Determining the Accuracy of Compliance Measurements on a Ventilator Using a Bench Model

Abstract

Background:

The accuracy of displayed measurements during mechanical ventilation is an important factor while assessing the progress of patients. Accurate lung compliance measurements can lead to an earlier recognition of ventilator induced lung injuries, along with indications of a patient’s improving condition. The purpose of this study was to compare multiple set static compliances (Cs) on the Michigan Instruments test lung (Michigan Instruments, Grand Rapids, Michigan) with the Servo-U (Gentinge, Stockholm, Sweden) ventilator, to determine the accuracy of the Servo-U’s displayed measurements. We hypothesized that there would be no significant difference between the control (test lung) and variable (Servo-U) during this bench study.

Methods:

Prior to each trial run, the test lung was calibrated using the MedGraphics (MedGraphics, St. Paul, Minnesota) 3 L syringe according to manufacturer’s guidelines, along with completing all pre-use checks on the Servo-U. The Servo-U was set to volume control mode with a tidal volume of 500 mL, respiratory rate of 12 breaths/min, PEEP of 5 cm H₂O, and inspiratory time of 1 second. Cs tested were 20, 50, and 80 mL/cm H₂O with a set resistance of 5 cm H₂O/L/s. The Servo-U was attached to the test lung and all Cs values were recorded during all trial runs using both a 3-s inspiratory and expiratory hold. Each run was performed in triplicate.

Results:

There was a significant difference between the set Cs on the test lung and the measured Cs on the Servo-U at all three set points. With set Cs of 20, 50, and 80 mL/cm H₂O, the mean displayed values were recorded at 22.6 (P = < .001), 60.1 (P = .006), and 93.4 (P < .001) mL/cm H₂O respectively.

Conclusions:

When compared to a set compliance on the test lung, the Servo-U did not demonstrate accuracy with measured compliance. Extended research should be considered to verify the accuracy of these results given the clinically significant difference between mechanically ventilated patient’s physical lung compliance and that of a test lung. The monitoring of intubated patients’ compliance values helps the practitioner to keep mechanical ventilation as safe and effective as possible.