TRACHEAL PRESSURE TRIGGERING A DEMAND-FLOW CONTINUOUS POSITIVE AIRWAYPRESSURE SYSTEM DECREASES PATIENT WORK OF BREATHING

Objectives: Triggering a ventilator ''ON'' at the carinal end of the e ndotracheal tube decreases imposed work of breathing by bypassing the resistance imposed by the breathing circuit and the endotracheal tube. We compared work of breathing during spontaneous ventilation between three methods of triggering the ventilator ''ON'': a) conventional pre ssure triggering from inside the ventilator; b) flow-by triggering; or c) tracheal pressure triggering at the carinal end of the endotrachea l tube. We hypothesized that the work of breathing would be substantia lly decreased with tracheal pressure triggering compared with conventi onal pressure and flow-by methods in patients receiving continuous pos itive airway pressure. Design: Clinical, prospective study. Setting: U niversity teaching hospital. Patients: Fourteen adults diagnosed with acute respiratory failure. Interventions: All patients were breathing spontaneously at an Flo(2) of 0.30 to 0.40 and received 5 cm H2O of co ntinuous positive airway pressure. Three different methods of triggeri ng the ventilator while set in the continuous positive airway pressure mode were administered in random order. Measurements and Main Results : Real-time measurements of esophageal pressure and tidal volume were integrated with a respiratory monitor (CP-100, Bicore, Riverside, CA) that uses the Campbell diagram to calculate total work of breathing. I mposed work of breathing was calculated by integrating tidal volume wi th the pressure at the carinal end of the endotracheal tube. Physiolog ic work of breathing was calculated by subtracting imposed work of bre athing from the total work of breathing. Breathing frequency, the inde x of rapid shallow breathing (breathing frequency/tidal volume), peak inspiratory flow rate demand, exhaled minute ventilation, and the dura tion of respiratory muscle contraction assessed by the ratio of inspir atory time to total cycle time were also measured. Data were analyzed by Friedman's repeated-measures analysis of variance on ranks. Alpha w as set at .05 for statistical significance. Imposed work of breathing decreased to approximately zero during tracheal pressure triggering. A s a result, total work of breathing decreased by similar to 40% compar ed with the flow-by and conventional methods. During tracheal pressure triggering only, airway pressure increased above baseline pressure to similar to 11 cm H2O, which resembled pressure-support ventilation. A lso, during tracheal pressure triggering, tidal volume and peak inspir atory flow rate were significantly increased, while the pressure time product and the index of rapid shallow breathing were significantly de creased. Hemodynamic status and oxygen saturation were not clinically affected. Conclusions: The tracheal pressure triggering of a demand fl ow continuous positive airway pressure system creates an effect simila r to pressure-support ventilation that significantly decreases imposed work of breathing and, thus, total work of breathing. We recommend mo ving the triggering site of the ventilator to the carinal end of the e ndotracheal tube.