To assess whether adjustments in the initial flow rate or breath termination criteria affected patient-ventilator synchrony, we studied the ventilatory pattern response to PS in 33 patients under two sets of circumstances: during seven different levels of delivered initial PS flow and during PS termination at 50 percent and at 25 percent of peak flow. In the study on initial PS flow, we found the following: (a) an optimal initial PS flow could be defined for a given level of PS that resulted in the patient obtaining maximal pressure and volume from the ventilator; (b) initial PS flows above and below this optimal flow were associated with faster breathing frequencies, shorter inspiratory times, smaller tidal volumes and a tendency for airway pressure to not reach the selected PS level; and (c) optimal initial PS flow was fastest in patients with the lowest compliances and the most active ventilatory drives. Changing PS termination criteria from 50 to 25 percent of peak flow had minimal effects on the ventilatory pattern or synchrony. We conclude that the initial PS flow to achieve the selected PS level is important in patient-ventilator synchrony but that termination criteria set between 25 and 50 percent of peak flow is not.