Transcranial Doppler evaluation of syncope: an application in aerospace physiology

A non-invasive method which combines the simultaneous measurement of mean arterial blood pressure (MBP), heart rate (HR) and mean cerebral blood flow velocity (MFV) was used to monitor patients with history of syncope, in horizontal and vertical posture tilt at 80 degrees. MFV in the right middle cerebral artery was measured using a transcranial Doppler instrument (TCD). MFV decreased concurrently with the onset of symptoms, and at the time of syncope reached an average of 68% below pre-tilt values. At the same time MBP showed an average decline of 25%, and HR increased by 38%. There was no correlation between MBP and MFV, at the onset of tilt, presyncope and syncope. MFV, but not HR or MBP, showed significant transition from one condition to the other. These data suggest that there may be a useful application of TCD measurements of MFV in aeromedical evaluation of syncope or syncopal tendency. These measurements would necessarily be used in conjunction with a tilt-table procedure. The possibility exists that MFV might be useful to preclude the actual occurrence of syncope in test subjects, or to show an abnormal tendency toward syncope, but will require more extensive testing than that carried out in the present study.