemodynamic responses after unclamping the aorta. These humoral factors and mediators, which may also play a role in organ dysfunction after aortic occlusion, include lactic acid, renin-angiotensin, oxygen free-radicals, prostaglandins, neutrophils, activated complement, cytokines, and myocardial-depressant factors. ^[242]

The avoidance of significant hypotension with unclamping requires communication with the surgical team, awareness of the technical aspect of the surgical procedure, and appropriate administration of fluids and vasoactive agents. It is essential that preoperative fluid deficits, intraoperative maintenance requirements, and replacement of blood loss be accomplished before unclamping. Vasodilators, if used, should be gradually reduced or discontinued before unclamping. Potent inhalational agents should be decreased. Moderate intravascular volume loading (approximately 500 mL) during the immediate prerelease period is indicated for infrarenal unclamping. ^{[231] [262]} More aggressive volume loading is required in the period immediately preceding supraceliac unclamping. Volume loading in an attempt to maintain an elevated central venous or pulmonary capillary wedge pressure during the cross-clamp period is not indicated and may result in significant overtransfusion of fluids and blood products. Gradual release of the aortic clamp and reapplication or digital compression if significant hypotension results are important measures in maintaining hemodynamic stability during unclamping. Although vasopressors are rarely required after release of the infrarenal clamp, they are frequently required after removal of supraceliac clamps. Caution must be observed when vasopressor support is used because profound proximal hypertension may occur if reapplication of the cross-clamp is required above the celiac axis. Hypertension should be avoided to prevent damage to or bleeding from the vascular anastomoses.