BACKGROUND: The patient's work of breathing(WOBp) during assisted ventilation may vary according to many factors including ventilatory demand of the patients and applied ventilatory setting by the physician. Pressure-controlled ventilation(PCV) which delivers gas with decelerating flow may better meet patients' demand to improve patientventilator synchrony compared with volume-controlled ventilation(VCV) with constant flow. This study was conducted to compare the difference in WOBp in two assisted modes of ventilation, PCV and VCV with constant flow. METHODS: Ten patients with respiratory failure were included in this study. Initially, the patients were placed on VCV with constant flow at low tidal volume(VT,LOW)(6-8 ml/kg) or high tidal volume(VT,HIGH)(10-12 ml/kg). After a 15 minute stabilization period, VCV with constant flow was switched to PCV and pressure was adjusted to maintain the same tidal volume(VT) received on VCV. Other ventilator settings were kept constant. Before changing the ventilatory mode, WOBp, VT, minute ventilation(VE), respiratory rate(RR), peak airway pressure (Ppeak), peak inspiratory flow rate(PIFR) and pressure-time product(PTP) were measured. RESULTS: The mean VE and RR were not different between PCV and VCV during study period. The Ppeak was significantly lower in PCV than in VCV during VT,HIGH ventilation(p<0.05). PIFR was significantly higher in PCV than in VCV at both VT (p<0.05). During VT,LOW ventilation, WOBp and PTP in PCV(0.80?0.37 J/min, 164.5?74.4 cmH2O.S) were significantly lower than in VCV(1.06+/-0.39J /min, 256.4+/-107.5 cmH2O.S)(p<0.05). During VT,HIGH ventilation, WOBp and PTP in PCV(0.33+/-0.14 J/min, 65.7+/-26.3 cmH2O.S) were also significantly lower than in VCV(0.40+/-0.14 J/min, 83.4+/-35.1 cmH2O.S)(p<0.05). CONCLUSION: During assisted ventilation, PCV with deccelerating flow was more effective in reducing WOBp than VCV with constant flow. But since individual variability was shown, further studies are needed to confirm these results.