Peak Flow Monitoring

Peak expiratory flow (PEF) is the maximum airflow a person produces during a forceful exhalation after taking a full breath in. When bronchial smooth muscle constricts, the airway lumen narrows and less air can move out quickly, so the PEF falls. The meter is detecting the narrowest point in the expiratory “bottleneck,” not measuring total lung volume or oxygen saturation. PEF is most useful for following variable airflow limitation in a person with asthma. It does not diagnose asthma by itself, explain every cause of a low reading, or replace assessment of respiratory distress. A patient may have a falsely low value because of a poor seal or weak effort; another patient may be seriously unwell even when the number is not yet dramatically low. Interpret the reading against the patient’s personal best, not against a generic “normal” value. A trend away from that individual baseline can reveal deteriorating control before the patient recognizes the change.