Validation of volume delivery with the use of heliox in mechanical ventilation

Using a mixture of helium and oxygen (heliox) while mechanically ventilating patients to relieve lower airway obstruction is commonly practiced in intensive care units. The use of heliox with commercially available mechanical ventilators is usually accomplished by connecting the heliox mixture to the air inlet of the ventilator. Since most ventilators do not compensate for the difference in gas densities, particular attention to the delivered tidal volume (V _T ) is required. We utilized a commercially available mechanical ventilator with an internal blending system that is capable of delivering heliox instead of medical air. It identifies and compensates for the gas mixture, theoretically enhancing stability in delivered and monitored parameters. Intubated, sedated male domestic pigs (n = 7) were ventilated with a mechanical ventilator equipped with an internal heliox blending system utilizing pressure assist control, pressure regulated volume control, and pressure support ventilation modes. Accuracy of volume delivery was assessed by comparing delivered V _T measured at the patient wye using the variable orifice flow sensor connected to the ventilator and a heated 0-35 L/min pneumotachograph that was calibrated for flow, pressure, and volume, with a 0.80/0.20 heliox mix and 0.50 oxygen. A paired t-test was utilized with a P < 0.05. Pigs mean weight 9.0 ± 0.9 kg. Mean exhaled tidal volume for all modes and was 66 ± 16 mL. When comparing all modes for the 0.50 oxygen to the heliox mix, we found that exhaled tidal volume % difference increased when using heliox ( P ≤ 0.037). This study confirms that clinicians should be vigilant in monitoring delivered V _T using a commercially available ventilator equipped with an internal heliox blending system. Accuracy of delivered V _T can vary greatly with the use of heliox in this system, as well as other configurations.