Heliox in acute severe asthma

Brief

A 35 year old asthmatic with multiple previous hospitals admissions for sudden onset severe asthma is brought into ED resus. She is talking in short sentences, saturating at 98% on a salbutamol nebuliser driven by oxygen, tachycardic at 130bpm and has a BP of 130/80. She tells you that Heliox is a part of her standard management plan.

Questions

1. What is Heliox
2. Which conditions can it be useful in?
3. What is the medical history of Heliox? What other uses does it have?
4. How/why does it work?

Answers

What is Heliox

Heliox is a mixture of helium and oxygen. For medical use the most commonly available formulation is 21/79 oxygen/helium, but 30/70 and 40/60 are also available. Different gas mixtures are available for non medical uses (see below).

Which conditions can it be useful in?

It can be useful in airway obstruction, asthma and COPD (diseases in which there is an increased work of breathing due to dysfunction in the large or medium airways.)

What is the medical history of Heliox? What other uses does it have?

Heliox use in asthma was described in the 1930s. Due to reduced availability of helium supplies over world war II and the widespread availability of bronchodilators usage of it tailed off.

Heliox also finds use in commercial deep diving where the gas mixtures are generally hypoxic (e.g. 10/90 oxygen to helium) to avoid nitrogen narcosis and reduce oxygen toxicity.

How/why does it work?

Helium itself is inert and does not act as a bronchodilator. However compared to air it has a similar viscosity but a lower density. This leads to increased laminar (smooth) and reduced turbulent flow in the airways.

The basic underlying pathology is asthma is narrowing of the airways - this will lead to increased airway resistance (as per Pouseille's law) - and we try to reverse this narrowing with our medical therapy. However whether the airflow is laminar (smooth) or turbulent also has a bearing on airway resistance.

The basic equation specifying whether flow will be laminar or turbulent is the Reynold's equation:

Where:

• Re is the Reynolds number
• p is the density of the fluid
• v is the velocity of the fluid
• d is the diameter of the pipe
• and μ is the dynamic viscosity

Basically if the Reynold's number is below 2300 there will be laminar flow. By dropping the density of the gas we are using we (hopefully) increase the calibre of the airways in which there will be laminar flow, reduce the total airway resistance and reduce the work of breathing.

This doesn't treat the underlying pathology - but may be a method of delaying intubation long enough for steroid and bronchodilator therapy to be effective. The laminar flow may also increase the delivery of nebulised drugs.