Effects of Inspiratory Rise Time on Triggering Work Load During Pressure-Support Ventilation: A Lung Model Study

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 H₂O, pressure-support of 5 cm H₂O and 10 cm H₂O, 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 (DT₁), and after the lowest P_ao (DT₂). 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 PTP₁ and PTP₂, at the point of the lowest P_ao.

RESULTS: Short inspiratory rise time reduced DT₂, PTP₁, and PTP₂, regardless of the pressure-support level, triggering sensitivity, or inspiratory drive. However, the inspiratory-rise-time setting did not affect DT₁. The PTP₁, PTP₂, and DT₂ 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.