Total inspiratory work with modern demand valve devices compared to continuous flow CPAP

The inspiratory work exerted by an electromechanical lung model in drawing a 500ml breath, was assessed by planimetry of pressure/volume loops for six commercial demand valve CPAP devices (Servo B and C from Siemens, EV-A and UV-2 from Drager, the Puritan Bennett 7200 and the Engstrom ERICA) and compared to the loading of a conventional high flow CPAP system. The effect of trigger sensitivity and inspiratory pressure support on inspiratory work was also investigated in some cases. The lung model allowed for calibrated changes in compliance and airway resistance. In the non-assisted CPAP mode, all machines required slightly larger amounts of inspiratory work than the continuous flow CPAP system. Most machines were comparable in performance but the ERICA and the Servo B required up to 22% more work than the continuous flow CPAP system and represented the maximal increase of total work due to any given machine. The greater part of total inspiratory work was due to lung compliance and airway resistance, factors external to the machines. Halving complicance doubled the work and exchanging a 7 for a 9 mm i.d. endotracheal tube in the circuit increased work by about 3% regardless of machine. Decreasing trigger sensitivity from 0 to 2 cm H2O for the Servo B increased work by up to 24%. Using 5 cm H2O of inspiratory pressure support decreased work for all machines up to 36% maximally. In conclusion, under the chosen experimental conditions the inspiratory work of breathing was only minimally increased with the following demand valve systems in comparison to a continuous flow CPAP system: Servo C, EV-A, UV-2, and Bennett 7200. The remaining required additional work could be eliminated by using inspiratory pressure support. However, triggering effort remains the unavoidable additional load of demand valve CPAP systems.