An Evaluation,in Four Ventilators,of Characteristics that Affect Work of Breathing

Abstract

We evaluated four current-generation mechanical ventilators in order to assess the degree and success of efforts by the manufacturers to reduce patient work of breathing during their use. Methods: Ventilators studied were the Puritan-Bennett 7200A, Siemens Servo 900C, BEAR 5, and Hamilton Veolar. We evaluated them for inspiratory and expiratory work imposed by machine components in the CPAP mode, for the greatest negative pressure relative to CPAP level required to achieve a standardized inspired volume (ΔPmax), and for response time to a standard inspiratory effort in the assist mode. Results: Inspiratory work added (imposed) by the ventilators as a per cent of normal inspiratory work ranged from 5% to 33% at 10 cm H₂O CPAP. All four ventilators compared favorably with inspiratory-work measurements made by the same method on a continuous-flow (40 L/min) reservoir circuit attached to the Puritan-Bennett MA-1 (31% work added at 10 cm H₂O CPAP), previously considered a low-work-of-breathing system. ΔPmax, which represents the maximum inspiratory-pressure change generated during a standardized inspired volume, ranged from -2.0 to -4.2 cm H₂O at 10 cm H₂O CPAP. Neither inspiratory work nor ΔPmax was decreased by adding 40 L/min of continuous flow in the CPAP mode on the BEAR 5, which is an optional setting on that ventilator. Expiratory work (the pressure-volume product resulting from airflow resistance) ranged from 0.08 to 0.13 joule per passive exhalation at 10 cm H₂O CPAP. When the delay in response time in the assist mode was measured, all four ventilators responded within 48-52 milliseconds, as contrasted to older ventilators' response times (BEAR 1 = 65 ms, MA-1 = 67 ms). Inspiratory work showed the greatest variation among the four ventilators, which all benefited from consideration of work of breathing in their designs.