Sixteen asthmatic patients who regularly showed early morning falls in peak expiratory flow rate of more than 25% were studied at 06.00 hours and 14.00 hours. At 06.00 hours considerable deterioration in static lung volumes and airways resistance occurred which was typical of an attack of acute asthma. Blood gas analysis showed less variation, with mild hypoxia and a wide alveolar-arterial oxygen tension gradient throughout the study. Diurnal variation was also seen in the response of maximal mid expiratory flow rates to the inhalation of helium/oxygen mixture in seven patients, but another seven were consistently non-responders. The overall pattern of results suggests that the calibre of both large and small airways decreased at night but improvement was more complete in large airways during the day. Despite these findings patients had few symptoms at 06.00 hours when most of them showed marked reversibility after inhaling aerosol salbutamol. This phenomenon might account for the sudden nature of some asthma deaths as these often occur in the early morning. If this is the case, the minority of patients whose early morning decline in lung function was not immediately reversible would appear to be at greatest risk.
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