peak expiratory flow
Peak expiratory flow is the single fastest gust of air you can produce when you blow out as hard and fast as you can — the top speed of your exhalation, reached in the first split-second of the blow. A cheap handheld meter reads it as a number you can check at home.
Because narrowed airways throttle that top speed, peak flow falls when asthma is poorly controlled and rises as treatment opens the airways. Patients often track it daily, watching for a morning dip or a falling trend that warns of an impending attack before they feel breathless.
Peak flow is convenient but blunt: it is very effort-dependent, reflects mainly the larger airways, and varies between devices, so it complements rather than replaces full spirometry. A reading is most useful when compared with the patient's own personal best rather than a population average.
A teenager whose personal best peak flow is 450 L/min wakes with a reading of 280 L/min (62 percent) — squarely in the yellow zone, prompting her to start the reliever inhaler in her action plan.
A morning dip in peak flow can warn of worsening asthma.
Asthma action plans often use peak flow zones — green, yellow and red bands based on a percentage of personal best — to tell patients when to step up treatment or seek help.