4.2 Heliox Therapy

Physical and Physiological Principles of Heliox

Heliox is a therapeutic gas mixture of helium and oxygen. It is used as a temporary, supportive therapy for patients experiencing acute, severe upper airway obstruction or severe bronchospastic disease (e.g., status asthmaticus) to decrease the work of breathing and prevent respiratory muscle fatigue.

The Physics of Laminar and Turbulent Flow

The therapeutic benefit of heliox is based entirely on the physical property of gas density. Helium is an inert, non-toxic gas with a molecular weight of 4 g/mol, compared to nitrogen (28 g/mol) and oxygen (32 g/mol). A mixture of 80% helium and 20% oxygen is approximately one-third as dense as ambient air.

• Reynolds Number ($Re$): Fluid dynamics dictates that gas flow through a tube is either laminar (smooth, straight streamlines) or turbulent (chaotic, eddy currents). The transition from laminar to turbulent flow is predicted by the Reynolds number: $Re = \\frac{\\rho \\cdot v \\cdot d}{\\eta}$ where $\rho$ is gas density, $v$ is velocity, $d$ is airway diameter, and $\eta$ is gas viscosity.
• Converting to Laminar Flow: When airways are severely narrowed due to edema, spasm, or stenosis, gas velocity increases, raising the Reynolds number and causing turbulent flow. Turbulent flow requires a high pressure gradient to move gas, which increases the work of breathing. Because heliox has an extremely low density ($\rho$), it significantly lowers the Reynolds number, converting turbulent flow back into smooth, laminar flow.
• Reducing Driving Pressure: According to Poiseuille’s Law, the pressure gradient ($\Delta P$) required to drive laminar flow is directly proportional to gas viscosity, not density, and is much lower than the pressure required for turbulent flow. This reduction in required driving pressure translates directly to a decrease in the patient's inspiratory effort and work of breathing.

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Dosing, Flow Calculations, and Delivery Systems

Heliox Mixtures and Conversion Factors

Heliox is available in standard cylinders in two primary concentrations. The clinical choice depends on the patient's oxygenation requirements:

• 80:20 Mixture (80% Helium / 20% Oxygen): Offers the lowest density and the maximum therapeutic benefit. It has a flowmeter conversion factor of 1.8.
• 70:30 Mixture (70% Helium / 30% Oxygen): Used if the patient requires a higher fraction of inspired oxygen. It has a conversion factor of 1.6.
• Clinical Note on Higher O2 Concentrations: Mixtures such as 60:40 (factor 1.4) or 50:50 are rarely used because the increased density of the higher oxygen concentration negates the physical benefits of the helium.

Flowmeter Calculations

Standard medical flowmeters are calibrated for the density of 100% oxygen. Because heliox is less dense, it passes through the flowmeter orifice faster than oxygen. To determine the actual flow delivered to the patient, the clinician must multiply the flowmeter reading by the conversion factor.

$\\text{Actual Flow} = \\text{Flowmeter Reading} \\times \\text{Conversion Factor}$

Conversely, to deliver a specific target flow, the clinician must divide the target flow by the factor to determine the flowmeter setting.

$\\text{Flowmeter Setting} = \\frac{\\text{Target Flow}}{\\text{Conversion Factor}}$

Delivery Methods

• Non-Rebreather Mask: For spontaneously breathing patients, heliox must be administered via a tightly fitting non-rebreather mask with a fully inflated reservoir bag. The mask must fit snugly to prevent the entrainment of room air. Entraining room air introduces nitrogen, which increases the density of the inhaled gas and renders the therapy ineffective.
• Aerosol Delivery: Heliox improves the deposition of aerosolized bronchodilators. In laminar flow, particles are less likely to deposit by inertial impaction in the upper airway, allowing more medication to reach the distal bronchioles. The nebulizer should be powered by the heliox gas source.

Mechanical Ventilation Challenges

Delivering heliox through a mechanical ventilator is highly complex:

• Flow Sensor Inaccuracy: Standard ventilators use heated-wire anemometers or differential pressure pneumotachometers to measure gas flow. These sensors are calibrated for the density and thermal conductivity of air and oxygen. Administering heliox through an unapproved ventilator will cause severe under-measurement of delivered tidal volumes, risking catastrophic over-delivery (volutrauma/barotrauma) and inaccurate trigger sensitivity.
• Approved Equipment: Heliox must only be delivered using ventilators specifically engineered and software-calibrated for helium mixtures (e.g., Getinge Servo-i/u). The ventilator's internal software dynamically adjusts its flow-delivery algorithms and volume measurements based on the set helium concentration.