MODEL RELEASED. Lung function test. Man breathing out through a peak flow meter. This device is used to monitor lung function by measuring the maximum rate that air can flow out of the lungs. The patient takes a deep breath and breathes out as hard as possible into the mouthpiece. A notch slides down the scale (black, at far right), giving a reading in litres of air per minute. This device may be used to detect constriction of airways in the lungs (broncho- spasm), such as occurs in asthma patients. The peak flow meter measures the severity of broncho- spasm and monitors the response the patient may show to bronchodilator drugs.