A Laboratory Evaluation of 2 Mechanical Ventilators in the Presence of Helium-Oxygen Mixtures

BACKGROUND: Helium-oxygen (heliox) mixtures are being used more frequently with mechanical ventilators. Newer ventilators continue to be developed that have not yet been evaluated for safety and efficacy of heliox delivery. We studied the performance of 2 previously untested ventilators (Servo-i and Inspiration) during heliox administration.

METHODS: We measured tidal volume (V_T) delivery, gas blending, gas analyzing, and pressure stability in the presence of heliox. A heliox (80% helium/20% oxygen) tank was attached to the 50-psi air inlet. We compared the set V_T (ie, set on the ventilator) and the exhaled V_T (measured by the ventilator) to the delivered V_T (measured with a lung model). Pressure measurements were also evaluated. We also compared the ventilator-setting fraction of inspired oxygen (F_IO2) to the F_IO2 measured by the ventilator and the F_IO2 measured with a supplemental oxygen analyzer.

RESULTS: Heliox significantly affected both the exhaled V_T measurement and the actual delivered V_T (p < 0.001) with both the Servo-i and the Inspiration. Neither peak inspiratory pressure (in the pressure-controlled ventilation mode) nor positive end-expiratory pressure were adversely affected by heliox with either ventilator. Introducing heliox into the gas-blending systems caused only a small error in F_IO2 delivery and monitoring.

CONCLUSIONS: Both ventilators cycled consistently with heliox mixtures. In most cases, actual delivered V_T can be reliably calculated if the F_IO2 and the set V_T or the measured exhaled V_T is known. With the Servo-i, at high helium concentrations the exhaled V_T measurement was unreliable and caused a high-priority alarm condition that couldn't be disabled. A supplemental oxygen analyzer is not necessary with either device for heliox applications.