INDEXES OF FLOW AND CROSS-SECTIONAL AREA OF THE MIDDLE CEREBRAL-ARTERY USING DOPPLER ULTRASOUND DURING HYPOXIA AND HYPERCAPNIA IN HUMANS

Background and Purpose This study examined changes in cross-sectional area of the middle cerebral artery as assessed by changes in Doppler s ignal power during hypoxia and hypercapnia. In addition, it examined t he degree of consistency among three indexes of cerebral blood flow an d velocity: the velocity spectral outline (<(V)over bar (p)>), the int ensity-weighted mean velocity (<(V)over bar (IWM)>), and an index of m iddle cerebral artery flow (P .<(V)over bar (IWM)>). P .<(V)over bar ( IWM)> was calculated as the product of <(V)over bar (IWM)> multiplied by the total power signal. Power is proportional to cross-sectional ar ea of the vessel; this calculation therefore allows for any changes in this variable. Methods Four protocols were used, each repeated six ti mes for six healthy adults aged 20.8 +/- 1.7 years (mean +/- SD). The first was a control protocol (A) with end-tidal PO2 (ETPO(2)) maintain ed at 100 mm Hg and ETPCO(2) at 1 to 2 mm Hg above eucapnia throughout . The second was a hypoxic step protocol (B) with ETPO(2) lowered from control values to 50 mm Hg for 20 minutes. The third was a hypercapni c step protocol (C) with ETPCO(2) elevated from control by 7.5 mm Hg f or 20 minutes. The fourth was a combined hypoxic and hypercapnic step protocol (D) lasting 20 minutes. A dynamic end-tidal forcing system wa s used to control ETPCO(2) and ETPO(2). Doppler data were collected an d stored every 10 milliseconds, and mean values were determined later on a beat-by-beat basis. <(V)over bar p>, <(V)over bar (IWM)>, power, and P .<(V)over bar (IWM)> were expressed as a percentage of the avera ge value over a 3-minute period before the step. Results In protocols A and B, there were no changes in power and there were no differences between <(V)over bar (p)>, <(V)over bar (IWM)>, and P .<(V)over bar (I WM)>. In C, at the relief from hypercapnia, there was a transient nons ignificant increase in power and a transient nonsignificant decrease i n both <(V)over bar (p)> and <(V)over bar (IWM)> compared with P .<(V) over bar (IWM)>. In D, during the stimulus period, <(V)over bar (p)> w as significantly higher than <(V)over bar (IWM)> (paired t test, P<.05 ), but both indexes were not different from P .<(V)over bar (IWM)>. In the period that followed relief from hypoxia and hypercapnia, the Dop pler power signal was significantly increased by 3.8%. During this per iod, <(V)over bar (p)> and <(V)over bar (IWM)> were significantly lowe r than P .<(V)over bar (IWM)>. Conclusions At the levels of either hyp oxia or hypercapnia used in this study, there were no changes in cross -sectional area of the middle cerebral artery, and changes in both <(V )over bar (p)> and <(V)over bar (IWM)> accurately reflect changes in P .<(V)over bar (IWM)>. With combined hypoxia and hypercapnia, however, at the relief from the stimuli when there is a very large and rapid d ecrease in P .<(V)over bar (IWM)> power is increased, suggesting an in crease in the cross-sectional area. During this period, changes in <(V )over bar (p)> and <(V)over bar (IWM)> underestimate the changes in P .<(V)over bar (IWM)>.