Changes in cerebral blood flow during hyperventilation and CO2‐breathing measured transcutaneously in humans by a bidirectional, pulsed, ultrasound doppler blood velocitymeter

Abstract

A bidirectional pulsed ultrasound Doppler system, which measures the instantaneous mean velocity across the lumen of a blood vessel was used to determine the relationship between alveolar PCO2 [CO2 partial pressure] (PACO2) and blood flow in the 4 arteries supplying the brain in humans. High and low PACO2 values were explored. Six subjects, 3 males and 3 females (22-40 yr) were studied by use of this non-invasive technique. To increase the PACO2, the subjects were breathing 4, 6 and 8% CO2 in air. PACO2 was reduced by voluntary hyperventilation down to a chosen end-expiratory PCO2 value of .apprx. 2.2 kPa [kilo Pascals]. A linear relationship was found between arterial blood flow expressed as a percentage of control level and PACO2 in the range 3.3-7.3 kPa. At the very lowest PACO2 values, a leveling off of the response, with flow values of 40-45%, was observed. The CO2-reactivities in the 6 persons varied between 28.1 and 30.0%/kPa. The time course and the magnitude of the flow response were similar in all 4 arteries.