Using Tracheal Pressure to Trigger the Ventilator and Control Airway Pressure During Continuous Positive Airway Pressure Decreases Work of Breathing

Abstract

We evaluated the difference in work of breathing (WOB) during spontaneous ventilation with continuous positive airway pressure (CPAP) among three methods of triggering the ventilator: conventional pressure triggering, tracheal pressure triggering, and flow-by triggering. In an in vitro model of the respiratory system consisting of a bellows (lungs) in a plastic canister (chest wall), spontaneous ventilation was simulated with a piston-driven pump (respiratory muscles). Data were recorded during CPAP of 5 cm H2O (model 7200ae ventilator, Puritan-Bennett, Overland Park, Kan) at peak sinusoidal inspiratory flow rate demands of 60 and 80 L/min and airway resistances of 5 and 20 cm H2O/L/s, with the demand flow system triggered by conventional pressure, tracheal pressure, or flow. Under each condition, tidal volume, pressure-time product (PTP), WOB, and changes in intrapleural pressure (Ppl) and airway pressure were recorded in real time by means of a computerized portable respiratory monitor (model CP-100, Bicore, Irvine, Calif). The Ppl was measured from within the canister, tidal volume by positioning a flow sensor between the Y-piece of the breathing circuit and the endotracheal tube (ETT), and airway pressure from a catheter attached to the flow sensor. The WOB was calculated by the monitor in real time. Changes in Ppl were greatest with conventional pressure triggering, less with flow-by triggering, and least with tracheal pressure triggering. The WOB was significantly lower (approximately 50%) with tracheal pressure triggering than with the other two methods. With tracheal pressure triggering only, an effect similar to that of pressure support ventilation (PSV) occurred, which accounted in part for the significant decrease in WOB. The PTP/breath ratio correlated strongly and was a good predictor of WOB (r2 = 0.95). Compared with conventional pressure and flow-by methods, triggering with tracheal pressure decreased WOB significantly. This method of triggering may improve patient-ventilator interaction.