INCREASED INITIAL FLOW-RATE REDUCES INSPIRATORY WORK OF BREATHING DURING PRESSURE SUPPORT VENTILATION IN PATIENTS WITH EXACERBATION OF CHRONIC OBSTRUCTIVE PULMONARY-DISEASE

Objective: To investigate whether the level of initial flow rate alter s the work of breathing in chronic obstructive pulmonary disease (COPD ) patients ventilated in pressure support ventilation (PSV). Design. P rospective study. Settings. Medical ICU in University hospital. Patien ts: Eleven intubated COPD patients. Methods: We modulated the initial flow rate in order to achieve seven different sequences. In each seque nce, the plateau pressure was reached within a predetermined time: 0.1 , 0.25, 0.50, 0.75, 1, 1.25 or 1.50 s. The more rapidly the pressure p lateau was achieved, the higher was the initial flow rate. In each pat ient, the pressure support level was an invariable parameter. The orde r of the seven sequences for each patient was determined randomly. Mea surements and results: Ten minutes after application of each initial f low rate, we measured the following parameters: inspiratory work of br eathing, electromyogram (EMG) of the diaphragm (EMGdi), breathing patt ern, and intrinsic positive end-expiratory pressure (PEEPi). Compariso n between the means for each sequence and each variable measured was p erformed by two-way analysis of variance with internal comparisons bet ween sequences by Duncan's test. The reduction of the initial flow rat e induced a progressive increase in the values of the work of breathin g, EMGdi, and mouth occlusion pressure (P 0.1). In contrast, the reduc tion of the initial flow rate did not induce any significant change in tidal volume, respiratory frequency or PEEPi. Conclusion: As the obje ctive of PSV is to reduce the work of breathing, it seems logical to u se the highest initial flow rate to induce the lowest possible work of breathing in COPD ventilated patients.