Estimation of cerebrovascular reactivity using transcranial Doppler, including the use of breath-holding as the vasodilatory stimulus.

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Abstract
A proportion of individuals with carotid artery stenosis show a reduced cerebrovascular reserve as measured by a reduced cerebral arterial vasodilatory response to carbon dioxide. Two methods of quantifying this vasodilatory response, using transcranial Doppler ultrasonography, have been in general use: the total range of vasodilation between hypocapnia, induced by hyperventilation, and hypercapnia induced by breathing carbon dioxide, and the response to breathing a fixed concentration of 5% carbon dioxide. We studied whether it is possible to use the rise in carbon dioxide occurring during breath-holding as the vasodilatory stimulus. Using transcranial Doppler, cerebral reactivity to carbon dioxide was measured in 23 subjects undergoing intravenous digital subtraction angiography of their carotid arteries for symptoms of cerebrovascular disease. A breath-holding method was compared with the two previous methods, which required administration of carbon dioxide. All three methods gave results that correlated highly significantly with the degree of carotid stenosis, although the correlation was highest when the full vasodilatory range was measured. This method was adopted as the gold standard, and the other methods were compared with it. The breath-holding method correlated at least as well (rho = 0.67) as the 5% CO2 method (rho = 0.64). It identified a similar group of low reactors to our gold standard method, whereas the 5% CO2 method gave some discrepant results. The breath-holding method offers potential as a convenient, well-tolerated screening method of assessing carbon dioxide reactivity not requiring the administration of carbon dioxide, although further validation against more established methods of measuring cerebrovascular reserve is first required.