Advanced, Clinical Subspecialties
• First and lightest noble gas.
• It is non-toxic, colorless, odorless, and tasteless.
• Extremely low boiling and melting point; therefore, it exists as a gas at most temperatures.
• Behaves like an ideal gas (i.e it obeys the Universal Gas Equation (PV=nRT))
• After hydrogen, Helium is the lowest density gas (0.179g/L at STP)
• It is nearly physiologically inert.
• Other than the risk of sudden hypoxia by inhaling pure helium, there is no evidence of deleterious effects on organ systems, carcinogenicity, or teratogenicity.
Helium-Oxygen Mixtures (Heliox):
During normal inspiration, airflow is laminar in small airways and turbulent in large airways due to their irregular walls and high velocity flow. Increased turbulent flow, as in airway obstruction, leads to increased airway resistance, work of breathing, hypoxia, and hypercapnia.
During turbulent airflow, flow is inversely proportional to density; therefore, the lower the density of the gas the higher the flow rate. Helium is eight times less dense than oxygen and Heliox is three times less dense than air. Thus, when mixed with oxygen. Helium reduces the density of the gas mixture, providing increased laminar flow and leading to decreased respiratory rate, work of breathing, and resulting hypoxia/hypercapnia (See Heliox: Airway resistance at https://www.openanesthesia.org/heliox\_airway\_resistance/). Furthermore, CO2 diffuses 4-5 times faster through Heliox leading to a reduction in hypercapnia and faster normalization of pH.
Additionally, increased laminar flow with helium-oxygen mixtures increases gas flow to distal airways. Helium mixtures can be used as a driving gas for nebulizers in bronchodilator therapy for asthma by increasing laminar flow which leads to increased and more uniform deposition of drugs in the distal airways.