Evaluation of Heat and Moisture Exhanger performance with two different methods in the same test apparatus

This study presents a new test apparatus for the evaluation of the humidification efficiency of heat and moisture exchangers (HMEs). These devices are widely used in invasive ventilation conditions to humidify and to heat medical gases inhaled by the patients. The new custom-made test apparatus allows simultaneous measurements with the two more frequent methods for in vitro tests: gravimetric, which is considered the reference method by the international standard ISO 9360, and hygrometric. In the reference method, the absolute humidity of the inhaled gas (HME moisture output) is determined by subtracting, from the absolute humidity of the exhaled gas (AHexh), the moisture loss measured with a precision balance. Consequently, AHexh is supposed to be stable throughout the test. In addition, ISO 9360 does not set a limit for the maximum AHexh in the measuring process. In the new experimental test apparatus AHexh is constantly monitored and the humidification efficiency of an HME is expressed as the ratio of the HME moisture output with respect to AHexh, the so-called relative efficiency. Results show that average values of AHexh and of the relative efficiency are very similar with both methods, confirming the good reliability of the new test apparatus.