MODEL RELEASED. Lung function test. Woman breathing into the tube of 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 tube. A reading in litres of air per minute is recorded on a drum (not seen). This device may be used to detect constriction of airways in the lungs (bronchospasm), such as occurs in asthma patients. The peak flow meter measures the severity of bronchospasm and monitors the response the patient may show to bronchodilator drugs.