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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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				S. Lavi, R. Egbarya, R. Lavi, and G. Jacob Role of Nitric Oxide in the Regulation of Cerebral Blood Flow in Humans: Chemoregulation Versus Mechanoregulation Circulation, April 15, 2003; 107(14): 1901 - 1905. [Abstract] [Full Text] [PDF]

				F. Pott, J. J. Van Lieshout, K. Ide, P. Madsen, and N. H. Secher Middle cerebral artery blood velocity during intense static exercise is dominated by a Valsalva maneuver J Appl Physiol, April 1, 2003; 94(4): 1335 - 1344. [Abstract] [Full Text] [PDF]

				T. Przyby{nmid}owski, M.-F. Bangash, K. Reichmuth, B. J Morgan, J. B Skatrud, and J. A Dempsey Mechanisms of the cerebrovascular response to apnoea in humans J. Physiol., April 1, 2003; 548(1): 323 - 332. [Abstract] [Full Text] [PDF]

				J. J. Van Lieshout, W. Wieling, J. M. Karemaker, and N. H. Secher Syncope, cerebral perfusion, and oxygenation J Appl Physiol, March 1, 2003; 94(3): 833 - 848. [Abstract] [Full Text] [PDF]

				M. R. Edwards, Z. L. Topor, and R. L. Hughson A new two-breath technique for extracting the cerebrovascular response to arterial carbon dioxide Am J Physiol Regulatory Integrative Comp Physiol, March 1, 2003; 284(3): R853 - R859. [Abstract] [Full Text] [PDF]

				R. Zhang, J. H. Zuckerman, K. Iwasaki, T. E. Wilson, C. G. Crandall, and B. D. Levine Autonomic Neural Control of Dynamic Cerebral Autoregulation in Humans Circulation, October 1, 2002; 106(14): 1814 - 1820. [Abstract] [Full Text] [PDF]

				M. R. Edwards, J. K. Shoemaker, and R. L. Hughson Dynamic modulation of cerebrovascular resistance as an index of autoregulation under tilt and controlled PETCO2 Am J Physiol Regulatory Integrative Comp Physiol, September 1, 2002; 283(3): R653 - R662. [Abstract] [Full Text] [PDF]

				T. E. Wilson, J. Cui, R. Zhang, S. Witkowski, and C. G. Crandall Skin cooling maintains cerebral blood flow velocity and orthostatic tolerance during tilting in heated humans J Appl Physiol, July 1, 2002; 93(1): 85 - 91. [Abstract] [Full Text] [PDF]

				P J Eames, M J Blake, S L Dawson, R B Panerai, and J F Potter Dynamic cerebral autoregulation and beat to beat blood pressure control are impaired in acute ischaemic stroke J. Neurol. Neurosurg. Psychiatry, April 1, 2002; 72(4): 467 - 472. [Abstract] [Full Text] [PDF]

				N. Bruder, D. Pellissier, P. Grillot, and F. Gouin Cerebral Hyperemia During Recovery from General Anesthesia in Neurosurgical Patients Anesth. Analg., March 1, 2002; 94(3): 650 - 654. [Abstract] [Full Text] [PDF]

				M. S. Olufsen, A. Nadim, and L. A. Lipsitz Dynamics of cerebral blood flow regulation explained using a lumped parameter model Am J Physiol Regulatory Integrative Comp Physiol, February 1, 2002; 282(2): R611 - R622. [Abstract] [Full Text] [PDF]

				R. Schondorf, R. Stein, R. Roberts, J. Benoit, and W. Cupples Dynamic cerebral autoregulation is preserved in neurally mediated syncope J Appl Physiol, December 1, 2001; 91(6): 2493 - 2502. [Abstract] [Full Text] [PDF]

				J. M. Serrador, S. J. Wood, P. A. Picot, F. Stein, M. S. Kassam, R. L. Bondar, A. H. Rupert, and T. T. Schlegel Effect of acute exposure to hypergravity (GX vs. GZ) on dynamic cerebral autoregulation J Appl Physiol, November 1, 2001; 91(5): 1986 - 1994. [Abstract] [Full Text] [PDF]

				S. Houtman, J. M. Serrador, W. N. J. M. Colier, D. W. Strijbos, K. Shoemaker, and M. T. E. Hopman Changes in cerebral oxygenation and blood flow during LBNP in spinal cord-injured individuals J Appl Physiol, November 1, 2001; 91(5): 2199 - 2204. [Abstract] [Full Text] [PDF]

				R. L. Hughson, M. R. Edwards, D. D. O'Leary, and J. K. Shoemaker Critical Analysis of Cerebrovascular Autoregulation During Repeated Head-Up Tilt Stroke, October 1, 2001; 32(10): 2403 - 2408. [Abstract] [Full Text] [PDF]

				K. Narayanan, J. J. Collins, J. Hamner, S. Mukai, and L. A. Lipsitz Predicting cerebral blood flow response to orthostatic stress from resting dynamics: effects of healthy aging Am J Physiol Regulatory Integrative Comp Physiol, September 1, 2001; 281(3): R716 - R722. [Abstract] [Full Text] [PDF]

				B. J. Carey, B. N. Manktelow, R. B. Panerai, and J. F. Potter Cerebral Autoregulatory Responses to Head-Up Tilt in Normal Subjects and Patients With Recurrent Vasovagal Syncope Circulation, August 21, 2001; 104(8): 898 - 902. [Abstract] [Full Text] [PDF]

				J. Sato, M. Tachibana, T. Numata, T. Nishino, and A. Konno Differences in the dynamic cerebrovascular response between stepwise up tilt and down tilt in humans Am J Physiol Heart Circ Physiol, August 1, 2001; 281(2): H774 - H783. [Abstract] [Full Text] [PDF]