SITE OF PRESSURE MEASUREMENT DURING SPONTANEOUS BREATHING WITH CONTINUOUS POSITIVE AIRWAY PRESSURE - EFFECT ON CALCULATING IMPOSED WORK OF BREATHING

Objective: To describe the importance of measuring pressure at the tracheal end of the endotracheal tube during spontaneous breathing with continuous positive airway pressure in order to correctly assess: a) the changes in airway pressure and b) the work imposed by the breathing apparatus. Design: Multitrial tests under simulated clinical conditions using a mechanical lung model. Setting: A research laboratory at a university medical center. Interventions: Spontaneous breathing with continuous positive airway pressure, at peak sinusoidal inspiratory flow-rate demands of 30 and then 60 L/min with sizes 6, 7, 8, and 9 mm internal diameter endotracheal tubes at each flow rate. Measurements and Main Results: Pressure, flow rate, and inhaled and exhaled volumes, during simulated spontaneous ventilation with continuous positive airway pressure were measured. Pressure was measured alternately at the "Y" piece of the breathing tubing of the continuous positive airway pressure system and at the tracheal end of the endotracheal tube to calculate the work imposed by the breathing circuit, endotracheal tube, and the total breathing apparatus. Greater changes in pressure and work were measured at the tracheal end of the endotracheal tube than at the "Y" piece of the breathing tubing for all test conditions. For example, at a peak inspiratory flow-rate demand of 30 L/min when pressures measured at the tracheal end of endotracheal tubes were compared with pressures measured at the "Y" piece, the total work imposed by the breathing apparatus increased by approximately 145% with a 6-mm tube, 95% with a 7-mm tube, 50% with an 8-mm tube, and 40% with a 9-mm tube (p < .05). Measuring pressure at the "Y" piece of the tubing results in significant underestimations of the changes in pressure and the work imposed, especially when the endotracheal tube has a small internal diameter and/or when the peak inspiratory flow-rate demand is high. Conclusions: The results indicate that pressure should be measured as close to the patient's airway as possible, i.e., at the tracheal end of the endotracheal tube, rather than using the traditional approach of measuring pressure and assessing work at the inspiratory or expiratory limbs, or "Y" piece of the breathing tubing.