Variability of middle cerebral artery blood flow with hypercapnia in women

We examined the effect of euoxic hypercapnia on middle cerebral artery (MCA) blood flow velocity waveform parameters in pre- and postmenopausal women by exposing 24 healthy women (12 pre-, 12 postmenopausal) to hypercapnia for 20 min. MCA blood flow velocity was measured continuously by transcranial Doppler ultrasound. The data were run through an algorithm that detected the feature points of the waveforms and then analyzed for statistically significant group differences. The changes in mean blood flow velocity with euoxic hypercapnia were not significant between the two groups. However, certain feature points, particularly the velocity of the reflected shoulder (V_REFLEC), increased (89.4 ± 14.6 to 110.0 ± 20.5 cm/s and 102.3 ± 14.1 to 125.1 ± 14.9 cm/s from euoxic eucapnia to euoxic hypercapnia in pre- and postmenopausal women, respectively), as did the augmentation index (79.9 ± 10.4 to 85.9 ± 12.6% and 114.7 ± 12.8 to 119.0 ± 12.6%) and pulsatility index (0.86 ± 0.18 to 0.74 ± 0.15 and 0.71 ± 0.11 to 0.66 ± 0.11). Furthermore, while systolic peak velocity (V_SYS) was the highest point of the waveform in premenopausal women, V_REFLEC was the highest point for the postmenopausal cohort. The implications of this finding become obvious when calculating pulsatility index (PI), the values of which varied significantly for the postmenopausal women, depending on whether V_SYS or the absolute maximum was used. These findings suggest that hypercapnia increases blood flow velocity waveform reflections, and that PI calculations, particularly for older age groups, may need to be considered more carefully, since these reflections often exceed the systolic peak velocity. (E-mail: poulin@ucalgary.ca)