The measuring principle of ELFA is to allow the subject to be tested to breathe  through a tube, the volume of which is precisely known. This allows a well-controlled amount of carbon dioxide to be rebreathed in each breath. The resulting increase in the expired carbon dioxide concentration is measured during a 30 second period and from the form of this curve one can determine both the effective lung volume (ELV) and the effective pulmonary blood flow. (EPBF) * It also allows the calculation of FRC (Functional Residual Capacity) and CO (Cardiac Output)

* (By effective one means the part that participates in the vital gas exchange process in the lung)