Doppler cardiac auscultation appears to be the most sensitive method for detecting air emboli before pathophysiologic alterations occur in the cardiorespiratory system. In more than 200 patients in our series monitored with this technic, the incidence approaches 26 percent for those undergoing operations in the sitting position. A Doppler fetal ultrasonic unit, modified to cut off in the presence of annoying radiofrequency interference generated by electrocautery units, currently in use, provides an acceptable method for monitoring, but still further refinements are under study.