Effects of controlled mechanical ventilation on diaphragm and inspiratory neuronal activity in the anaesthetised rat

Mechanical ventilation is applied in patients to decrease the mechanical load of respiratory muscles. Apnea is a phenomenon often observed postmechanical ventilation. The mechanism of post-mechanical ventilation apnea is unclear, although 'inertia' has been a suggested mechanism. Our work (i) examined the effects of controlled mechanical ventilation on diaphragm activity in the rat, and (ii) document the response of inspiratory neurons in the nucleus tractus solitarius (NTS) to mechanical ventilation. Changes in diaphragmatic electromyogram and activity of the inspiratory neurones located in the caudal medullary NTS were studied during controlled mechanical ventilation at varying volumes and frequencies. Nembutal anaesthetised (70 mg/kg, ip followed by intravenous supplementation) spontaneously breathing rats (n=6) of either sex weighing 300-400 gm were used for the study. Mechanical ventilation at volumes twice greater than the tidal volume and a frequency 1.5-2 times the spontaneous breathing frequency (representing the threshold levels) caused inhibition of the diaphragm. Inspiratory neuronal activity was inhibited in response to the combined threshold levels of mechanical volume and frequency. Cessation of diaphragm and neuronal activity persisted for a period of time after the mechanical ventilator was removed from the animal. Apnea was never seen when only the volume or the frequency of the ventilator was singularly manipulated. Inhibition of neuronal activity was seen prior to the inhibition of the diaphragm activity. Return of neuronal activity always preceded that of diaphragm activity. The longer the period of apnea during mechanical ventilation, the longer the post mechanical ventilatory apnea was seen. These findings suggest that 1. Both volume and frequency components of ventilation are essential for the generation of apnea during controlled mechanical ventilation. 2. A threshold in the volume and frequency is pertinent in generating apnea during mechanical ventilation 3. The inspiratory neurons located in the caudal medullary solitary nucleus mediate the apnea produced.