The level of imposed work of breathing (WOB) is important for patient-ventilator synchrony and during weaning from mechanical ventilation. Triggering methods and the sensitivity of demand system are important determining factors of the imposed WOB. Flow triggering method is available on several modem ventilator and is believed to impose less work to a patient-triggered breath than pressure triggering method. We intended to compare the level of imposed WOB on two different methods of triggering and also at different levels of sensitivities on each triggering method (0.7 L/min vs 2.0 L/min on flow triggering ; -1 ㎝H2O vs -2 ㎝ H2O on pressure triggering).

The subjects were 12 patients (64.8α4.2 yrs) on mechanical ventilation and were stable in respiratory pattern on CPAP 3 ㎝H2O. Four different triggering sensitivities were applied at random order. For determination of imposed WOB, tracheal end pressure was measured through the monitoring lumen of Hi-Lo Jet tracheal tube (Mallincrodt, New York, USA) using pneumotachograph/pressure transducer (CP-100 pulmonary monitor, Bicore, Irvine, CA, USA). Other data of respiratory mechanics wert also obtained by CP-100 pulmonary monitor.

The imposed WOB was decreased by 37.5% during 0.7 L/min on flow triggering compared to -2 ㎝H2O on pressure triggering and also decreased by 14% during -1 ㎝H2O compared to -2 ㎝H2O on pressure triggering (p<0.05 in each). The PTP(Pressure Time Product) was also decreased significantly during 0.7 L/min on flow triggering and -I ㎝H2O on pressure triggering compared to -2 ㎝H2O on pressure triggering(p<0.05 in each). The proportions of imposed WOB in total WOB were ranged from 37% to 85% and no significant changes among different methods and sensitivities. The physiologic WOB showed no significant changes among different triggering methods and sensitivities.

To reduce the imposed WOB, flow triggering with sensitivity of 0.7 L/min would be better method than pressure triggering with sensitivity of -2 ㎝ H2O.