![]() We then applied these analyses to a cohort of patients with known localized cerebrovascular compromise, and compared the findings with those in healthy subjects. The aim of the study was to develop a method of separating CVR into (1) a dynamic component providing a voxelwise measurement of the speed of the cerebrovascular response to hypercapnia and (2) a static component providing a measure of the steady-state reactivity of the vasculature. Further details of the system are provided in Materials and Methods section. The rebreathing aspect also reduces the gradient between P ETCO 2 and PaCO 2. This system enables the implementation of an abrupt (within 1 to 2 breaths) step increase in P ETCO 2, which is sustained constant and independent of the pattern or extent of breathing. 16 In contrast, sequential gas delivery breathing circuits administer a gas composition supplied by a computerized gas blender, followed by rebreathing of previously exhaled gas. 19 With either of these methods P ETCO 2 is not a reliable indicator of PaCO 2, 20, 21 so that neither the time course nor the magnitude of the stimulus is precisely known, and therefore difficult to synchronize with the vascular response. 18 The latter produces a biexponential rise as the CO 2 is first ‘washed in' to the lung, gradually rising to an asymptote as CO 2 elimination comes into equilibrium with the body CO 2 production. This is not provided by breath holding or even an abrupt change in inspired PCO 2. The simplest stimulus that is useful for measuring the speed of the CBF vasodilatory response is a step (i.e., square wave) increase in PaCO 2. 10, 15, 16 In the course of a recent study of the amplitude of the BOLD signal response to hypercapnia in patients with cerebrovascular steno-occlusive disease, 17 we also noticed that the time course of signal changes can vary greatly throughout the brain within gray and white matter in healthy subjects, and between healthy subjects and patients, particularly in locations close to those experiencing vascular steal.Ī Step Hypercapnia to Measure the Speed of Cerebrovascular Response 14Ĭerebral blood flow vasodilatory response times have also been found to be consistently longer in patients with mild dementia and Alzheimer's disease than in age-matched, healthy subjects. 12, 13 CVR also varies within individual subjects between gray and white matter. The time course of middle cerebral artery flow velocity in response to a step hypercapnic stimulus has some variation between healthy subjects. The speed of Cerebrovascular Reactivity ResponseĪnother assessment of vascular function that may be useful is the rate of CBF increase in response to an abrupt vasodilatory stimulus. τ and ssCVR are respectively the dynamic and static components of measured CVR. We found that both prolongations of τ and reductions in ssCVR (compared with the reference atlas) were associated with the reductions in CVR on the side of the lesion. Additionally, the slope of the regression between S and the convolved PCO 2 represents the steady-state CVR (ssCVR). The τ corresponding to the best fit between S and the convolved PCO 2 was used to score the speed of response. We convolved the PCO 2 with a set of first-order exponential functions whose time constant τ was increased in 2-second intervals between 2 and 100 seconds. We applied an abrupt boxcar change in PCO 2 and monitored S. Our aim was to further characterize CVR into dynamic and static components and then study 46 healthy subjects collated into a reference atlas and 20 patients with unilateral carotid artery stenosis. We define cerebral vascular reactivity (CVR) as the ratio of the change in blood oxygen level-dependent (BOLD) magnetic resonance imaging (MRI) signal (S) to an increase in blood partial pressure of CO 2 (PCO 2): % Δ S/Δ PCO 2 mm Hg. ![]()
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