Response to hypercapnia and exercise hyperpnea in graded anesthesia

We examined the relationship between response to hypercapnia and ventilatory response to exercise under graded anesthesia in eight dogs. The response to hypercapnia was measured by the CO₂ rebreathing method under three grades of chloralose-urethan anesthesia. The degrees of response to hypercapnia (ΔV̇E/ΔPET(CO₂), l·min^-1·Torr^-1) in light (L), moderate (M), and deep (D) anesthesia were 0.40 ± 0.05 (mean ± SE), 0.24 ± 0.03, and 0.10 ± 0.02, respectively, and were significantly different from each other. Under each grade of anesthesia, exercise was performed by electrically stimulating the bilateral femoral and sciatic nerves for 4 min. The time to reach 63% of full response of the increase in ventilation (τV̇E) after beginning of exercise was 28.3 ± 1.5, 38.1 ± 5.2, and 56.0 ± 6.1 s in L, M, and D, respectively. During steady-state exercise, minute ventilation (V̇E) in L, M, and D significantly increased to 6.17 ± 0.39, 5.14 ± 0.30, and 3.41 ± 0.16 l·min^-1, from resting values of 3.93 ± 0.34, 2.97 ± 0.17, and 1.69 ± 0.14 l·min^-1, respectively, while end-tidal CO₂ tension (PET(CO₂) in L decreased significantly to 34.8 ± 0.9 from 35.7 ± 0.9, did not change in M (38.9 ± 1.1 from 38.9 ± 0.8), and increased significantly in D to 47.3 ± 1.9 from 45.1 ± 1.7 Torr. These findings indicate that reduced response to hypercapnia is accompanied by a decrease of ventilatory response to exercise as anesthesia deepens.