MODEL RELEASED. Lung function test. Young girl breathes into the mouthpiece of a peak flow meter, in a general practice surgery. A peak flow meter is a device used to monitor lung capacity by measuring the maximum rate that air can flow out of the lungs. The patient takes a deep breath and then breathes out as hard as possible through the tube. The device gives a reading in litres of air per minute. It is commonly used to diagnose lung disorders such as asthma which constricts the airways in the lungs (bronchospasm), making breathing difficult. The peak flow meter can measure the severity of bronchospasm and the patient's response to bronchodilator drugs.