Reversal of arterial-to-expired CO2 partial pressure differences during rebreathing in goats

Whether CO2 partial pressure (PCO2) in expired gas may exceed that in arterial blood has been controversial. We measured arterial PCO2 (Paco2) and end-tidal PCO2 (PETco2) in four awake goats during air breathing and during hyperoxic CO2 rebreathing in various conditions of acid-base balance. During air breathing, Paco2 was slightly higher than PETco2; i.e., the mean (+/- SE) difference, Paco2 - PETco2, was positive by + 2.36 +/- 0.53 Torr (P less than 0.001). In contrast, during CO2 rebreathing with the same techniques of measurement, this difference was always negative (mean +/- SE = -11.63 +/- 0.22 Torr, P less than 0.001), and it widened as Paco2 increased with rebreathing. Magnitude of the negative difference during rebreathing was too great to be accounted for by incorrect assumptions or measurement error, even if reasonable contributions from all known sources of error were concurrently invoked. We conclude that during hyperoxic CO2 rebreathing in goats, PETco2 exceeds Paco2.