Total versus tube-related additional work of breathing in ventilator-dependent patients

Background: In tracheally intubated or tracheostomized spontaneously breath ing patients, tube resistance can highly increase the patient's work of bre athing. In this study we focused upon the relationship between total (WOBto t) and tube-related additional inspiratory work of breathing (WOBadd) and c ompared different ventilatory modalities for proper tube compensation. Methods: In ten tracheostomized spontaneously breathing patients we measure d WOBtot and WOBadd in the continuous positive airway pressure (CPAP) mode, under inspiratory pressure support of 5, 10, and 15 cmH(2)O in the pressur e support ventilation (PSV) mode, and under flow-adjusted pressure support in the automatic tube compensation (ATC) mode. WOBadd and WOBtot were calcu lated on the basis of measured tracheal pressure and esophageal pressure, r espectively. Inspiratory peak tracheal pressure above PEEP was taken as an estimate of pressure support beyond mere tube compensation (i.e., overcompe nsation). Results: The percentage of the tube-related WOBadd On WOBtot in the CPAP mo de was 52%. It decreased with increasing pressure support in the PSV mode f rom 32% (PSV 5 cmH(2)O) to 17% (PSV 15 cmH(2)O). WOBadd was only 15% of WOB tot in the ATC mode. In contrast to the other ventilatory modes, reduction of WOBadd in the ATC mode was achieved with the smallest amount of overcomp ensation, i.e. with minimal pressure assist beyond mere tube compensation. Conclusion: In tracheally intubated or tracheostomized spontaneously breath ing patients, adequate compensation of tube resistance (i.e, with minimal o vercompensation and minimal undercompensation) is best done by the ATC mode . (C) Acta Anaesthesiologica Scandinavica 44 (2000).