Peak Flow Monitoring

Peak expiratory flow (PEF) is the maximal flow rate achieved during a forced exhalation that begins with a full inspiration. A handheld meter captures how quickly air leaves the lungs, not how much air the patient can exhale overall. Airway caliber strongly influences the result, so bronchospasm, mucosal edema, and mucus in asthma can lower PEF. The reading also depends on lung volume, respiratory muscle strength, and voluntary effort. PEF is therefore effort-dependent and reflects predominantly large-airway flow. It is useful for tracking change, but it is not a complete measure of airflow obstruction, oxygenation, or asthma control. A falling value may represent worsening airway narrowing, but it may also result from a weak blow, a poor mouth seal, the tongue blocking the mouthpiece, coughing into the meter, respiratory muscle weakness, chest wall disease, or obesity. In severe asthma, PEF can underestimate the true degree of airflow limitation. The number must be interpreted beside the patient: work of breathing, speech, mental status, respiratory rate, wheezing or a quiet chest, oxygen saturation, and response to treatment matter more than a device...