Background: The rise in inspiratory flow is important during patient-triggered ventilation. Many ventilators incorporate a function to control the time to reach the targeted airway pressure (inspiratory rise time). However, it has not been clarified how inspiratory rise time affects inspiratory work load under various ventilator settings. In a bench study we investigated the effect of inspiratory rise time on inspiratory work load during pressure-support ventilation (PSV).
Methods: We studied 6 ICU ventilators. We measured flow and pressure at the airway opening (P(ao)) at PEEP of 5 cm H(2)O, pressure-support of 5 cm H(2)O and 10 cm H(2)O, 4 triggering sensitivities, and inspiratory drives 300 mL, 500 mL, and 700 mL. The inspiratory-rise-time setting was not consistent between the ventilators, and we chose 3 inspiratory-rise-time levels with each ventilator. The inspiratory delay time (DT) was defined as the time between the onset of inspiration and the return of P(ao) to baseline, and was divided into 2 parts at the point of the lowest P(ao): before the lowest P(ao) (DT(1)), and after the lowest P(ao) (DT(2)). As an indicator of inspiratory work load we calculated the pressure-time-product (PTP) of the P(ao) over the DT. PTP was also divided into PTP(1) and PTP(2), at the point of the lowest P(ao).
Results: Short inspiratory rise time reduced DT(2), PTP(1), and PTP(2), regardless of the pressure-support level, triggering sensitivity, or inspiratory drive. However, the inspiratory-rise-time setting did not affect DT(1). The PTP(1), PTP(2), and DT(2) values differed significantly among the ventilators. A combination of short inspiratory rise time, high PSV, and sharp triggering sensitivity resulted in the smallest PTP and DT values.
Conclusions: Short inspiratory rise time decreased inspiratory work load, regardless of the pressure-support level, triggering sensitivity, or inspiratory drive. Inspiratory work load can be maximally lowered by a combination of a short inspiratory rise time, a sharp triggering sensitivity, and a high inspiratory pressure-support level for a given patient's inspiratory effort.