Airway pressure curve: a good tool to measure neural inspiratory time?

Considerable asynchrony during pressure-support ventilation has been reported. While the beginning of active inspiration is usually identifiable in the airway pressure (Paw) curve (the inspiratory trigger), there is still a need for accurate, non-invasive methods to identify the end of inspiration. To test the hypothesis that inspiration, particularly the end of inspiration, can be estimated from the Paw curve, we compared indirect measurements based on Paw with simultaneous direct electromyography of the diaphragm (EMGdi).

We prospectively studied 10 patients during the weaning period after cardiac surgery. Inspiratory pressure support was set at 20, 15, 10, and 5 cm H(2)O; 25 respiratory cycles were analyzed at each pressure level. Recording of the electromyogram was obtained with electrodes inserted into the diaphragm during surgery.

The start and end of inspiration were identified in the Paw curve in 99% and 98% of the 1000 cycles analyzed, respectively, and were coincident with the electromyogram in 62% and 53% of the cycles, respectively. The inspiratory time estimated from the Paw curve was well correlated (r=0.94, P<0.0001) with the electromyogram.

The end of neural inspiration (EMGdi) can be easily and with little error recognized from the Paw curve alone in patients with normal ventilatory mechanics who receive pressure-support ventilation.