ESTIMATION OF VESSEL FLOW AND DIAMETER DURING CEREBRAL VASOSPASM USING TRANSCRANIAL DOPPLER INDEXES

OBJECTIVE: An important limitation of transcranial Doppler (TCD) ultra sonography is its inability to directly measure blood flow or vessel d iameter. To extend the ability of TCD ultrasonography, indices were de rived from an intensity-weighted mean of the entire Doppler spectrum. The objective of this article is to test the behavior of these Indices under conditions of diameter constancy (hyper- and hypoventilation) a nd when vessel diameter decreases (vasospasm). METHODS: A flow index ( FI) was calculated by averaging several heartbeats of spectral data an d calculating the first spectral moment. An area index (Al) was define d as the FI divided by the mean velocity, motivated by the knowledge t hat vessel flow is the product of vessel diameter and mean velocity. T o test the FI and the AI under conditions of diameter constancy, middl e cerebral artery Doppler signals were obtained from 20 patients durin g conditions of hypercarbia, hypocarbia, and normocarbia. To test the ability of these indices to evaluate a decrease in vessel diameter, si gnals from 41 sites on 23 arteries were obtained from patients who und erwent both TCD and angiographic studies on two separate occasions aft er the occurrence of subarachnoid hemorrhage. The changes in the AI we re compared with the arterial diameters measured from angiograms.RESUL TS: The FI was proportional to the mean velocity in the cohort of heal thy patients (r = 0.97). The AI changed by less than 3% in the same co hort. The AI predicted the direction of the diameter change in all ves sels showing angiographic changes in area. Changes in the AI and the m easured angiographic changes in cross-sectional areas were correlated (overall, r = 0.90; with two outlines removed, r = 0.86). CONCLUSION: This variant of the intensity-weighted mean predicts changes in vessel cross-sectional area under conditions of changes in CO2 and cerebral vasospasm. This preliminary study suggests that careful use of this to ol may provide accurate evaluation of cerebral blood flow through the large vessels and quantitative changes in diameter, which occur freque ntly after subarachnoid hemorrhage.