The pathophysiology of asthma is characterized by a pattern of lung dysfunction that includes [1,2]:
- Airflow limitation that reverses with bronchodilator administration.
- Variable airflow limitation, which can be either circadian or episodic in nature.
- Airways hyperresponsiveness, which is an excessive decrease in airflow in response to specific stimuli or "triggers" (see "Risk factors for asthma").
Unlike other pulmonary diseases, asthma cannot be identified by a definitive pathologic picture or one diagnostic test. (See "Diagnosis of asthma in adolescents and adults".) Rather, the diagnosis of asthma is based upon an appropriate clinical history and characteristic findings from a series of pulmonary function tests [1-3]. These tests most often include different measures of airflow, bronchodilator responses, lung volumes, and the diffusing capacity. (See "Overview of pulmonary function testing in adults".)
MEASURES OF AIRFLOW LIMITATION
The detection of airflow limitation in the patient with asthma can be accomplished with a variety of techniques. The most common are the measurement of peak expiratory flow rate (PEFR), forced expiratory volume in one second (FEV1), and the flow-volume relationship [3,4]. It is important to realize that these endpoints are not necessarily equivalent because of the complex physiology of the lung and the clinical state of the patient.
Peak expiratory flow rate — Peak expiratory flow rate (PEFR) is measured during a maximally forceful and rapid exhalation that immediately follows a maximal inhalation. The utility of PEFR to detect the presence of airflow limitation is not particularly good, since the variability of PEFR among individuals is very large (+30 percent) [3-5]. However, PEFR is a useful method of monitoring changes or trends in the patient's lung function [2,3,6-10].