Frequency and volume thresholds for inhibition of inspiratory motor output during mechanical ventilation

Abstract

We quantified volume and frequency thresholds necessary for the inhibition of respiratory motor output during prolonged normocapnic mechanical ventilation in healthy subjects during wakefulness ( n = 7) and NREM sleep ( n = 5). Subjects were ventilated at eupneic frequency (f r) with 3 min step-wise increases in tidal volume (V t), or at eupneic V t with step-wise increases in f r, or by combinations of these two parameters. Inhibition of respiratory motor output was determined using mask pressure and, when available, esophageal pressure and diaphragmatic EMG. During wakefulness, the volume threshold (at eupneic f r) averaged 969 ± 94 ml or 1.3–1.4 times the average eupneic tidal volume; the frequency threshold (at eupneic V t was 14.1 ± 0.7 min −1 or 1.2 times the average eupneic frequency. The volume threshold was reduced when MV was provided at an f r above the eupneic value, and the frequency threshold was decreased when MV was provided at a V t above the eupneic level. During NREM sleep ( n = 5) the volume threshold for inhibition was 835 ± 108 ml or 1.4–1.5 times eupneic V t. The inhibitory thresholds for V t and f r 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 highly sensitive to changes in ventilator V t, f r and their combination.