TCD uses relies on the Doppler shift of sounds waves reflected off moving RBCs to measure blood flow velocity in the cerebral vessels. The most commonly monitored arteries are the middle cerebral artery and the internal carotid. Velocity increases during systole and decreases during diastole and these velocities are plotted against time, the plot resembles an arterial waveform.
Assuming the diameter of the artery is unchanged (and the angle of insonation of the Doppler probe is constant), changes in blood flow velocity in the cerebral arteries are assumed to reflect changes in cortical cerebral blood flow. Both absolute and relative decreases in velocity can suggest decreases in cerebral blood flow and the potential for cerebral ischemia.
Moritz et al. (Anesthesiology 107(4): 563-569, 2010) simultaneously compared TCD, near-infrared spectroscopy (NIRS), stump pressure (SP) measurement, and somatosensory evoked potentials (SSEP) in 48 awake patients undergoing carotid surgery during regional anesthesia. The percent decrease in TCD had the greatest ability to detect neurologic changes. The authors wrote “The best results were found for TCD%, but there was no significant difference compared with SP and NIRS%.”
TCD can also be used to detect patients at risk for hyperperfusion syndrome after CEA. An increase in mean flow velocity of more than 175% predicted the development of hyperperfusion-related postoperative symptoms in one study.