Significance of gaseous microemboli in the cerebral circulation during cardiopulmonary bypass in dogs

Background. Gaseous microemboli during cardiac surgery may damage the brain by reducing cerebral blood flow (CBF). We examined whether the incidence of gaseous microemboli during 150-minute hypothermic (28°C) cardiopulmonary bypass (CPB) adversely affects CBF (radioactive microspheres). Methods and Results. Thirty anesthetized dogs were placed on CPB using bubble oxygenators with 50% O₂ (n=10) or 100% O₂ (n=10) to produce a wide range in the number of gaseous microemboli or membrane oxygenators with 50% O₂ (n=10) to avoid microemboli. The number of carotid artery microemboli occurring in a 1-minute interval was counted using a 5-MHz Doppler probe every 15 minutes for the duration of CPB, which lasted 258±5 minutes. With bubbled 100% O₂, the number of microemboli averaged 4.1±1.7 emboli per minute on normothermic bypass and increased with cooling to 18.3±4.9 emboli per minute (P<.001). With bubbled 50% O₂, the microemboli number was greater on normothermic bypass (19.8±9.8 emboli per minute, P=.0653 compared with bubbled 100% O₂) and increased with cooling (100.3±18.7 emboli per minute, P<001) to a greater extent than with bubbled 100% O₂ (P<.001). In contrast, with membrane 50% O₂, the emboli number was small (0.6±0.1 emboli per minute) and did not change with CPB temperature. CBF values were not reduced after termination of CPB, even when compared with prebypass values, being 48.3±7.5 mL/min per 100 g (bubbled 50% O₂), 49.6±4.1 mL/min per 100 g (bubble 100% O₂), and 44.5±2.8 mL/min per 100 g (membrane 50% O₂, P=.7581). Similarly, regional perfusion to the cerebellum, hippocampus, and caudal brainstem was not adversely affected by microemboli. After CPB, cortical biopsies demonstrated no difference among groups with respect to lactate (P=.1753), energy charge (P=.5179), and brain water content (P=.939). Retinal histopathology indicated no differences among groups. Conclusions. These results indicate that (1) the incidence of gaseous microemboli during hypothermia increases when a bubble oxygenator is used, and (2) global CBF and regional brain perfusion are not adversely affected by numerous gaseous microemboli.