MRI measures of middle cerebral artery diameter in conscious humans duringsimulated orthostasis

Background and Purpose-The relationship between middle cerebral artery (MCA ) flow velocity (CFV) and cerebral blood now (CBF) is uncertain because of unknown vessel diameter response to physiological stimuli. The purpose of t his study was to directly examine the effect of a simulated orthostatic str ess (lower body negative pressure [LBNP]) as well as increased or decreased end-tidal carbon dioxide partial pressure (PETCO2) 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 CO, (hypercapnia), or they hyperventilated to approximate to 25 mmHg PETCO2 (hypocapnia). In the LBNP protocol, subje cts experienced 10 minutes each of -20 and -40 mm Hg lower body suction. CF V and diameter of the MCA were measured by transcranial Doppler and MRI, re spectively, 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 PETCO2 (24+/-2 mmHg, 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, PET CO2 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 wer e not detected. Therefore, we conclude that relative changes in CFV were re presentative of changes in CBF during the physiological stimuli of moderate LBNP or changes in PETCO2.