Ventilatory response to phasic contraction and passive movement in graded anesthesia

The ventilatory response to electrically induced contraction (EIC) and passive movement (PM) of hindlimb muscles at different levels of anesthesia was studied in 11 chloralose-urethan anesthetized dogs with and without rhizotomy. The level of anesthesia was assessed by corneal reflexes and measurements of the ventilatory response to hypercapnia. Muscle contraction was induced by electrically stimulating the peripheral cut ends of the sciatic and femoral nerves for 4-5 min, and PM was induced manually at the same frequency and amplitude as during EIC. In spinal intact dogs (n = 7), initial rapid increases in minute ventilation (V̇E) during EIC and PM were found in both light and deep anesthesia. Further increases in V̇E above the initial rise were seen during EIC but not PM. The initial rapid increases in V̇E did not differ between the two anesthetic levels. The steady-state ventilatory response during EIC decreased as anesthesia deepened but did not do so during PM. Rhizotomy (n = 4) abolished the initial rapid increase in V̇E during EIC and PM and the steady-state V̇E response to PM at both anesthetic levels. These results suggest that the transitional ventilatory response is neurally mediated from the muscles and not affected by the level of general anesthesia. Additionally, the anesthesia-induced reduction of ventilatory response may be due to depression of responsiveness to CO₂ rather than to the inspiratory motoneuron pathway.