Use of the rapid/shallow breathing index as an indicator of patient work of breathing during pressure support ventilation

Background. Measuring patient work of breathing (WOBpt) has been suggested to provide safe, aggressive weaning from mechanical ventilation. We compared WOBpt and pressure-time-product (PTP) to routine weaning parameters [breath rate (f), tidal volume (V-T), frequency/tidal volume ratio (f/V-T)] at different levels of pressure support ventilation (PSV). Methods. Fifteen patients in the surgical intensive care unit requiring prolonged weaning (more than 3 days) were entered in the study. A balloon-tipped esophageal catheter was placed and position confirmed by inspection of pressure and flow waveforms. Each patient was randomly assigned to breathe with 5, 10, 15, and 20 cm H2O of PSV. After 30 minutes, 40 breaths were recorded and analyzed. Measurement of WOBpt, PTP, f, V-T, and f/V-T were made using the Bicore CP-100 monitor. Mean volues for each parameter were calculated. PTP and WOBpt were plotted against f/V-T to determine correlation coefficient. Results. PTP, WOBpt, and f/V-T decreased in a stepwise fashion as PSV was increased. The f/V-T correlated most closely with WOBpt (r = 0.983) and PTP (r = 0.972). Monitoring f alone also correlated with WOBpt (r = 0.894) and PTP (r = 0.881). All patients were weaned from the ventilator (mean duration, 22 +/- 5.9 days). Nine patients required tracheostomy before liberation from the ventilator. Conclusions. Direct measurement of WOBpt is invasive, expensive, and may be confusing to clinicians. Monitoring f/V-T may be useful when changing PSV during weaning.