Received December 20, 2006
Revised January 31, 2007
Accepted after revision March 16, 2007

¹ University of Otago

^* To whom correspondence should be addressed. E-mail: philip.ainslie{at}stonebow.otago.ac.nz.

	Abstract

The reduction in cerebrovascular reactivity to CO2 and/or endothelial function that occurs in the early hours after waking are potential causes for the increased risk for cardiovascular events at this time point. It is unknown if cerebral autoregulation is reduced in the morning. We tested the hypothesis that early morning reduction in endothelium-dependent vascular reactivity would be linked to changes in cerebrovascular reactivity to CO2 and CA. Overnight changes in a dynamic cerebral autoregulation index (ARI) were determined from continuous recordings of blood flow velocity in the middle cerebral artery (MCAv) and arterial blood pressure (BP) during transiently induced hypotension in 20 individuals. Frontal cortical oxygenation (near infrared spectroscopy) and cerebral haemodynamics were also monitored during hypercapnia and before and during 3-min of active standing. Brachial artery flow-mediated endothelium dependant vasodilation (FMD) and endothelium-independent dilation (NFMD) were also monitored. From evening to morning, there was a significant lowering in ARI [5.3±0.5 vs. 4.7±0.6 (aU); P<0.05], cerebrovascular reactivity to CO2 [5.3±0.6 vs. 4.6±1.1 (%/mmHg); P<0.05], and FMD [7.6±0.9 vs. 6.0±1.4 (%); P<0.05]. The lowered FMD was related to the decrease in cerebrovascular reactivity to CO2 (r=0.76; P<0.05). Transient reductions in morning MCAv and cortical oxy-hemoglobin concentrations were observed upon resuming a supine-to-upright position (P<0.05 vs. evening). The early morning reduction in cerebral autoregulation may facilitate the onset of cerebrovascular accidents; this may be of particular relevance to at-risk groups, especially upon resuming the upright position.

Key Words: Brain, Carbon dioxide, Endothelium

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