Helium–oxygen (He–O₂) enhances oxygenation and increases carbon dioxide clearance in mechanically ventilated patients

Introduction

Helium is eight times less dense than nitrogen and only 10% more viscous. As a result of these physical properties it produces significantly higher gas flows for the same differential pressure gradient. This coupled with the fact that as a carrier gas He facilitates faster diffusion makes it a potentially useful adjunct in the ventilatory support of patients with acute respiratory failure. Substituting He for nitrogen has been shown to be of considerable benefit in the management of acute upper airway obstruction from a wide variety of causes. There is also a growing body of evidence for its use in acute severe asthma and decompensated COPD. We previously conducted a pilot study of He–O₂ in patients with acute respiratory distress syndrome (ARDS) and found that it led to a significant improvement in gas exchange in the majority of subjects. Having resolved a number of technical problem regarding the use of He–O₂ we have gone on to perform a larger cross over study in a wider variety of patients and present our preliminary findings here.

Methods

All patients who were mechanically ventilated on our unit were eligible. Exclusion criteria included haemodynamic instability, active weaning of respiratory support and imminent deterioration. All patients were ventilated in a pressure control mode. Patients were observed for a 15 min period on their established ventilatory regime of N₂–O₂. They were then switched to He–O₂ and observations repeated after 15 and 60 min and then every 60 min for a maximum of 360 min. The trial was terminated when no further change in the partial pressures of arterial oxygen (PaO₂) and carbon dioxide (PaCO₂) were scene. Patients were then re-established on N₂–O₂ and observed for a further 60–120 min. No alterations in ventilatory parameters were made unless warranted by changes in arterial blood gases. Ventilatory and haemodynamic parameters were continuously monitored throughout. The ventilator flowmeter was calibrated for use with He–O₂ as previously described.

Results

Six out of eight patients showed a significant improvement in PaO₂ and PaCO₂ within 15 min. Most of those studied showed further improvements at the successive observation time points. There were small improvements in respiratory mechanics, but these were insufficient to explain the improvements in gas exchange. There were no significant haemodynamic changes seen. The worse the derangement of gas exchange at study outset, the greater the magnitude of improvement seen on He–O₂.

Conclusions

This study adds to the growing body of evidence that He–O₂ may be a useful adjunct to mechanical ventilation, especially in the most severe cases of respiratory failure.