The postural reduction in middle cerebral artery blood velocity is not explained by PaCO2

Abstract

In the normocapnic range, middle cerebral artery mean velocity (MCA Vmean) changes approximately 3.5% per mmHg carbon-dioxide tension in arterial blood (PaCO2) and a decrease in PaCO2 will reduce the cerebral blood flow by vasoconstriction (the CO2 reactivity of the brain). When standing up MCA Vmean and the end-tidal carbon-dioxide tension (PETCO2) decrease, suggesting that PaCO2 contributes to the reduction in MCA Vmean. In a fixed body position, PETCO2 tracks changes in the PaCO2 but when assuming the upright position, cardiac output (Q) decreases and its distribution over the lung changes, while ventilation (VE) increases suggesting that PETCO2 decreases more than PaCO2. This study evaluated whether the postural reduction in PaCO2 accounts for the postural decline in MCA Vmean). From the supine to the upright position, VE, Q, PETCO2, PaCO2, MCA Vmean, and the near-infrared spectrophotometry determined cerebral tissue oxygenation (CO2Hb) were followed in seven subjects. When standing up, MCA Vmean (from 65.3+/-3.8 to 54.6+/-3.3 cm s(-1) ; mean +/- SEM; P<0.05) and cO2Hb (-7.2+/-2.2 micromol l(-1) ; P<0.05) decreased. At the same time, the VE/Q ratio increased 49+/-14% (P<0.05) with the postural reduction in PETCO2 overestimating the decline in PaCO2 (-4.8+/-0.9 mmHg vs. -3.0+/-1.1 mmHg; P<0.05). When assuming the upright position, the postural decrease in MCA Vmean seems to be explained by the reduction in PETCO2 but the small decrease in PaCO2 makes it unlikely that the postural decrease in MCA Vmean can be accounted for by the cerebral CO2 reactivity alone.

Hemodynamics and brain blood flow during posture change in younger women and postmenopausal women compared with age-matched men

“…Lower PET CO 2 and MCA velocity (mean, systolic, and diastolic) were observed with sitting and standing, consistent with previous observations (25,53). This reduction of PET CO 2 could be a consequence of reduced venous return with standing, especially in light of maintained alveolar ventilation (25).…”

“…Carbon dioxide has long been known to cause vasodilation of the cerebral arteries (25,28,46,53), while cerebral autoregulation can control brain blood flow over a range of changing perfusion pressure (9, 24, 35, 57). In the present study, the transitions from supine to sitting to standing intentionally manipulated BP MCA , but simultaneous changes in PET CO 2 with postural change complicated the interpretation of the MCA velocity response.…”

“…While the effect of age or sex on cerebral blood flow has been investigated previously, this study was designed to concurrently observe both systemic and cerebral hemodynamic changes in younger and older men and women during orthostatic stress, while considering the effects of both cerebral perfusion pressure and CO 2 on brain blood flow. We investigated the effects of both cerebral perfusion pressure and end-tidal PCO 2 (PET CO 2 ), since the latter might overestimate changes in arterial PCO 2 , and the reduction of PET CO 2 with posture change might not fully explain lower brain blood flow in upright posture (25,53). It was designed to investigate both the sexually dimorphic responses to orthostatic stress and the effects of female sex hormones by comparing pre-and postmenopausal women to each other and to age-matched men.…”

Identical pattern of cerebral hypoperfusion during different types of syncope

“…Although our comparisons seem to contradict two other studies in which end-tidal PCO 2 changes overestimated arterial PCO 2 during tilt, 50,51 there are some relevant experimental differences between these studies and ours. For example, end-tidal PCO 2 and arterial PCO 2 comparisons were made at rest and during standing upright 51 and during 10 min of 851 upright tilt.…”

“…Although our comparisons seem to contradict two other studies in which end-tidal PCO 2 changes overestimated arterial PCO 2 during tilt, 50,51 there are some relevant experimental differences between these studies and ours. For example, end-tidal PCO 2 and arterial PCO 2 comparisons were made at rest and during standing upright 51 and during 10 min of 851 upright tilt. 50 Because of the lesser degree of orthostatic stress compared with that of this study, the degree of hypocapnia in these studies was rather modest (BÀ1.1 to À3.0 mm Hg) compared with this study (BÀ1.3 to À17.3 mm Hg).…”