Relationship between work of breathing provided by a ventilator and patients' inspiratory drive during pressure support ventilation; Effects of inspiratory rise time

Inspiratory drive and work of breathing provided by a ventilator (WOBv) dur ing pressure support ventilation (PSV) were examined in 15 patients. At PSV 10 and 15 cm H2O during CPAP 5 cm H2O patients with low P-0.1 (<4.2cm H2O, n = 9) showed WOBv 0.57 and 0.92 J/l, those with high P-0.1 (>4.2cm H2O, n = 6) showed 0.31 and 0.62 J/l respectively,. WOBv was smaller and pressure -time product of oesophageal pressure (PTP) was significantly larger in hig h P-0.1 patients. Peak inspiratory flow for low P0.1 patients increased as PSV level increased but high P0.1 patients showed no significant change. In a lung model, effects of inspiratory rise time (IRT) and PSV were studied at high and low inspiratory drives by using ventilators with (Servo 300) an d without (Mallinckrodt 7200a) adjustable IRT With 7200a, PSV 10 cm H2O dur ing low drive was compared with PSV 10 and 15 cm H2O during high drive. In Servo 300, PSV 10 cm H2O (IRT 0.6 and 0.0 see) during low drive was compare d with PSV 10 cm H2O (IRT 0.6 and 0.0 see) and PSV 15 cm H2O (IRT 0.6 sec) during high drive. Raising PSV and shortening IRT both increased peak inspi ratory flow. Initial inspiratory flow increased in inverse proportion to IR T, but higher PSV had a little effect. WOBv with high drive was less than w ith low drive. Higher PSV preserved WOBv by increasing tidal volume. Shorte ning IRT recruited WOBv by increasing initial inspiratory flow without chan ging airway pressure and tidal volume. Compared with higher PSV, shorter IR T reduced PTP more. In conclusion, WOBv decreased as inspiratory drive incr eased due to inability to increase inspiratory flow. Increasing initial ins piratory flow was more effective than raising PSV to preserve inspiratory a ssistance of PSV at high inspiratory drive.