Transcranial Doppler Monitoring During Cardiopulmonary Bypass

In Response: We would agree with Dr. Edmonds that a monitor that could measure the adequacy of cerebral perfusion during cardiopulmonary bypass (CPB) would have significant "clinical utility." At present, the nature of transcranial Doppler (TCD) technology is promising in that it provides a continuous measurement of blood velocity. However, we were not able to document a good correlation between the TCD velocity and cerebral blood flow during hypothermic CPB [1]. Therefore, we would urge caution in equating TCD velocity with cerebral blood flow during nonpulsatile CPB. Dr. Edmonds highlights several clinical situations where the continuous monitoring capability of TCD may have specific usefulness. Aortic or venous cannula malposition, which would compromise cerebral perfusion, or flow reversal during retrograde cerebral perfusion may be detected by TCD, but these are unusual events that can be discovered by other means or are uncommon in clinical practice. We would agree that TCD is a very useful technique for detection of cerebral embolic events and could improve the efficacy of de-airing and patient management. However, emboli detection was not addressed by our study. Our study did not measure the clinical utility of TCD velocity in cardiac surgery, however difficult that is to define. Regardless, any test to be used as a measure of the adequacy of cerebral perfusion should have a good correlation with measured cerebral blood flow. We did not find that correlation. Furthermore, the clinical utility of the TCD for improving clinical outcome or intraoperative management will need to be addressed in studies of a different design. While our study and that of Weyland et al. [2] did not document a strong correlation between TCD velocity and cerebral blood flow during hypothermic nonpulsatile CPB, it was not our intent to "denigrate" this technique. Dr. Edmonds' points are well taken and we look forward to further developments in TCD technology, which will provide it with further applicability in the operating room. Gregory A.