Quantitative evaluation of pulmonary stretch receptor activity during high-frequency ventilation

The purpose of this study was to determine the neural output of pulmonary stretch receptors (PSRs) in response to conditions that, in previous studies (J. Appl. Physiol. 65: 179-186, 1988 and Respir. Physiol. 80: 307-322, 1990), produced apnea in anesthetized cats. These conditions included changes in airway pressure (Paw; 2 or 6 cmH₂O), stroke or tidal volume (1-4 ml/kg), frequency [conventional mechanical ventilation (CMV) vs. high-frequency ventilation (HFV) at 10, 15, and 20 Hz], and levels of inspired CO₂ (0, 2, and 5%). These data were needed to assess properly the specific contribution of the PSRs to the apnea found with certain combinations of the above variables. Each PSR was subjected to HFV over a range of mechanical and chemical settings, and its activity was recorded. PSRs exhibited continuous activity associated with pump stroke in 11 of 12 fibers tested. PSRs fired more rapidly when mean Paw was 6 cmH₂O [45.3 ± 0.8 (SE) impulses/s] than when it was 2 cmH₂O (31.7 ± 0.9 impulses/s, P = 0.0001). At both pressure, PSR activity increased as the volume of inflation, or tidal volume, was increased from 1 to 4 ml/kg. At Paw of 2 cmH₂O, the number of impulses per second for HFV was not different from that for CMV (averaged over the respiratory cycle), under conditions previously demonstrated as apneogenic for both modes of ventilation. Therefore the absolute amount of information being sent to the brain stem processing centers via PSRs during HFV did not differ from that during CMV. Thus any PSR contribution to HFV-induced apnea must have been the result of changes in the pattern of the signal or the central nervous system's processing of it rather than an increase in the amount of inhibitory afferent signal.