Cerebral Perfusion and Metabolism during Experimental Extracorporeal Circulation
Sammanfattning: Neurologic injuries are major causes of mortality and morbidity after cardiac surgery. This thesis aimed to investigate cerebral metabolism and perfusion abnormalities in pigs during hypothermic circulatory arrest, selective antegrade cerebral perfusion (SACP) and extracorporeal circulation following progressive venous stasis.Hypothermic circulatory arrest induced a metabolic pattern consistent with overt ischaemia, which was absent following SACP. In contrast, metabolism during SACP was influenced by the perfusate temperature, where a colder perfusate (20 °C) preserved cellular metabolism and membrane integrity better than a warmer perfusate (28 °C).The minimum SACP flow required to maintain metabolism during hypothermia at 20 °C was investigated with magnetic resonance imaging, protein S100β, near infrared spectroscopy and microdialysis. The findings suggested an ischaemic threshold close to 6 ml/kg/min in the present models. Furthermore, regional differences in perfusion with a hemispheric distribution were apparent at all flow levels and differed from earlier studies where the differences were uniform and followed a neuranatomical pattern.Venus stasis following superior vena cava congestion produced measurable signs of impaired cerebral perfusion and patterns of cerebral ischaemia were evident in individual animals. As venous pressure increased, the mean arterial pressure stayed more or less unchanged, generating reduced cerebral perfusion pressure and consequently an increased risk of ischaemia, which may impair cerebral perfusion, especially in cases of compromised arterial flow during extracorporeal circulation.In conclusion, cerebral metabolism and perfusion are influenced by temperature, SACP flow levels and venous congestion. In clinical practice, the regional differences in perfusion during SACP may be of pathogenic importance in focal cerebral ischaemia. Furthermore, the reduced superior vena cava cannula flow may pass undetected during bicaval cardiopulmonary bypass if the superior vena cava flow is not specifically monitored.
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