DECREASING IMPOSED WORK OF THE BREATHING APPARATUS TO ZERO USING PRESSURE-SUPPORT VENTILATION

Objectives: To apply pressure-support ventilation with the goal of decreasing the imposed work of the breathing apparatus (endotracheal tube, breathing circuit tubing, and the ventilator's demand-flow system) to zero and to evaluate a clinical method of measuring the imposed work of breathing. Design: A prospective evaluation of adult and pediatric patients receiving mechanical ventilatory support. Setting: A surgical and a pediatric intensive care unit in a university hospital. Patients: Fifteen patients (11 adult and four pediatric), who were diagnosed with acute respiratory failure from various etiologies, and who were intubated and spontaneously breathing, received continuous positive airway pressure and pressure-support ventilation. Measurements and Main Results: Imposed work of the breathing apparatus was calculated by integrating pressure measured at the tracheal end of the endotracheal tube from a narrow air-filled catheter and the change in volume from a miniature pneumotachograph (flow sensor) positioned between the ''Y'' piece of the breathing circuit and the endotracheal tube. Pressure and volume signals were directed to a computerized, portable respiratory monitor (Bicore Monitoring Systems) that provides real-time display of the pressure-volume (work) loops and calculation of the imposed work. Imposed work was measured at 0 cm H2O pressure-support ventilation, and then incremental levels of pressure-support ventilation were applied until the imposed work decreased to zero. Imposed work decreased in a quadratic fashion after incremental levels of pressure-support ventilation (r = -.83 [r2 = .691; p <.001). At pressure-support ventilation level of 0 cm H2O, the imposed work was 0.60 +/- 0.17 joule/L. At mean pressure-support ventilation levels of 13.5 +/- 4.8 cm H2O, imposed work decreased to 0 joule/L. Conclusions. Ideally, the imposed work of the breathing apparatus should be zero to decrease the afterload on the ventilatory muscles and, thus, the patient's work of breathing. Eliminating the imposed work is achieved using appropriate levels of pressure-support ventilation. We describe an easily applied, practical method of measuring imposed work using a commercially available, portable, bedside respiratory monitor. We recommend that all patients diagnosed with respiratory failure and compromised pulmonary mechanics and who are intubated and breathing spontaneously, receive at least a minimal level of pressure-support ventilation that results in zero breathing apparatus-imposed work of breathing.