Abstract
BACKGROUND: Levels of work of breathing imposed (WOB(i)) by variations in ventilator design have been addressed by a number of investigators. We conducted this study to assess the effects of tracheal pressure-triggering (TPT) on the WOB(i) and to compare it with conventional pressure-triggering (CPT). MATERIALS and METHODS: Sine wave breaths of ≃ 500 mL, with mean flows of 30 and 60 L/min, were inspired through a test ventilator (Servo 900 C) set in the continuous positive airway pressure (CPAP) mode (CPAP, 5 cm H2O). WOB(i) was measured during CPT and TPT. In addition, we simulated 500-mL breaths through the test ventilator with its trigger sensitivity set at -10 cm H2O for both CPT and TPT. Pressure was measured at the Y-piece and within the tracheal lumen. Flow was measured at the Y-piece and volume obtained from the digital integration of flow. WOB(i) was calculated from the area enclosed by the pressure-volume plot (in that part of the pressure-volume loop where the pressure was below the level of PEEP). RESULTS: WOB(i) (including that contributed by the tracheal tube) was less during TPT, mean (SD) 258 (42) mJ/L than with CPT, 477 (24) mJ/L at 60 L/min, p < 0.05. However, for breaths simulated at flows of 60 L/min, mean inspiratory pressures within the tracheal lumen and at the Y-piece were greater during TPT than during CPT, p < 0.05 for both. Under the test conditions used to measure trigger delay and trigger work, trigger delay was similar during TPT and CPT, 280 (20) ms and trigger work was similar for the two triggering modes 1.34 (0.3) cm H2O · s vs 1.31 (0.25) cm H2O · s. CONCLUSIONS: Decrease in WOB(i) during TPT is not due to an alteration in triggering function per se but to an inadvertent pressure assist and is no more advantageous than application of pressure support to compensate for apparatus work.