This technique is a closed-circuit system where a spirometer is filled with a mixture of helium (He) and oxygen.
The helium spreads into the lungs of the patient, and settles at a new concentration (C2).
Because there is no leak of substances in the system, the amount of helium remains constant during the test, and the FRC is calculated by using the following equation:
In patients with obstructive pulmonary diseases the measurements of the helium dilution technique are not reliable because of incomplete equilibration of the helium in all areas of the lungs.
A simplified helium dilution technique may be used as an alternative to quantitative CT scans to assess end-expiratory lung volumes (EELV) among patients who are on mechanical ventilation with diagnosis of ALI/ARDS according to a cross-sectional study.
[1] The results show a good correlation [EELV(He)=208+0.858xEELV(CT), r=0.941, p < 0.001] between the two methods, and the helium dilution technique offers the advantages of lower cost, decreased transportation of critically ill patients, and reduced radiation exposure.
This study's results may have limited generalizability due to its specificity to the ALI/ARDS population and its small sample size (21 patients).