Abstract
In the normocapnic range, middle cerebral artery mean velocity (MCA V mean) changes ∼ 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 \((\dot {Q})\) decreases and its distribution over the lung changes, while ventilation \((\dot {V}_{{\rm E}})\) 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, \(\dot {V}_{{\rm E}},\) \(\dot {Q},\) PETCO2, PaCO2, MCA V mean, and the near-infrared spectrophotometry determined cerebral tissue oxygenation (CO2Hb) 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 cO2Hb (−7.2±2.2 μmol l−1 ; P<0.05) decreased. At the same time, the \(\dot {V}_{{\rm E}}/\dot {Q}\) 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 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.
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Immink, R.V., Secher, N.H., Roos, C.M. et al. The postural reduction in middle cerebral artery blood velocity is not explained by PaCO2 . Eur J Appl Physiol 96, 609–614 (2006). https://doi.org/10.1007/s00421-006-0136-6
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DOI: https://doi.org/10.1007/s00421-006-0136-6