Cerebral perfusion during cardiopulmonary bypass with special reference to blood flow
Abstract: Cardiopulmonary bypass (CPB) is an important method that enables open heart surgery. There is a risk of neurological complications, and efforts to minimize those include optimization of the cerebral perfusion during CPB. This thesis focuses on such optimization of flow conditions in case of obstructed venous drainage, carotid stenosis and during selective antegrade cerebral perfusion (SACP).In a pig model of impaired venous drainage from the superior vena cava (SVC), stepwise obstruction increased the central venous pressure (CVP) and caused impaired oxygenation. Cerebral micro-dialysis revealed ischemic responses in some but not all of the pigs.Further experiments, using the same model, aimed to restore cerebral perfusion pressure (CPP) reduced by 75% superior venous obstruction. Both vasopressor treatment and increased venous drainage were effective in normalizing the CPP and improving the cerebral oxygenation. The intracranial pressure was elevated in the vasopressor group, but no signs of brain damage were observed.The arterial flow during CPB can be altered between pulsatile and non-pulsatile profiles. Switching between these modes was performed during CPB in 20 patients with or without carotid stenosis. The effects on cerebral oxygenation and mean arterial pressure (MAP) were examined. The MAP was significantly lowered by pulsatile flow, but the flow profile did not affect the cerebral oxygenation. No differences were seen between patients with or without carotid stenosis.SACP is used to ensure the cerebral perfusion during deep hypothermic circulatory arrest (HCA). The cerebral blood flow (CBF) was examined using positron-emission tomography (PET) technique in 8 pigs divided into HCA and HCA+SACP groups. The CBF was downregulated by 70% to 0.10 ml/cm3/min by 20°C hypothermia. A pump flow of 6 ml/kg/min preserved the CBF level without signs of cerebral desaturation. The fluorodeoxyglucose (FDG) uptake after re-warming to 37°C was similar after SACP compared with HCA alone.In conclusion, experimental SVC obstruction may impair the cerebral perfusion. Vasopressors can restore the CPP during SVC obstruction and improve cerebral oxygenation. In patients, pulsatile flow can lower the MAP in absence of effects on the cerebral oxygenation. During experimental HCA, SACP at 6 ml/kg/min can preserve the CBF at 0.10 ml/cm3/min.
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