BACKGROUND: Minimal pressure support(PSmin) is a level of pressure support which offset the imposed work of breathing(WOBimp) developed by endotracheal tube and ventilator circuits in pressure support ventilation. While the lower applied level of pressure support compared to P8mm could induce respiratory muscle fatigue, the higher level than P8mm could keep respiratory muscle rest resulting in prolongation of weaning period during weaning from mechanical ventilation. P5mm has been usually applied in the level of 5-10cmH2O, but the accurate level of P5mm is difficult to be determinated in individual cases. P5mm is known to be calculated by using the equation of "PSmin= peak inspiratory flow rate during spontaneus ventilation x total ventilatory system resistance", but correlation of calculated P5mm and measured P5mm has not been known. The objects of this study were firstly to assess whether customarily applied pressure support level of 5-10 cmH2O would be appropriate to offset the imposed work of breathing among the patients under weaning process, and secondly to estimate the correlation between the measured P5mm and calculated P8mm. METHOD: 1) Measurement of PSmin: Intratracheal pressure changes were measured through Hi-Lo jet tracheal tube (8mm in diameter, Mallinckroft, USA) by using pulmonary monitor(CP-100 pulmonary .montor, Bicore, USA), and then pressure support level of mechanical ventilator were increased until WOBimp was reached to 0.01 J/L or less. Measured P5mm was defined as the lowest pressure to make WOBimp 0.01 J/L or less. 2) Calculation of PSmin: Peak airway pressure(Ppeak), plateau airway pressure(Pplat) and mean inspiratory flow rate of the subjects were measured on volume control mode of mechanical ventilation after sedation. Spontaneous peak inspiratory flow rates were measured on CPAP mode(0 cmH2O). Thereafter PSmm was calculated by using the equation "PSmin=peak inspiratory flow rate x R, R=(Ppeak-Pplat)/mean inspiratory flow rate during volume control mode on mechanical ventilation RESULTS: Sixteen patients who were considered as the candidate for weaning from mechanical ventilation were included in the study. Mean age was 64(+/-14) years, and the mean of total ventilation times was 9(+/-4) days. All patients except one were males. The measured PSmm of the subjects ranged 4.0-12.5cmH2O in 14 patients. The mean level of PSmm was 7.6(+/-2.5 cmH2O) in measured PSmm, 8.6(+/-3.25 cmH2O) in calculated PSmm. Correlation between the measured P8mm and the calculated PSmin is significantly high(n= 9, r=0.88, p=0.002). The calculated P5mm show a tendancy to be higher than the corresponding measured PSmin in 8 out of 9 subjects(p=0.09). The ratio of measured P5mm/calculated PSmin was 0.81(+/-0.05). CONCLUSION: Minimal pressure support levels were different in individual cases in the range from 4 to 12.5 cmH2O. Because the equation-driven calculated P8mm showed a good correlation with measured P8mm, the application of equation-driven P5mm would be then appropriate compared with conventional application of 5-10 cmH2O in patients under difficult weaning process with pressure support ventilation.