Tuesday, March 21, 2017

Heliox & Mechanical Power

PulmCCM - By Jon-Emile S. Kenny - March 20, 2017
"Of the countless things taught to me by Dr. Chitkara at the Palo Alto VA Health Care System, one that sticks is the difference between density-dependent and viscosity-dependent airflow. He often used the chronic bronchitic suffering through the viscous, humid New York City summers as a teaching example. The importance of gas density and viscosity is also frequently encountered when considering the properties and benefits of helium-oxygen gas mixtures [i.e. ‘Heliox’]. This brief review highlights the viscosity and density characteristics of gas within the context of the mechanical power applied to the lung; as such, roles for Heliox may extend beyond that of a rescue therapy for severe obstructive airways disease... 
Caveats, Concerns, Conclusion 
Lowering gas density has salutary effects on both inspiratory and expiratory phases of the respiratory cycle. By reducing Ppeak, PEEPi and Pplat, dynamic and elastic work is reduced. Accordingly, the total mechanical power applied to the lung skeleton is diminished which may mitigate VILI. Lung units of differing time-constants have been found in ARDS, so this physiology may be germane beyond obstructive airways disease. The obvious drawback is that the density-dependent benefits of Heliox come at the expense of FiO2 and will limit application to severely hypoxemic patients. Accordingly, Heliox as an adjunct to extracorporeal oxygenation may hold clinical merit. Regardless, as a lesson in applied physiology this thought experiment – I think – remains worthwhile; thanks Dr. C!"