Dose maintenance for Partial Liquid Ventilation: passive heat-and-moisture exchangers

Partial Liquid Ventilation (PLV), a promising method for the treatment of acute lung failure, has been evaluated in many animal studies. It has recently progressed to the point of controlled clinical trials in which patients of all ages on conventional mechanical ventilation (CMV) have their lungs substantially filled with a perfluorochemical (PFC) liquid, perflubron (PFB). During PLV, it is desirable to both maintain humidification and minimize the evaporation of PFB in order to maintain a desired dose in the lung and to reduce dose consumption and redosing effort. Heat-and-moisture exchangers (HMEs) have been used for years as a passive means of minimizing water vapor loss from the respiratory tract during CMV support of intensive care and surgical patients. In the current study, research was undertaken to leverage the operating principles of existing HMEs such that specialized "fluorophilic" HMEs (FHMEs), devices optimized for both water and PFB conservation, could be realized. A patient simulator (involving both water vapor and PFB vapor sources) was constructed and used in the in-vitro evaluation of various FHME concepts. Dose-retention efficiencies were determined with the aid of an infrared instrument and a digital thermohygrometer. Although no larger than commercial HMEs in terms of dead space (gas-occupying volume), efficient FHMEs resulted, offering less flow resistance (delta P) than their commercial counterparts. Additionally, the presence of PFB vapor did not appear to compromise the water-exchange efficiency of certain HME configurations. One promising FHME design was also tested in swine undergoing 12-hour PLV treatments. A mean conservation efficiency of 63% at an average tidal volume of 550 mL was shown, although somewhat lower efficiencies may result in adult patients because efficiency was found to trend downward with increasing tidal volume. The use of an FHME is expected to sustain dose levels in patients for longer periods with less frequent dosing and reduced dose consumption, saving treatment labor and cost.