Monitoring Cerebral Oximetery During Cardiopulmonary Byapss

A common reported use of ScO2 is to monitor adequacy of cerebral perfusion during cardiopulmonary bypass. Major changes in CBF are known to occur with change from pulsatile to non-pulsatile flow with the initiation of CPB. These changes may be reflected by changes in ScO2. In a recent observational study in 18 children, changes were observed before, during and after bypass. Before bypass, 10 children showed signficant (>15%) falls in ScO2 achieving absolute values of <35% in 5 patients. These desaturations were most commonly associated with manipulations of the position of the heart itself or with venous cannulation for bypass. During bypass and cooling, there was a significant rise in saturation (mean absolute increase of 18%). If circulatory arrest was used during surgery, ScO2 decayed with time. The rate of decay was related to the starting temperature (0.25% per minute if <20 degrees and 2% per minute if >20 degrees). Reperfusion resulted in an immediate increase in saturation followed by a gradual decrease.

Another recent study compared SjvO2 with ScO2 during bypass in 19 patients. The SjvO2 showed two distinct phases of change during bypass. During hypothermia, all patients had a relatively high SjvO2 averaging 80% and no patient showed any cerebral lactate production. During rewarming, however, 16 of 19 patients showed SjvO2 desaturation with 11 of these showing increased cerebral lactate production (anaerobic metabolism). The ScO2 trended with SjvO2 in most patients but the authors concluded that the SjvO2 was a better indicator of cerebral well-being.

A third study of 40 children undergoing open heart procedures also compared SjvO2 with ScO2. For all patients combined, the correlation between the measurements was 0.69 (p<.0001). For individual children, the correlation was also excellent. However, at low values of SjvO2, ScO2 tended to be significantly higher. The converse was true at very high values of SjvO2. These findings together with other studies in the literature suggest that ScO2 may have value to monitor cerebral well-being during cardiopulmonary bypass. Clearly, however, the role of ScO2 in monitoring the brain during bypass is not yet defined or established. Many more studies are needed before any firm conclusions may be drawn about the utility of this monitor.