Pulmonary stretch receptor discharge patterns in eupnea, hypercapnia, and hypoxia.

The discharge properties of pulmonary stretch receptors (PSR) were studied in spontaneously breathing, pentobarbital sodium-anesthetized cats. During eupneic breathing, 105 of 116 PSR (both tonically and phasically active) were recruited in the first third of inspiration; none were recruited in the last third. Linear equations adequately expressed the relation between instantaneous discharge frequency and inspired volume in eupnea. During CO2 rebreathing, both tidal volume and peak PSR discharge frequency were inversely related to inspiratory duration. At fixed volumes less than 40 ml above functional residual capacity, instantaneous PSR discharge frequency either did not change or decreased with increases in flow. Above 40 ml, increases in discharge frequency accompanied increases in flow (0.033 spikes/s per ml/s). During progressive hypocapnic hypoxia, discharge frequency increased, on average, at all volumes with increases in flow (0.206 spikes/s per ml/s). During both conditions, as with eupnea, increases in frequency were linearly related to increments in tidal volume. Therefore, tidal volume alone can be used to estimate PSR feedback to the respiratory centers, provided that its instantaneous value is appropriately scaled to account for the different effects of CO2 and hypocapnic hypoxia on PSR discharge.