Correlation between cerebral tissue oxygen saturation and central venous oxygen saturation during off-pump coronary artery bypass graft surgery
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Currently, off-pump coronary artery bypass surgery (OPCAB) is a selectively employed technique for myocardial revascularization used in the majority of heart units worldwide. This strategy obviates the documented deleterious effects of cardiopulmonary bypass. However the occurrence of neurological sequelae associated with OPCAB ranges from minor cognitive dysfunction to major stroke. Haemodynamic instability throughout the positioning, stabilization and interruption of coronary blood flow are regarded as important factors that affect the performance of off-pump surgery. Fluctuations during the perioperative period, in particular manipulation of the heart could result in temporary brain hypoperfusion and neurological sequelae. To predict those patients that are predisposed to cerebral complications, investigators have used neurological monitoring, in particular Near-infra red spectroscopy (NIRS) during cardiac surgery. Aims and Objectives of the study This prospective, observational study was carried out to assess the correlation between cerebral oxygen saturation and central venous saturation during OPCAB surgery. Central venous saturation is an important variable used to assess global tissue perfusion and could therefore be advocated as a surrogate measure of cerebral oxygen saturation. In addition variables such as mean arterial (MAP) pressure, heart rate (HR), patient oxygen saturation (SpO2), partial pressure of carbon dioxide (PcvCO2), haematocrit (Hct) and lactate were also measured to determine if they were independent predictors of cerebral desaturation. This study is one of the first done in the South African population group. iv Methodology Twenty patients undergoing OPCAB surgery from the Cardiothoracic unit at Inkosi Albert Luthuli Central Hospital, Durban, South Africa were recruited in the trial. Cerebral somasensors were placed on the patients forehead to measure left and right cerebral saturations. These sensors were linked by cables to the cerebral monitor (NIRS), INVOS model 5100C. Eight time periods throughout the surgical procedure whereby patients would be haemodynamically unstable were identified. These time periods included, post induction and pre sternotomy, pre and post placement of swabs beneath the heart, pre and post placement of the stabilizer device (Octopus), pre and post snaring of the LAD (left anterior branch of the coronary arteries), pre anastomosis and during anastomosis of the coronary arteries, second sample during anastomosis and post anastomosis, pre and post removal of swabs from beneath the heart, pre and post transfer of the patient to the ICU bed. These time periods constituted the sampling period pre and post manoeuvres. Eight paired measurements, i.e., MAP, PaCO2, HR, Hct, lactate, SpO2, central venous saturation (ScvO2) and cerebral oxygen saturation (rSo2) per patient were taken during these time periods. Recording of cerebral saturations and blood samples from the central venous line were taken during these eight time periods in order to determine the correlation between central venous and cerebral oxygen saturations. Results Strong positive correlations between central venous saturation and cerebral saturation presented in majority of the sampling time periods throughout the study (post induction and pre sternotomy, post placement of swabs beneath the heart, post snaring of the LAD (left anterior branch of the coronary arteries, pre anastomosis and during anastomosis of the coronary arteries, second sample during v anastomosis, pre and post transfer of the patient to the ICU bed). The positive correlation indicates that central venous saturation can be used as a surrogate measure of cerebral oxygen saturation during OPCAB surgery. Conclusion The absence or poor correlation of MAP, HR, PcvCO2, heamatocrit, lactate, and patient saturation to cerebral saturation in this study suggests that insertion of a central venous line (CVP) during OPCAB should be a fundamental clinical requirement.