Respiratory control during volume-cycled ventilation in normal humans

The purpose of this study was to evaluate the extent of inhibition to respiratory rhythm associated with high volumes of ventilation during volume-cycled mechanical ventilation (neuromechanical inhibition). Two approaches were used. 1) In 18 normal awake subjects, ventilator tidal volume (VT) in the assist/control mode (A/C) was increased in steps from the minimum tolerable level up to 80% of the subject's inspiratory capacity or ventilator's maximum VT. We looked for appearance of intermittent apnea or a reduction in spontaneous rate (f). 2) Another 18 normal awake subjects were placed on controlled mechanical ventilation (CMV). When apnea was established, we abruptly terminated CMV and measured the time before the appearance of the next spontaneous effort. In the assist mode (protocol 1), we did not observe intermittent apnea because VT was increased from [from 944 +/- 198 to 1,867 +/- 277 (SD) ml], and there was only a modest reduction in f (14.1 +/- 3.9 to 12.4 +/- 4.0 breaths/min). End-tidal PCO2 (PETCO2) decreased precipitously as VT was increased. In protocol 2, we did not observe apnea after discontinuation of CMV in any subject. Total breath duration of the first breath after discontinuation did not differ significantly from total breath duration during A/C in the same subjects (4.84 +/- 2.2 vs. 5.2 +/- 2.0 s). This similarly applied regardless of route of breathing (nose vs. mouth) or PETCO2 level at time of discontinuation. We conclude that neuromechanical inhibition is quite weak and provides very little negative feedback that may help control PCO2 in the face of excessive VT and f demands of the subject.