Determination of Minimal Pressure Support Level During Weaning from Pressure Support Ventilation

Abstract

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