Abstract
BackgroundThe middle cerebral artery supplies long end-artery branches to perfuse the deep white matter and shorter peripheral branches to perfuse cortical and subcortical tissues. A generalized vasodilatory stimulus such as carbon dioxide not only results in an increase in flow to these various tissue beds but also redistribution among them. We employed a fast step increase in carbon dioxide to detect the dynamics of the cerebral blood flow response.Methodology/Principal FindingsThe study was approved by the Research Ethics Board of the University Health Network at the University of Toronto. We used transcranial ultrasound to measure the time course of middle cerebral artery blood flow velocity in 28 healthy adults. Normoxic, isoxic step increases in arterial carbon dioxide tension of 10 mmHg from both hypocapnic and normocapnic baselines were produced using a new prospective targeting system that enabled a more rapid step change than has been previously achievable. In most of the 28 subjects the responses at both carbon dioxide ranges were characterised by more complex responses than a single exponential rise. Most responses were characterised by a fast initial response which then declined rapidly to a nadir, followed by a slower secondary response, with some showing oscillations before stabilising.Conclusions/SignificanceA rapid step increase in carbon dioxide tension is capable of inducing instability in the cerebral blood flow control system. These dynamic aspects of the cerebral blood flow responses to rapid changes in carbon dioxide must be taken into account when using transcranial blood flow velocity in a single artery segment to measure cerebrovascular reactivity.
Highlights
The anatomy of the cerebral vasculature is complicated, involving groups of long end-arteries, such as those supplying the deep white matter, and well collateralized vascular networks, such as those supplying the grey matter and subcortical white matter [1]
In all tests the step changes in PETCO2 were completed within 2 breaths, i.e. the second breath of the hypercapnic period was at the final value
The dynamic characteristics of the responses differed between subjects, as well as between the two CO2 step ranges within a subject, the majority of subjects at both ranges of CO2 step stimuli had Middle cerebral artery blood velocity (MCAv) responses that were divisible into two phases delineated by the early decline of the initial response to a visible nadir
Summary
The anatomy of the cerebral vasculature is complicated, involving groups of long end-arteries, such as those supplying the deep white matter, and well collateralized vascular networks, such as those supplying the grey matter and subcortical white matter [1]. These vasculatures differ as well in their hemodynamic response to a global carbon dioxide (CO2) stimulus with respect to the magnitude, time course, and speed of response. A generalized vasodilatory stimulus such as carbon dioxide results in an increase in flow to these various tissue beds and redistribution among them. We employed a fast step increase in carbon dioxide to detect the dynamics of the cerebral blood flow response
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