Mechanical ventilator design and function: the trigger variable.

Because of the design characteristics, flow-triggering appears to offer measurable advantages over pressure-triggering, particularly during spontaneous breathing. During the trigger phase, flow-triggering provides a relatively shorter time delay than pressure-triggering. A trigger sensitivity that does not cause autocycling can be set while a short time delay is maintained. It remains to be determined whether flow-triggering has less effect on the pressure-time product than pressure-triggering. During the post-trigger phase, the relatively optimal flow delivery with flow-by results in the maintenance of airway pressure at or above the end-expiratory airway pressure level. This accounts for the lower level of inspiratory muscle work observed with flow-by over that observed with demand-flow. Whether inspiratory muscle work on a demand-flow system with optimal flow delivery will be similar to that on flow-by is not known. With a flow-by or demand-flow system, the circuit pressure-sensing site influences the flow-pressure control algorithm in the post-trigger phase only. In microprocessor-based ventilators, the shortcomings seen with pressure-triggering during the post-trigger phase can unquestionably be overcome with a better ventilator algorithm design or the application of a small amount of pressure support. However, during the trigger phase, the impact of this effort is less clear.