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This Article Full Text Full Text (PDF) All Versions of this Article: 92/4/769    most recent expphysiol.2006.036814v1 Alert me when this article is cited Alert me if a correction is posted Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Ainslie, P. N. Articles by Taylor, R. D. Search for Related Content PubMed PubMed Citation Articles by Ainslie, P. N. Articles by Taylor, R. D.

¹ Department of Physiology² School of Physiotherapy, University of Otago, Dunedin, New Zealand ³ Department of Medical and Surgical Sciences, Dunedin School of Medicine, University of Otago, Dunedin, New Zealand

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 whether 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 cerebral autoregulation (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-dependent vasodilatation (FMD) and endothelium-independent dilatation (NFMD) were also monitored. From evening to morning, there was a significant lowering in ARI (5.3 ± 0.5 versus 4.7 ± 0.6 a.u.; P < 0.05), cerebrovascular reactivity to CO₂ (5.3 ± 0.6 versus 4.6 ± 1.1% mmHg^–1; P < 0.05) and FMD (7.6 ± 0.9 versus 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 oxyhaemoglobin concentrations were observed upon resuming a supine-to-upright position (P < 0.05 versus 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.

(Received 20 December 2006; accepted after revision 16 March 2007; first published online 23 March 2007)
Corresponding author P. N. Ainslie: Department of Physiology, University of Otago, Dunedin, New Zealand. Email: philip.ainslie{at}stonebow.otago.ac.nz

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