BACKGROUND We have consistently argued that mild asthma is an important underlying aetiological factor in patients with severe symptomatic hyperventilation. While hyperventilation has been demonstrated in acute asthma, there have been few studies in mild chronic asthma, and mechanisms are uncertain.

RESULTS Despite the absence of current asthmatic symptoms, no clinical evidence of hyperventilation, and normal lung function in the patients with asthma, Paco ₂ and Petco ₂ were significantly (p<0.01) lower in the patients than in the control group (mean (SD) Paco ₂ 4.96 (0.43) kPa for patients versus 5.27 (0.38) kPa for controls (mean difference 0.31 kPa, 95% confidence interval (CI) 0.06 to 0.56, p<0.02)). Petco ₂ was very similar to Paco ₂ in both groups (mean (SD) Petco ₂ 4.89 (0.47) kPa for the patients and 5.28 (0.40) for the controls (mean difference 0.39 kPa, 95% CI 0.12 to 0.66, p<0.01)). There was no significant difference in ventilation or respiratory pattern between the two groups. The reduced Paco ₂ in the asthmatic patients correlated significantly with the concentration of methacholine provoking a fall in FEV₁ of more than 20% (PC₂₀) (r = 0.56, p<0.01) but not with any aspect of lung function, eosinophil count, or anxiety/depression.

CONCLUSION Mild asymptomatic asthma is not associated with clinically significant hyperventilation but is associated with a significant reduction in both arterial and end tidal Pco ₂ which relates to airway hyperresponsiveness rather than to the degree of airway obstruction or mucosal inflammation. Anxiety and depression appear not to be implicated.