Body temperature influences regional tissue blood flow during retrograde cerebral perfusion

Objective: This study compared the cerebral microcirculation during retrograde cerebral perfusion with that during antegrade cardiopulmonary bypass under normothermic and hypothermic conditions. Methods: Brain tissue blood flow was measured with the hydrogen-clearance and colored microsphere (15 and 50 μm) methods during antegrade cardiopulmonary bypass and retrograde cerebral perfusion. Measurements were performed during normothermia (37°C), moderate hypothermia (28°C) and deep hypothermia (20°C) in groups of mongrel dogs (n = 8). Results: During antegrade cardiopulmonary bypass, the microsphere method showed a significant decrease in cerebral blood flow as body temperature decreased (40.1 ± 20.8 ml/min/100 gm at 37°C, 16.2 ± 18.0 ml/min/100 gm at 20°C with 50 μm microspheres). At 20°C, the cerebral blood flow measured with the 15 μm microspheres was significantly lower than that assessed with the hydrogen-clearance method (11.3 ± 7.0 vs 24.8 ± 7.0 ml/min/100 gm). During retrograde cerebral perfusion, the microsphere method also showed a significant decrease in cerebral blood flow with cooling. At 37°C, the cerebral blood flow measured with the 15 μm microspheres (0.8 ± 0.7 ml/min/100 gm) was significantly lower than that assessed with the hydrogen-clearance method (10.1 ± 3.5 ml/min/100 gm). At both 28°and 20°C, the hydrogen-clearance method showed significantly higher cerebral blood flow (10.1 ± 5.8 and 8.2 ± 3.7 ml/min/100 gm) than did the 50 μm microspheres (1.8 ± 0.6 and 1.0 ± 0.8 ml/min/100 gm) and 15 μm microspheres (0.23 ± 0.14 and 0.18 ± 0.15 ml/min/100 gm). Conclusion: (1) Cerebral blood flow that shunts to capillaries is increased during antegrade cardiopulmonary bypass under deep hypothermia. (2) During retrograde perfusion, the majority of the blood flow shunts away from brain capillaries, even under normothermic conditions, and blood flow through large venoarterial shunts increases as body temperature decreases. Although the cerebral microcirculation during retrograde perfusion is decreased, retrograde perfusion provides some degree of oxygenation to the body.