Continuous monitoring of mixed venous oxygen saturation during aortic operations

OBJECTIVE: To systematically analyze the changes in mixed venous oxygen saturation (delta SvO2) during aortic operations with tube, aortobi-iliac, and aortobifemoral grafts. DESIGN: Survey of consecutive patients. SETTING: Teaching community hospital. PATIENTS: Thirty-one patients (22 male, 9 female, mean age 67 +/- 10 yrs), undergoing elective operations for aortic aneurysms (n = 25) and aortoiliac occlusive disease (n = 6). INTERVENTIONS: SvO2 was recorded throughout the operation. Cardiac output, mean pulmonary arterial pressure, arterial oxygen saturation (SaO2), and arterial pH were measured before and immediately after the unclamping of the aortic graft. RESULTS: In all patients, unclamping the aorta resulted in a marked reduction of mean SvO2, with no change in the cardiac output or SaO2. The unclamping of tube grafts was associated with a significant reduction in arterial pH (p less than .01) and in SvO2 (p less than .001), when compared with unclamping of bifurcation grafts. A significant (p less than .05) increase in mean pulmonary arterial pressure was observed after unclamping the aorta in patients with tube grafts. Despite a longer clamp time, unclamping the second limb of a bifurcation graft resulted in a smaller delta SvO2, when compared with that observed after unclamping the first limb (12% vs. 6%; p less than .01). The delta SvO2 after unclamping limb II was only 2% in aortobifemoral grafts and 9% in aortobi-iliac grafts. CONCLUSIONS: Reperfusion via extensive pelvic and lumbar collaterals in patients with aortoiliac occlusive disease reduces the delta SvO2 after aortic unclamping. Monitoring the changes in SvO2 during different types of aortic reconstruction helps to define precisely the physiologic alterations that occur in the course of these operations.