Peak expiratory flow occurs within the first second of a forced expiratory maneuver. Maximum flow occurs within the first few hundred milliliters of volume expired from total lung capacity and is volume and effort dependent. To measure peak flow accurately, the exhalation must begin at maximum inspiration and must be performed with maximum effort. In contrast to spirometry, there is no requirement for a prolonged smooth exhalation, which is an advantage for asthmatics. (Asthmatics often cough immediately following a forced expiratory effort, which can interfere with attempts to obtain acceptable spirometry.) Reduced peak expiratory flow is considered a valid measure of airflow obstruction, and it correlates well with FEV1.
Peak flow meters are lightweight, compact, inexpensive ($15–$50) instruments that are small enough to be carried in a purse or coat pocket. Hospitals use peak flow meters for bedside monitoring of asthma severity and response to bronchodilators. Current asthma management guidelines recommend daily home monitoring of peak flow using a peak flow meter (NHLBI, 1991). Peak flow measurements are used extensively in clinical trials of asthma therapy.
Efforts are proceeding to improve peak flow meters. The European Respiratory Society and the American Thoracic Society are developing standards for their construction and use. In addition, peak flow meters with a computer chip to allow recording of the time of peak flow effort should be on the market soon. This innovation will increase the cost of peak flow meters but will be a significant advantage for clinical studies.
The accuracy of peak flow meters varies among models. Table 5-9 shows technical standards suggested by the National Asthma Education Panel of the National Heart, Lung, and Blood Institute (NHLBI, 1991). Deficiencies in commonly used meters are outweighed by their benefits (Lebowitz, 1991; NHLBI, 1992). Although a flow range of 0–720 liters per minute has been recommended, some units are highly inaccurate within this range. Nevertheless, peak flow meters are invaluable for correlating changes in respiratory obstruction with a variety of activities and events experienced by the patient.
Current guidelines suggest that peak flows should be reproducible within 5 percent or 10 liters per minute (NHLBI, 1991). Reproducibility of peak flow measurements using mini-Wright peak flow meters was determined by having 10 subjects perform 30 forced expiratory maneuvers. Coefficients of variation ranged from 2 to 14 percent (Lebowitz et al., 1982). Significant training effects were seen during the first 2 days of a fortnight's study, and Quackenboss and colleagues (1991b) excluded these data from consideration in an epidemiological study. Measurement by standard and mini-Wright peak flow meters has been found to be stable over 6 months (Morrill et al., 1981; Van As, 1982). This stability over time may be more important