A laboratory evaluation of 2 mechanical ventilators in the presence of helium-oxygen mixtures

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.

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(IO(2))) to the F(IO(2)) measured by the ventilator and the F(IO(2)) measured with a supplemental oxygen analyzer.

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.

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.