A device to test how the lung is working (see also PULMONARY FUNCTION TESTS, LUNG VOLUMES) to assess the effects of airway disease or the progress of treatment – a procedure called spirometry. It records the total volume of air breathed out – the forced vital capacity. The machine also records the volume of air breathed out in one second – the forced expiratory volume. In diseases such as ASTHMA, in which the airways are obstructed, the ratio of the forced expiratory volume to the forced vital capacity is reduced. (See RESPIRATION.)