FREQUENCY AND VOLUME THRESHOLDS FOR INHIBITION OF INSPIRATORY MOTOR OUTPUT DURING MECHANICAL VENTILATION

We quantified volume and frequency thresholds necessary for the inhibi tion of respiratory motor output during prolonged normocapnic mechanic al ventilation in healthy subjects during wakefulness (n = 7) and NREM sleep (n = 5). Subjects were ventilated at eupneic frequency (fR) wit h 3 min step-wise increases in tidal volume (VT), or at eupneic VT wit h step-wise increases in fR, or by combinations of these two parameter s. Inhibition of respiratory motor output was determined using mask pr essure and, when available, esophageal pressure and diaphragmatic EMG. During wakefulness, the volume threshold (at eupneic fR) averaged 969 +/- 94 ml or 1.3-1.4 times the average eupneic tidal volume; the freq uency threshold (at eupneic VT was 14.1 +/- 0.7 min(-1) or 1.2 times t he average eupneic frequency. The volume threshold was reduced when MV was provided at an fR above the eupneic value, and the frequency thre shold was decreased when MV was provided at a VT above the eupneic lev el. During NREM sleep (n = 5) the volume threshold for inhibition was 835 +/- 108 ml or 1.4-1.5 times eupneic VT. The inhibitory thresholds for VT and fR were reproducible upon repeat trials within subjects. We conclude that inhibition of respiratory motor output during prolonged normocapnic mechanical ventilation in wakefulness or NREM sleep is hi ghly sensitive to changes in ventilator VT, fR and their combination.