Arterial to end-tidal CO2 gradients during spontaneous breathing, intermittent positive-pressure ventilation and jet ventilation

Arterial to end-tidal CO2 tension gradients were measured in 18 dogs during spontaneous breathing (SB), intermittent positive-pressure ventilation (IPPV), and both low-frequency and high-frequency jet ventilation (LFJV and HFJV). The dogs were anesthetized with nembutal and permitted to breathe spontaneously through an 8-mm internal diameter endotracheal tube; blood gas tensions, cardiac output, and end-tidal CO2 partial pressure (Petco2) were measured. IPPV, LFJV, and HFJV were then instituted in a random sequence and measurements repeated. Pao2, Paco2 and cardiac output were similar during all four ventilatory modes. The mean Paco2 differed significantly (p< .001) from Petco2 during IPPV, LFJV, and HFJV but not during SB. The mean Paco2-Petco2 gradient was 3.7 ± 1 (sd), 12.6 ± 5.0, and 24.3 ± 8 torr during IPPV, LFJV and HFJV, respectively. The large gradients during LFJV and HFJV were not produced by dilution of tracheal CO2 by entrained air or by oxygen delivered by the jet. These results suggest that both LFJV and HFJV may be associated with a large Paco2-Petco2 gradient.