Breathing pattern and diaphragm EMG after SO2 in rabbit intra- or extrathoracic airways

In anesthetized rabbits under SO2 block of slowly adapting stretch receptors (SAR) in intrathoracic airways TI increased by 49.7 +/- 6.9% and VT by 19.5 +/- 3.7%. TE decreased for small increases of TI and vice versa. Slope of moving average of diaphragm EMG (M.A.) decreased by 16.7 +/- 4.1% from 0 to 30% TI control, and increased by 26.3 +/- 8.1% from 30 to 70%. Considering that discharge from intrathoracic SAR increases during inspiration, these changes of M.A. indicate that their input facilitates inspiration at its beginning, but inhibits it later on, in line with previous conclusions based on SAR block which included 2/3 of extrathoracic trachea (ETT). Neither SO2 nor pressures from -40 to +40 cm H2O in ETT changed breathing pattern or M.A.. Hence, SAR of ETT do not affect parameters studied in rabbits. When extrathoracic airways were exposed to SO2 TI increased by 30.5 +/- 8.2%, TE by 37.9 +/- 5.1%, VT did not change. Hence, some receptors rostral to trachea, involved in respiratory control, are affected by SO2.