Interpretation ofthevariability ofpeakflow rates in chronic bronchitis

Increased diurnal variation ofexpiratory flowrates hasbeendocumented inpatients withchronic bronchitis, butthis could besecondary tothedisease processofbronchitis rather than anassociated disease-namely, asthma. Peakexpiratory flow wasmeasured twice daily before and after inhalation of200jigsalbutamol in34subjects withchronic bronchitis. TheFEV1ranged from 38% to 121%predicted. Diurnal variation (expressed as highest-lowest/highest (%))was increased in18subjects, allbutthree ofwhomhadairflow obstruction andan increase inmeth- acholine airway responsiveness. There was only aweakcorrelation between diurnal variation and airway responsiveness (r= - 054) ortheseverity oftheairflow obstruction. Thisfinding, together withtheoccurrenceofan increase indiurnal variation without anincrease inmethacholine airway responsiveness inthree subjects, suggests that theincreased diurnal variation inchronic bronchitis may have adifferent underlying mechanism fromthat inasthma. Anincreased diurnal variability ofpeakexpiratory flow(PEF)hasbeendocumented inpatients with chronic bronchitis andairflow obstruction,1 3butit isnotclear whether this isduetoassociated asthma or issecondary toother factors, suchasexpectoration of sputum4 ormagnification ofnormal variability dueto thesmall diameter oftheairways (Poiseuille's law).5 Interpretation ofthis increased variability iscompli- cated bysemantics, because asthma isdefined func- tionally as"variable airflow obstruction."6 This definition, however, doesnottakeinto account the underlying pathology. Indifferent diseases theevents initiating thevariability ofairway narrowing maybe different, eventhough theendresult could besimilar variability inairflow. Recent studies haveshownthatmediator release occurs inasthmatic subjects inresponse tostimuli suchasexercise79 orisocapnic hyperventilation of coldair,8 andincreased "releasability" ofmediator containing cells maybeafundamental abnormality in asthma.9 -lBronchoconstriction inresponse toexer- cise orisocapnic hyperventilation therefore islikely to indicate thepresence ofasthma, although theabsence