Erschienen in:
01.03.2006 | Original Article
The postural reduction in middle cerebral artery blood velocity is not explained by PaCO2
verfasst von:
R. V. Immink, N. H. Secher, C. M. Roos, F. Pott, P. L. Madsen, J. J. van. Lieshout
Erschienen in:
European Journal of Applied Physiology
|
Ausgabe 5/2006
Einloggen, um Zugang zu erhalten
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.