Central Nervous System
Processed EEG (BIS)
Cerebral desaturations appear to be related to neurological outcome, with cumulative area of rSO2 < 40% being the most significant predictor for postoperative impairments [Yao FS et al. J Cardiothorac Vasc Anesth 18: 552, 2004]. Whether or not they change management is controversial. Murkin JM et al. randomized 200 patients to either no oximetry-based interventions or an algorithmic approach designed to maintain rSO2 > 75% of baseline (including positional changes, vasopressors, blood transfusions, and manipulation of [CO2]), and found that their rSO2-based protocol reduced ICU LOS and major morbidity/mortality [Murkin JM et al. Anesth Analg 104: 51, 2007; FREE Full-text at Anesthesia & Analgesia]. Importantly, the control group did not follow any protocol (ex. based on MAP, ScvO2, etc.), so, based on Murkin et al.’s data, it is impossible to differentiate between the cerebral oximeter and the increased standardization of care.
ScvO2 assesses global oxygen delivery but cannot accurately detect perterbations in regional DO2/VO2 matching [Schmid FX et al. Thorac Cardiovasc Surg 51: 306, 2003]. Normal SvO2 is ~ 75% (which corresponds to an extraction ratio [ER] of ~ 25%). An ER approaching 50% is thought to portend the onset of critically-low oxygen delivery / ischemic injury, however Ranucci et al. found that during CPB, neither ScvO2 or calculated ER were strongly correlated with hyperlactatemia (defined as > 3 mM, correlation coefficients -0.19 and 0.19, respectively [p = 0.06 for both]) [Ranucci M et al. Ann Thorac Surg 81: 2189, 2006]
Despite this data, many perfusionists aim for an arbitrary ScvO2 of 70% or higher. If perfusion is managed based on ScvO2, an understanding of DO2 and VO2 is essential
DO2 is the volume of oxygen (per time) provided by the pump, i.e. flow rate x [Hgb] x saturation. The critical level of DO2 is debatable, and is proportionate to CPB temperature. Ranucci et al., in a retrospective review of 1,048 cardiac anesthetics, found that a DO2 (indexed to BSA) of less than 272 ml/min/m2 was the single best predictor for renal failure following CPB (AUC 0.76, sensitivity 68%, specificity 68%, p = 0.011) [Ranucci M et al. Ann Thorac Surg 80: 2213, 2005]
Oxygen consumption. ~ 130 mL/min/m2 in an awake adult, closer to 100 mL/min/m2 under general anesthesia and falls even lower during hypothermic CPB (VO2 decreases by 5% for every degree C). When DO2 is less than VO2, anaerobic metabolism and potentially ischemia ensue
No commercially available monitor is able to accurately measure brain temperature. Since brain temperature cannot be assessed using current technology, the next best metric is core temperature, which requires up to three measures to reliably assess.