1. Transcranial Doppler measurements of the mean blood flow velocity and the resistance index in the middle cerebral artery are used to assess cerebral haemodynamics. The prerequisite for the use of these parameters in clinical pharmacology studies is an evaluation of their repeatability and spontaneous variation with time.

2. Repeatability of transcranial Doppler measurements of mean blood flow velocity and resistance index was investigated in healthy subjects by calculation of the repeatability coefficient as defined by the British Standards Institution. Intra-observer repeatability (comparison of two successive determinations by one observer, n = 30 subjects), between-observer repeatability (comparison of two successive measurements each performed by a different observer, n = 30) and long-term repeatability (comparison of two determinations performed at the same hour with a 1 week interval by one observer, n = 14) were assessed. In addition, the spontaneous evolution with time of mean blood flow velocity and resistance index was determined over an 8 h period (n = 14).

3. Repeatability coefficients for intra-observer repeatability, between-observer repeatability and long-term repeatability were 4.83, 4.59 and 3.32 cm/s for mean blood flow velocity (normal value = 61.2 ± 10.2 cm/s) and 2.62, 3.12 and 3.49% for resistance index, respectively (normal value = 53.6 ± 5.9%), indicating that transcranial Doppler measurements are repeatable enough to be used over periods of time of up to 1 week in clinical pharmacology studies.

4. Finally, a time effect was detected for mean blood flow velocity, indicating that this parameter undergoes diurnal variation.

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