[abstract] CORRELATION OF MIDDLE CEREBRAL ARTERY BLOOD FLOW VELOCITIES WITH 133^Xe CEREBRAL BLOOD FLOW PERFUSION RATES DURING OXYGEN BREATHING IN MAN OVER A WIDE RANGE OF ARTERIAL pCO2 VALUES

Abstract

Transcranial doppler (TCD) ultrasonography provides a potentially useful method for measuring an index of cerebral blood flow (CBF) during underwater immersion and exposure to increased ambient pressure. This method was calibrated against a standard method for CBF determination by measuring velocity of blood flow through the left middle cerebral artery (MCA) simultaneously with 133^Xe determinations of regional CBF perfusion rates over a wide range of arterial pCO2 in eight subjects. Arterial pCO2 was varied by administration of 4% and 6% CO2 in 02 and by voluntary, controlled hyperventilation while breathing 02. Average arterial pCO2 ranged from a low value of 25.3mm Hg during hyperventilation on 02 to a high value of 49.9 mm Hg while breathing 6% CO, in 02. Corresponding average values of MCA blood flow velocity were 42.8 and 94.2 cm/sec, respectively, while global CBF perfusion rates ranged from 34.4 to 92.4 ml/100 gm/min. Regional CBF perfusion rates for the left MCA distribution were nearly identical to the global perfusion rates. TCD measurements of MCA blood flow velocity correlated significantly with CBF perfusion rate over the entire range of arterial pCO2. When CBF perfusion rates and MCA blood flow velocities are expressed as percent changes from control values measured during 02 breathing at rest, changes in perfusion rates significantly exceed velocity changes at higher levels of arterial pCO2 by an average factor of 2.1, but are not significantly different during hyperventilation to a lower pCO2. These data indicate that TCD measurements of MCA blood flow velocity provide a useful index of CBF perfusion rate during 02 breathing at rest, and are also consistent with MCA vasodilation during exposure to hypercapnia. Supported in part by Navy NMRDC Contract N00014-92-J-1275, NASA Contract NAG 9-527, and NOAA Contract NA-90AA-UR064.