Cross-clamping of the descending thoracic aorta leads to the asymmetrical distribution of propofol during cardiopulmonary bypass surgery

BACKGROUND: We hypothesized that cross-clamping of the descending thoracic aorta (CcDTA) would result in significant changes in plasma propofol concentrations (Cp) proximal and distal to the cross-clamp. We investigated the effect of CcDTA on Cp centrally and distally, including the pulmonary artery and the cardiopulmonary bypass (CPB) cannula. METHODS: The bispectral index (BIS) was recorded during CcDTA in eight patients undergoing thoracic aortic surgery using target-controlled total intravenous anesthesia with propofol. The calculated Cp was maintained at 3 µg/ml. Cp was measured in blood samples drawn from the right radial artery, left dorsalis pedis artery, pulmonary artery, and the long venous CPB cannula. RESULTS: Complete data were obtained from six patients. BIS decreased significantly in all cases 5 minutes after initiating CcDTA. BIS continued to decrease in association with increasing propofol concentrations. During CcDTA, Cp in samples from the radial and pulmonary arteries (3.5 ± 0.50 and 2.9 ± 0.63 µg/ml, mean ± SD) was significantly higher than in samples from the dorsalis pedis artery and the venous cannula (1.1 ± 0.22 and 1.4 ± 0.02 µg/ml) (P < 0.05). CONCLUSIONS: The results suggest that almost all of the blood returning from the superior vena cava during CcDTA directly enters the pulmonary circulation without mixing with blood from the inferior vena cava. Observed changes in anesthetic blood concentrations could be due to the presence of a split circulation and asymmetrical distribution of propofol induced by CcDTA and CPB.