Apnoea following normocapnic mechanical ventilation in awake mammals: a demonstration of control system inertia.

1. Inhibition of inspiratory muscle activity from volume-related feedback during mechanical ventilation has been shown previously. To determine if this neuromechanical inhibition displays a memory effect, the duration of expiration immediately following cessation of mechanical ventilation was assessed in eight normal subjects. The subjects were passively mechanically ventilated via a nasal mask until the end-tidal CO2 (PET,CO2) was a minimum of 30 mmHg and inspiratory effort was no longer detected, as evidenced by stabilization of mouth pressure and disappearance of surface diaphragm EMG activity. The ventilator output was held constant at a mean tidal volume (VT) of 1.0 l and breath duration of 4.6 s and PET,CO2 was increased 1-1.5 mmHg/min (via increased inspired CO2 fraction, FI,CO2) until inspiratory muscle activity returned. The PET,CO2 at which activation first occurred was defined as the CO2 recruitment threshold (PCO2,RT). The mechanical ventilation protocol was repeated and the PET,CO2 increased 1-1.5 mmHg/min until it was a mean of 1.1 mmHg above spontaneous PET,CO2 and 3.6 mmHg below PCO2,RT. After 4-6 min of mildly hypercapnic mechanical ventilation, the mechanical ventilation was terminated. 2. Following termination of mechanical ventilation, the duration of the subsequent apnoea was 14.6 +/- 2.8 s (mean +/- S.E.M.) or 453 +/- 123% > spontaneous TE and 178 +/- 62% > the TE chosen by the subject during 'assist control' ventilation at VT = 1.0 l. 3. To test the hypothesis that the apnoea following cessation of mechanical ventilation was due to a vagally mediated memory effect, the study was repeated in five double-lung transplant patients with similar PCO2,RT to normal subjects. These pulmonary vagally denervated patients also displayed an apnoea (14.5 +/- 4.0 s) upon cessation of mechanical ventilation (at a PET,CO2 2.0 mmHg > eupnoea and 2.4 mmHg < PCO2,RT), that was 367 +/- 162% > spontaneous TE. 4. We also found significant apnoea in the awake dog immediately following mildly hypercapnic passive mechanical ventilation, and this was similar before and after bilateral vagal blockade (15.7 +/- 1.3 and 19.7 +/- 4.7 s, respectively). 5. We conclude that neuromechanical inhibition of inspiratory muscle activity, produced by passive mechanical ventilation at high VT, exhibits a memory effect reflected in TE prolongation, which persists in the face of substantial increases in chemoreceptor stimuli. This effect is not dependent on vagal feedback from lung receptors. 6. We hypothesize that this persistent apnoea represents an inherent 'inertia', characteristic of the ventilatory control system.(ABSTRACT TRUNCATED AT 400 WORDS)