Decent data indicate that during assisted mechanical ventilation the diaphragm continues to contract after triggering the ventilator, dispelling the commonly held belief that diaphragmatic muscle contraction ceases after the machine is triggered. Variations in airway pressure and measurements of the mechanical work of breathing have been cited by some authors as evidence of continued inspiratory muscle contraction. Ayres et al showed that patients with COPD who were ventilated with intermittent positive-pres-sure breathing have lower airway pressures during assisted breaths than during controlled unassisted breaths of the same tidal volume. The same observation has been made in normal humans and in animals. Marini and associates demonstrated that patients with pulmonary disease may perform significant amounts of muscle work during assisted mechanical ventilation. These data provide strong indirect evidence for continued diaphragmatic contraction beyond triggering the ventilator; however, the duration of this contraction and the factors influencing the duration of inspiratory muscle contraction have not been well characterized during mechanical ventilation in humans with illness.
The duration of diaphragmatic contraction can be accurately determined by examination of the diaphragmatic EMG. Evidence indicates that inspiration is terminated and diaphragmatic contraction turned off in response to volume-related stretch receptor feedback.* For this reason, and in order to study the effects of delivered flow rate (volume/time), we chose to express contraction duration in terms of inspired volume. Contraction duration, therefore, is characterized by the inspired volume at peak electromyographic activity and is referred to as the volume at diaphragmatic off-switch (Vofl). This investigation was undertaken to confirm that persistent diaphragmatic contraction occurs during ventilator-assisted breaths in patients with pulmonary disease and, further, to determine if alterations in ventilator-delivered tidal volume or flow rate affect the duration of diaphragmatic contraction.