A new method for correcting middle cerebral artery flow velocity for age by calculating Z-scores

For the interpretation of middle cerebral artery flow velocity (MCAFV) measurements by transcranial Doppler clinicians should take normal aging into account.

MCAFV measurements can be corrected for age by a transformation to Z-scores, expressing in standard deviations the distance of a measurement to the regression line as function of age. Z-scores for different parameters can be combined into a single plot in order to facilitate clinical interpretation.

In a retrospective analysis 41 subjects were identified with normal autonomic responses, normal CO2-reactivity and satisfactory transcranial Doppler data. Age related changes were determined for: maximal change in flow velocity (FV) during stroke onset (Acc), maximal FV during the first (Sys1) and second phase (Sys2) of systole and mean diastolic FV 560 ms after stroke onset (Dias@560). Patients were supine and breathing normally. Sys1, Sys2 and Dias@560 decreased significantly with age: -0.58 cm/s/yr, -0.36 cm/s/yr. and -0.38 cm/s/yr., respectively. Sys1 decreased more rapidly than Sys2 and Dias@560 so that in the young the MCAFV signal is more Sys1 dominant, whereas in the elderly it becomes more Sys2 prevalent. The effect of CO2-reactivity was assessed by transformation to Z-scores and combining different parameters within a single radar plot.

In contrast to standard MCAFV measurements the calculation of Z-scores takes aging into account and standardizes the signal variation.

Graphically combining Z-scores for different MCAFV parameters makes the signal more accessible for rapid clinical interpretation.