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Arterial CO2 response to low levels of inspired CO2 in awake beagle dogs.

We have previously shown, using a repeated-measures experimental design, that 1% inspired CO2 with a partial pressure of 7 Torr at sea level results in an increased end-tidal CO2 pressure (PETCO2) in awake beagle dogs (22), suggesting hypercapnia rather than the isocapnia found by some investigators in human and nonhuman subjects. Because PETCO2 may, not equal arterial PCO2 (PaCO2), we examined the steady-state PaCO2 during air and 1% CO2 inhalation periods in three awake beagle dogs having exteriorized carotid arterial loops and an intact airway, and breathing through a low dead-space respiratory mask. Six low-level CO2 inhalation experiments were performed in three dogs with two experimental sequences in each dog on separate days. An experimental session consisted of alternating control and CO2 inhalation states for up to 10 low-CO2 and 11 control conditions, resulting in a total of 75 control and 57 low-level CO2 inhalation observations. Ten minutes were allowed to reach steady state in each condition. Blood samples (1.5 ml) drawn anaerobically over a 0.5-min time period from an indwelling arterial catheter were immediately analyzed with a radiometer blood gas system. The 1% inhaled CO2 produced a significant increase of 0.88 Torr in the steady-state PaCO2, compared with bracketing controls (t = 5.82, P less than 0.05, df = 2). We conclude that 1% inhaled CO2 results in hypercapnia detectable by a repeated-measures experimental design.