Can peak expiratory flow measurements estimate small airway function in asthmatic children?

Background: Asthma is characterized in part by small airways dysfunction. Peak expiratory flow (PEF) measurement has been suggested by all international guidelines as an important tool in asthma management. The correlation between PEF and FEV₁ but not with forced expired flow at 50% of vital capacity (FEF₅₀) is well-established. Study objective: To determine the value of PEF measurement as a predictor of small airways status as expressed by FEF₅₀. Design: Analysis of the ssociation between PEF and FEF₅₀ in single and multiple determinations. Patients: One hundred eleven asthmatic children (mean age, 11.8 years), grouped in the following way according to FEV₁ values: within normal range (n = 46); mildly reduced FEV₁ (n = 44); and moderately/severely reduced FEV₁ (n = 21). Results: Overall, FEF₅₀ and PEF were significantly correlated (r = 0.49; p < 0.0001). However, in 41.6% of the patients, the actual FEF₅₀ differed by > 20% from the calculated FEF₅₀. PEF has a high specificity (82.4%) but a poor sensitivity (51.7%) to detect FEF₅₀ status. PEF was better able to reflect abnormal FEF₅₀ in the patients with more severe asthma and to reflect normal FEF₅₀ values in the healthier patients. In patients with multiple measurements (n = 40), the correlation between FEF₅₀ and PEF was significantly better than that derived from a single determination (multiple measurements r = 0.77; single measurement, r = 0.49). Conclusions: Although PEF is an important tool in the management of asthmatic patients, it does not yield a complete picture because it is not sensitive in detecting small airways function. It is best used at home along with regular spirometry measurements at the clinic. PEF may serve as a better index of changes in small airways function once an individual regression is determined.