Relationship between different measurements of respiratory function in asthma.

Peak expirator flow rate (PEFR) and forced expiratory volume in 1 s (FEV1) were measured in 630 asthmatics at routine attendance (actual function). The highest potential PEFR and forced vital capacity (FVC) were established with corticosteroids if necessary (maximum function). Maximum function assesses the persistent component, and the ratio of actual maximum function is a measure of the potentially reversible component of obstruction relieved at attendance. Actual PEFR and FEV1 were closely related (r = 0.85), and maximum PEFR and FVC somewhat less so (r = 0.58). Mean values for actual/maximum function were similar in both sexes by both methods. Persistent obstruction and control of reversible wheeze were not closely related, particularly in long-standing asthma. In routine clinics, spirometry adds little to PEFR although in females it was more sensitive than PEFR in detecting persistent obstruction. Dissociation between control and persistent obstruction, particularly in long-standing asthma, suggests that poor control is not important in determining the development of persistent obstruction.