Although the concept of ventilator circuit compression is well known, it is not fully appreciated clinically. We compared the compression volume of five adult disposable ventilator circuits and a nondisposable circuit.
Methods: Five brands of disposable circuits (Inspiron, Intertech, Marquest, Seamless, and U-Mid) and one nondisposable brand (Bennett) were used. The circuits were attached to the outlet of a Bennett MA-1 ventilator in the standard manner, and the filter and humidifier were bypassed to eliminate their contribution to compression volume. The ventilator delivered 1 L of gas to a Michigan Instruments Test Training Lung at a flow of 60 L/min and a rate of 12/min. A valve system was placed between the Y-connector and the TTL to partition circuit compression volume from the volume delivered to the test lung. The compression factor was calculated by dividing the compression volume by the ventilating pressure. Five compliance settings were used on the TTL (0.1, 0.05, 0.03, 0.02, and 0.015 L/cm) to simulate different ventilating pressures. Pressure at the Y-connector, compressible volume, and volume delivered to the TTL were measured using a calibrated Timeter RT200 volume-pressure analyzer. Five measurements were made with each brand. A new circuit was used for each run with a disposable circuit, and a different circuit from the respiratory care department stock was used for each run with a nondisposable circuit.
Results: There were significant differences between the compression factors at different TTL compliance settings (p less than 0.001), and brands of circuits evaluated (p less than 0.001). There was a significant interaction effect for compression factors between TTL compliance setting and circuit brands (p less than 0.001). The compression factors for all disposable circuits were greater than the compression factors for the nondisposable circuits (p less than 0.05).
Conclusions: There were differences in circuit compression factors as a function of the ventilating pressure and the brand of circuit. Although these differences are too small to be clinically important in many cases, they may be important during ventilation with low tidal volumes and high pressures.