Small airway dysfunction may explain part of why obese patients with asthma have more attacks

Design

Multicentre cross-sectional observational study with mediation analysis

Population

1,169 adults with asthma

Primary outcome

Associations between BMI, oscillometry-defined small airway dysfunction and severe exacerbations

Effect

SAD with obesity aOR 2.11; with severe exacerbations aOR 2.01; mediated 26–41% of the obesity link

A multicentre observational study of 1,169 adults with asthma combined spirometry with impulse oscillometry, which detects small airway dysfunction that spirometry often misses.

Small airway dysfunction was more common with a BMI of 30 or more, and oscillometry worsened steeply beyond a BMI of about 28 to 30. It was associated with obesity (adjusted OR 2.11) and with severe exacerbations in the previous year (adjusted OR 2.01). In mediation analysis, it accounted for about 26% to 41% of the association between obesity and exacerbations. Spirometry added little.

This is cross-sectional and the exacerbations were in the past year, so it cannot show that small airway dysfunction causes attacks or that treating it prevents them. It does suggest that in an obese patient with frequent exacerbations and near-normal spirometry, the small airways deserve a look, and that weight loss and inhaler choice may both matter.

• Consider oscillometry, where available, in obese patients with asthma who exacerbate despite near-normal spirometry.
• Treat obesity as part of asthma management in patients with frequent exacerbations.
• Check inhaler technique, as delivering drug to the small airways depends on it.
• Do not rely on normal spirometry alone to reassure an obese patient who keeps exacerbating.

Why it matters

It points to a measurable, possibly treatable reason obese patients with asthma do worse.

Don't overread it

This was observational; it does not show that treating small airways would prevent exacerbations.

The statistics, in plain English

Mediation analysis estimates how much of one association runs through a third factor. Saying 26% to 41% of the obesity link runs through small airways is a statistical estimate from observational data, not proof of a causal pathway.

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