Transcranial Doppler Monitoring During Cardiopulmonary Bypass

To the Editor: The article by Nuttall et al. [1] found poor correlation between estimates of end-artery cerebral blood flow and transcranial Doppler (TCD)-derived basal artery flow velocity during cardiopulmonary bypass. However, since the authors did not measure clinical utility, their conclusion of limited TCD usefulness seems unjustified. TCD value during cardiac surgery may be assessed independently of flow-velocity relationships. In sharp contrast to flow estimates, TCD provides continuous detection of unanticipated perfusion-related abnormalities. Aortic perfusion cannula malposition may disrupt carotid flow [2]. During repair of an aortic arch dissection, retrograde pressurization of the false lumen may restrict true lumen and carotid flow [3]. In either case, a peak velocity drop identifies the problem. A sudden decrease in end-diastolic velocity prebypass characterizes venous cannula malposition [2]. This same pattern seen post-bypass suggests cerebral edema and the need for ultrafiltration and/or neuroprotectant administration. Detection of flow reversal is important during the initiation of retrograde cerebral perfusion, a new alternative to circulatory arrest for aortic aneurysm repair. Because of the substantial variability in cerebral venous anatomy [4], monitoring of flow reversal verifies bihemispheric retrograde perfusion [2]. Finally, TCD is exquisitely sensitive to gaseous emboli, permitting instantaneous detection of massive air embolism [2]. In each of these clinical situations, I have found TCD monitoring to be literally brain saving. Since flow and velocity estimates measure different aspects of perfusion in distinct vascular territories, their close correlation, especially during nonpulsatile perfusion, would be a physiologic curiosity. The lack of such correlation, by itself, seems insufficient justification to denigrate the only available direct continuous monitor of cerebral perfusion. The author thanks Medasonics for providing the transcranial Doppler monitors used in the cited study. The author has no economic interest in this or any other manufacturer of ultrasound equipment. Financial support for the cited study was provided by Cadwell Laboratories, the Alliant Community Trust, and the Rudd Heart and Lung Institute of the Jewish Hospital Health Care System, Louisville, KY. Harvey L. Edmonds, Jr., PhD Department of Anesthesiology University of Louisville Louisville, KY 40292