Effect of partial spinal cord ablation on exercise hyperpnea in ponies.

We addressed the role of spinal afferent information in the exercise hyperpnea. Arterial PCO2 (PaCO2) was assessed in 10 normal ponies during low (1.8 mph 7% or 17% grade) or moderate (6 mph 7% grade) treadmill exercise. After control studies, bilateral spinal ablation (SA) of the dorsolateral sulcus and dorsolateral funiculus at L2 was performed in seven ponies. In normal ponies within the first 90 s of exercise, PaCO2 initially decreased 2.7, 4.1, and 5.2 Torr below rest at the three work loads, respectively (P less than 0.05). PaCO2 thereafter increased toward resting levels but remained 1.1, 2.9, and 4.9 Torr below rest during the steady state of exercise (P less than 0.05). One month post-SA, PaCO2 at the exercise onset decreased 1.5, 2.3, and 5.2 Torr and in the steady state was 0.7, 1.9 and 4.9 Torr below rest at the three work loads, respectively. The changes in PaCO2 from rest to exercise (delta PaCO2) were calculated for each pony in the rest to work transition and between rest and steady-state exercise and then averaged for each group of ponies. After SA, delta PaCO2's were significantly less than pre-SA only in the rest to work transition at the low work loads (P less than 0.05). No differences were found in steady-state delta PaCO2's. The modest attenuation of the exercise hypocapnia at the lower workloads in SA ponies suggests that spinal afferent information does play some role in the exercise hyperpnea in awake ponies. Our data probably underestimate this role because our SA surgery is only a partial deafferentation.