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

In the normocapnic range, middle cerebral artery mean velocity (MCA V-mean) changes similar to 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 V-mean and the end-tidal carbon-dioxide tension (PETCO2) decrease, suggesting that PaCO2 contributes to the reduction in MCA V-mean. In a fixed body position, PETCO2 tracks changes in the PaCO2 but when assuming the upright position, cardiac output ((Q)over dot) decreases and its distribution over the lung changes, while ventilation ((V)over dot(E)) increases suggesting that PETCO2 decreases more than PaCO2. This study evaluated whether the postural reduction in PaCO2 accounts for the postural decline in MCA V-mean. From the supine to the upright position, (V)over dotE, (Q)over dot, PETCO2, PaCO2, MCA V-mean, and the near-infrared spectrophotometry determined cerebral tissue oxygenation (CO(2)Hb) were followed in seven subjects. When standing up, MCA V-mean (from 65.3 +/- 3.8 to 54.6 +/- 3.3 cm s(-1) ; mean +/- SEM; P < 0.05) and cO(2)Hb (-7.2 +/- 2.2 mu mol 1(-1) ; P < 0.05) decreased. At the same time, the (V)over dot(E)/(Q)over dot 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 V-mean seems to be explained by the reduction in PETCO2 but the small decrease in PaCO2 makes it unlikely that the postural decrease in MCA V-mean can be accounted for by the cerebral CO2 reactivity alone.