Journal of Applied Physiology, Vol 58, Issue 5 1654-1658, Copyright © 1985 by American Physiological Society
Arterial-alveolar CO2 equilibration in exercising dogs during prolonged rebreathing
J. A. Loeppky, P. Scotto, H. Rieke, M. Meyer and J. Piiper
Arterial-alveolar equilibration of CO2 during exercise was studied by
normoxic CO2 rebreathing in six dogs prepared with a chronic tracheostomy
and exteriorized carotid loop and trained to run on a treadmill. In 153
simultaneous measurements of PCO2 in arterial blood (PaCO2) and end-tidal
gas (PE'CO2) obtained in 46 rebreathing periods at three levels of
mild-to-moderate steady-state exercise, the mean PCO2 difference
(PaCO2-PE'CO2) was -1.0 +/- 1.0 (SD) Torr and was not related to O2 uptake
or to the level of PaCO2 (30-68 Torr). The small negative PaCO2-PE'CO2 is
attributed to the lung-to-carotid artery transit time delay which must be
taken into account when both PaCO2 and PE'CO2 are continuously rising
during rebreathing (average rate 0.22 Torr/s). Assuming that blood-gas
equilibrium for CO2 was complete, a lung-to-carotid artery circulation time
of 4.6 s accounts for the observed uncorrected PaCO2-PE'CO2 of -1.0 Torr.
The results are interpreted to indicate that in rebreathing equilibrium
PCO2 in arterial blood and alveolar gas are essentially identical. This
conclusion is at variance with previous studies in exercising humans during
rebreathing but is in full agreement with our recent findings in resting
dogs.
