Tracheal constrictor drive above the apneic threshold in anesthetized dogs

1 Department of Anaesthesia, Mount Sinai Hospital, University of Toronto, Toronto, M5G 1X5; and 2 Department of Physiology, Queen's University, Kingston, Ontario, Canada K7L 3N6 We have previously shown that raising arterial P CO 2 (Pa CO 2 ) by small increments in dogs ventilated below the apneic threshold (AT) results in almost complete tracheal constriction before the return of phrenic activity (Dickstein JA, Greenberg A, Kruger J, Robicsek A, Silverman J, Sommer L, Sommer D, Volgyesi G, Iscoe S, and Fisher JA. J Appl Physiol 81: 1844-1849, 1996). We hypothesized that, if increasing chemical drive above the AT mediates increasing constrictor drive to tracheal smooth muscle, then pulmonary slowly adapting receptor input should elicit more tracheal dilation below the AT than above. In six methohexital sodium-anesthetized, paralyzed, and ventilated dogs, we measured changes in tracheal diameter in response to step increases in tidal volume (V T ) or respiratory frequency (f) below and above the AT at constant Pa CO 2 (~40 and 67 Torr, respectively). Increases in V T (400-1,200 ml) caused significantly more ( P = 0.005) tracheal dilation below than above AT (7.0 ± 2.2 vs. 2.8 ± 1.0 mm, respectively). In contrast, increases in f (14-22 breaths/min) caused similar ( P = 0.93) tracheal dilations below and above (1.0 ± 1.3 and 1.0 ± 0.8 mm, respectively) AT. The greater effectiveness of dilator stimuli below compared with above the AT is consistent with the hypothesis that drive to tracheal smooth muscle increases even after attainment of maximal constriction. Our results emphasize the importance of controlling P CO 2 with respect to the AT when tracheal smooth muscle tone is experimentally altered. slowly adapting receptors; apnea; airway smooth muscle tone; respiratory drive; tone; upper airway