BACKGROUND: Patient-ventilator dyssynchrony due to an unresponsive ventilator demand-flow system can result in fatigue and failure to wean from mechanical ventilation. PURPOSE: We sought to evaluate the response times (Tr) of several ventilators to weak inspiratory efforts at various pressure support (PS) ventilation levels and to gauge the effects of bias flow (BF, intended to be used as a system leak compensator and to reduce the subjective sense of delay in breathing against a closed system) on Tr when sensitivity is maximized. METHODS: Flow-triggering (FT) was evaluated in the Siemens SV300 and Puritan-Bennett 7200ae (7200FT), and pressure-triggering (PT) was evaluated in the Newport E200, Bird VIP, and Puritan-Bennett 7200ae (7200PT). PS was used to eliminate target pressure variables that would favor some ventilators and have a negative impact on others. Patient efforts were simulated by using a ventilator to drive one compartment of a simulator, adjusted to produce inspiratory efforts of 2 L/min, 5 L/min, or 25 L/min in the dependent compartment. Tr began when pressure rose in the drive compartment and ended when pressure in the patient compartment returned to baseline. RESULTS: The E200 was quicker to respond than other ventilators for all endotracheal tube (ETT) sizes. VIP exhibited marked difficulty in triggering at 2 L/min inspiratory flow. VIP's response time was 40-75 ms slower than the E200. SV300 was 10-48 ms slower than E200. 7200PT was faster than 7200FT. BF had a significant impact on some ETT-PS combinations. PS at 3 cm H2O and 2.5 mm ETT resulted in a 30 ms increase in Tr. CONCLUSIONS: Results indicate BF levels should be tailored to individual patient requirements, particularly in neonatal and pediatric applications. We conclude that when a combination of BF, continuously adjustable sensitivity control, and proximal pressure monitoring are used FT provides no measurable advantage over PT. The site and method of measurement as well as the interaction of demand system components determines the total response to inspiratory efforts.
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