Definition: Peak Flow MeterPeak Flow MeterPeak flow meters help people with asthma know how well their lungs are working. A peak flow meter can tell if someone's airways have narrowed even before the person coughs, wheezes, or has trouble breathing. To use the peak flow meter, a person blows into it and then checks the reading. It's portable, doesn't need to be plugged in, and you don't need a doctor or nurse to use it. Some people use a peak flow meter at home to see how well they're keeping their asthma under control.