Transient influence of end-tidal carbon dioxide tension on the postural restraint in cerebral perfusion

Abstract

In the upright position, cerebral blood flow is reduced, maybe because arterial carbon dioxide partial pressure (PaCO2) decreases. We evaluated the time-dependent influence of a reduction in PaCO2, as indicated by the end-tidal Pco2 tension (PetCO2), on cerebral perfusion during head-up tilt. Mean arterial pressure, cardiac output, middle cerebral artery mean flow velocity (MCA Vmean), and dynamic cerebral autoregulation at supine rest and 70° head-up tilt were determined during free breathing and with PetCO2 clamped to the supine level. The postural changes in central hemodynamic variables were equivalent, and the cerebrovascular autoregulatory capacity was not significantly affected by tilt or by clamping PetCO2. In the first minute of tilt, the decline in MCA Vmean (10 ± 4 vs. 3 ± 4 cm/s; mean ± SE; P < 0.05) and PetCO2 (6.8 ± 4.3 vs. 1.7 ± 1.6 Torr; P < 0.05) was larger during spontaneous breathing than during isocapnic tilt. However, after 2 min in the head-up position, the reduction in MCA Vmean was similar (7 ± 5 vs. 6 ± 3 cm/s), although the spontaneous decline in PetCO2 was maintained ( P < 0.05 vs. isocapnic tilt). These results suggest that the potential contribution of PaCO2 to the postural reduction in MCA Vmean is transient, leaving the mechanisms for the sustained restrain in MCA Vmean to be identified.