[Quantitative comparison of ventilator-induced work during simulated CPAP in eight demand-flow valve ventilators]

The ventilator-induced work during continuous positive airway pressure (CPAP) mode in demand-valve ventilators was evaluated by using a piston pump as a simulator for active breathing. A piston pump delivered and withdrew a stroke volume of 500 ml at rates of 10, 20 and 40 cycle.min-1 with a sinusoidal waveform. A hot-wire flowmeter and a differential pressure transducer were interposed between the pump and ventilators and their signals were fed to a microcomputer to display a pressure-volume loop. The area of the loop was divided into the four parts. Inspiratory work associated with the opening of the demand valve was represented by the area of baseline airway pressure (BPa) during inspiration. The remaining area during inspiration reflected the work done by the ventilators. Expiratory work for overcoming the flow resistance of the expiratory apparatus was represented by the area above BPa during exhalation. The fourth was the area below the baseline during exhalation. The Puritan-Bennett 7200a, the Bear 5, the Siemens Servo 900C, the Hamilton Veolar, the Bird 6400ST, the Engström Erica, the Dräger EV-A, and the CPU-1 were examined at varying CPAP and pressure support levels. Because of demand valve oscillation throughout inspiration, the inspiratory workload of the Bear 5, the Siemens Servo 900C, the Hamilton Veolar, the Bird 6400ST, and the Dräger EV-A could not be calculated. Expiratory flow-resistive work was higher in the Siemens Servo 900C and the Bird 6400ST than the others. The present system can assess the entire performance of ventilators, and may serve to compare ventilators' performance.