MODEL RELEASED. Lung function test. A 6-year-old girl breathes out through a peak flow meter while her doctor watches. This device is used to monitor breathing in victims of asthma by measuring the maximum rate that air can flow out of the lungs. The patient first zeros the black notch on the calibrated scale, takes a deep breath, and then breathes out as hard as possible through the tube. The black notch slides down the scale, giving a reading in litres of air per minute. Asthma causes constriction of airways in the lungs (broncho- spasm), making breathing difficult. The peak flow meter measures the severity of bronchospasm and monitors the response to bronchodilator drugs.