Abstract

Historically serial PEF measurements have been used to assess variation in airway calibre over a period of time in the monitoring and diagnosis of asthma. Some have questioned whether the FEV1 which is thought to better reflect the size of small airways would be a better indicator of changes in airway calibre. This study aims to compare the average diurnal variation (% predicted) in PEF and FEV1 with both indices measured from the same exhalation manoeuvre. Subjects attending a specialist clinic for investigation of occupational asthma were asked to serially record their lung function using a MicroMedical diarycard spirometer. The device records and stores PEF, FEV1 and FVC by directly measuring flow and then calculating volume. Subjects were instructed to produce 3 maximum forced exhalations for at least 2 seconds following a maximal inhalation, every 2 hours from waking to sleeping for at least 21 days. Following instruction subjects were observed performing 10 practice manoeuvres in clinic before returning home. Each reading was screened to make sure that the FVC exceeded the FEV1. The best PEF and FEV1 from each set of 3 was used in calculations provided that the next best reading was within 20l/min for PEF and 5% or 0.1L for FEV1. Records with <21 days were excluded, as well as days with <4 readings. The diurnal variation was calculated for each day of the record as amplitude % predicted. The diurnal variation for each eligible day was used to calculate the mean diurnal variation for PEF and FEV1. 31 subjects kept records fulfilling these criteria. The mean number of days with =4 readings was 16.5 (SD 6.3). The mean daily diurnal variation for FEV1 was 12.13% and 12.95% for PEF (paired t test p=0.23). 10 records had a mean diurnal variation >15% PEF with one extra record having diurnal variation >15% FEV1. There is no evidence that FEV1 is more sensitive in measuring variation in airway calibre than PEF.

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