Peak flow meter: Peak flow meter a small hand-held device that measures how fast air comes out of the lungs when a person exhales forcefully. This measurement is called a peak expiratory flow (PEF) and is measured in liters per minute (lpm). A person’s PEF may drop hours or even days before asthma symptoms are noticeable. Readings from the meter can help the patient recognize early changes that may be signs of worsening asthma. A peak flow meter can also help the patient learn what trigger their symptoms and understand what symptoms indicate that emergency care is needed. Peak flow readings also help the doctor decide when to stop or add medications.