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(Stroke. 2000;31:1672.)
© 2000 American Heart Association, Inc.

Original Contributions

MRI Measures of Middle Cerebral Artery Diameter in Conscious Humans During Simulated Orthostasis

Jorge M. Serrador, MSc; Paul A. Picot, PhD; Brian K. Rutt, PhD; J. Kevin Shoemaker, PhD Roberta L. Bondar, MD, PhD

From the Neurovascular Research Laboratory (J.M.S., P.A.P., J.K.S., R.L.B.), School of Kinesiology, and the Robarts Research Institute (B.K.R.), The University of Western Ontario, London, Ontario, Canada.

Correspondence to Dr Kevin Shoemaker, Neurovascular Research Laboratory, School of Kinesiology, The University of Western Ontario, London, Ontario, Canada N6A 3K7. E-mail kshoemak{at}julian.uwo.ca

Background and Purpose—The relationship between middle cerebral artery (MCA) flow velocity (CFV) and cerebral blood flow (CBF) is uncertain because of unknown vessel diameter response to physiological stimuli. The purpose of this study was to directly examine the effect of a simulated orthostatic stress (lower body negative pressure [LBNP]) as well as increased or decreased end-tidal carbon dioxide partial pressure (P_ETCO₂) on MCA diameter and CFV.

Methods—Twelve subjects participated in a CO₂ manipulation protocol and/or an LBNP protocol. In the CO₂ manipulation protocol, subjects breathed room air (normocapnia) or 6% inspired CO₂ (hypercapnia), or they hyperventilated to 25 mm Hg P_ETCO₂ (hypocapnia). In the LBNP protocol, subjects experienced 10 minutes each of -20 and -40 mm Hg lower body suction. CFV and diameter of the MCA were measured by transcranial Doppler and MRI, respectively, during the experimental protocols.

Results—Compared with normocapnia, hypercapnia produced increases in both P_ETCO₂ (from 36±3 to 40±4 mm Hg, P<0.05) and CFV (from 63±4 to 80±6 cm/s, P<0.001) but did not change MCA diameters (from 2.9±0.3 to 2.8±0.3 mm). Hypocapnia produced decreases in both P_ETCO₂ (24±2 mm Hg, P<0.005) and CFV (43±7 cm/s, P<0.001) compared with normocapnia, with no change in MCA diameters (from 2.9±0.3 to 2.9±0.4 mm). During -40 mm Hg LBNP, P_ETCO₂ was not changed, but CFV (55±4 cm/s) was reduced from baseline (58±4 cm/s, P<0.05), with no change in MCA diameter.

Conclusions—Under the conditions of this study, changes in MCA diameter were not detected. Therefore, we conclude that relative changes in CFV were representative of changes in CBF during the physiological stimuli of moderate LBNP or changes in P_ETCO₂.

Key Words: cerebral blood flow • hypotension, orthostatic • middle cerebral artery • ultrasonography, Doppler, transcranial

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