Objective: To examine patient-ventilator interaction during pressure support ventilation in critically ill patients when they were ventilated: (1) by a new system (Vision, Respironics) which uses the flow waveform as a method of triggering and cycling; and (2) by a new generation ventilator (Evita 4, Drager) which uses the traditional flow triggering (2 l/min) and cycling criterion (25% of peak flow).
Design: Prospective clinical and bench study.
Methods: Twelve mechanically ventilated patients were studied at three levels of pressure support, applied randomly with both ventilators. The two systems of triggering were further studied at controlled levels of dynamic hyperinflation and respiratory drive using an active lung model.
Results: Patients' breathing patterns, respiratory effort, and arterial blood gases were not affected by the type of ventilator. The flow waveform method of triggering was more sensitive to patient effort than the flow triggering, resulting in less ineffective effort but a greater number of auto-triggerings. At controlled levels of dynamic hyperinflation and inspiratory effort the simulated patient effort needed to trigger the ventilator was considerably less with the flow waveform method of triggering than that with the flow triggering. The flow waveform method of cycling resulted in mechanical breaths with similar characteristics to those that used the traditional flow criterion of breath termination.
Conclusions: We conclude that the flow waveform method of triggering improves the ventilator function and decreases the patient effort during the triggering phase. This system is highly sensitive, but under certain circumstances may be unstable.