Alveolar-arterial oxygen gradient in asthma

Introduction Measuring small airway function is difficult. Current measures do not test gas transfer. We set out to establish if the A-a oxygen gradient differed with asthma severity. Methods Normal controls, mild/moderate people with asthma, and people with severe asthma (by ATS criteria) were recruited. Asthma was defined as; Methacholine PC 20 1 reversibility or PEF variability u003e20%. Mild/moderate asthma was diagnosed if no major and only one minor criterion for severe asthma were met. Controls had normal spirometry and Mch PC 20 . A-a gradient was measured on ear lobe capillary blood, taken on room air, and calculated using the ideal air equation, corrected for barometric pressure. 5 flow FeNO was also measured. Results The A-a oxygen gradient was higher in severe asthma compared with controls (mean difference 2.15 KPa, p=0.01 [95% CI 0.95 to 3.3]) and was also higher in mild/moderate asthma compared with normal controls (mean difference 1.29 KPa, p=0.024 [95% CI 0.17 to 2.4]). There was a trend for increased A-a gradient with increasing asthma severity independent of age, spirometry and gender; p=0.012 for trend by severity, effect size 0.82KPa, (95% CI 0.19, 1.6). Conclusion Severe asthma is associated with an increasing A-a gradient. The A-a gradient may reflect distal airway inflammation in severe asthma.