Factors affecting assessment of cerebral autoregulation using the transient hyperaemic response test.

The transient hyperaemic response in the middle cerebral artery blood flow velocity on the release of brief compression of the ipsilateral common carotid artery has been validated as an indicator of cerebral autoregulation. We evaluated, in three stages, the effect of experimental factors such as duration of compression of the common carotid artery and magnitude of the decrease in blood flow velocity during common carotid artery compression on the transient hyperaemic response. In stage 1, 13 healthy volunteers underwent six transient hyperaemic response tests each; two tests each for either 3, 6 or 10 s duration of compression of the common carotid artery. In stage 2, 10 volunteers underwent four transient hyperaemic response tests each; two tests each for either 10 or 15 s duration of compression of the common carotid artery. In stage 3, data from the transient hyperaemic response tests using 10 s compression from the 23 volunteers who participated in stages 1 and 2 were analysed to evaluate the relationship between magnitude of decrease in blood flow velocity at the onset of compression and the transient hyperaemic response. The transient hyperaemic response ratio (blood flow velocity after the release of compression/baseline blood flow velocity) increased significantly when the duration of common carotid artery compression increased from 3 to 6 s, or from 6 to 10 s (stage 1); increase in the duration from 10 to 15 s did not have any significant effect (stage 2). The transient hyperaemic response ratio correlated significantly with the magnitude of decrease in blood flow velocity after compression, up to the values of the compression ratio (percent decrease in blood flow velocity at the onset of compression) of 40% but not more (stage 3). We conclude that experimental factors such as duration of common carotid artery compression and magnitude of the decrease in blood flow velocity during common carotid artery compression can significantly influence the transient hyperaemic response. These factors should be controlled if the transient hyperaemic response test is used for a comparison between repeated measurements. A compression time of 10 s and a compression ratio of 40% or more, allow maximum expression of the hyperaemic response in healthy volunteers.