Elevated imposed work of breathing masquerading as ventilator weaning intolerance

Abstract

Objective: To test the hypothesis that, if apparent ventilatory insufficiency observed during a weaning or preextubation trial is due to a significant contribution of imposed work of the endotracheal tube and breathing apparatus (WOBimp), and the patient's actual physiologic work of breathing (WOBPhys) is not excessive, it should be possible to extubate these patients safely. Design: Prospective descriptive study. Setting: University hospital trauma intensive care unit. Patients: A total of 28 (17% of all ventilated patients) adults intubated for 48 h or longer, who developed tachypnea (40±9 breaths/min) but whose blood gas exchange tact predefined extubation criteria, were evaluated over a 3-month period. Interventions: Using a microprocessor-based monitor (Bicore Monitoring Systems Inc, Irvine, Calif) total patient work of breathing (WOBTOT) was determined by integrating the change in intraesophageal pressure with tidal volume measured with a miniature pneumotachograph positioned at the airway opening. If the patient's WOBTOT was equal to or greater than 0.8 J/L, WOBImp was determined by integrating the changes in carinal pressures with tidal volume. If neither the patient's WOBTOT or WOBPhys was excessively greater than that of spontaneous breathing at rest (ie, <0.8 J/L: normal range, 0.5 to 0.6 J/L), the patient was extubated. Measurements and results: Breathing frequency, peak inspiratory flow rate (PIFR), auto-Peep (PEEPa), dynamic compliance (CDXN) WOBTOT, WOBImp, resistance to expiratory airway flow (RAWE) were measured, and WOBPhys calculated (WOBTOT minus WOBImp). The means and SDs were calculated, and data were analyzed by unpaired t test and linear regression. Six patients (5%) were found to have WOBTOT of <0.8 J/L and were successfully extubated without determination of WOBImp. Twenty-one patients were found to have an elevated WOBTOT (1.6±0.83 J/L), and had WOBImp measured. In these 21 patients, WOBImp (1.1±0.64 J/L) was twice WOBPhys (0.5±0.26 J/L). Extubation was successful in 20 of 21 patients in which WOBPhys was determined not to be excessive (ie, <0.8 J/L). The last patient had an elevated WOBPhys (1.4 J/L) and was not extubated until his disease improved later. Overall, reintubation rate was 4%. Conclusion: Increased WOBTOT may be misinterpreted as a patient failure (ie, tachypnea) and weaning halted or extubation not done, prolonging intubation. The ability to measure the contribution of WOBImp to WOBTOT can identify those patients who may be safely extubated when WOBPhys (WOBTOT minus WOBImp) is acceptable and the apparent ventilatory insufficiency is related to significant WOBImp.