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A new asthma severity index: a predictor of near-fatal asthma?

Dept of Respiratory Medicine, Tan Tock Seng Hospital

CORRESPONDENCE: J. Abisheganaden, Dept of Respiratory Medicine, Tan Tock Seng Hospital, 11 Jalan Tan Tock Seng, Singapore, 308433. Fax: 65 3577871

Keywords: Excessive bronchoconstriction, intubation, near-fatal asthma

Received: August 25, 2000
Accepted March 3, 2001

Bronchial hyperresponsiveness (BHR), measured as the provocative dose of inhaled histamine or methacholine required to produce a 20% fall in forced expiratory volume in one second (FEV₁) (PD₂₀), is widely used as one of the indices of asthma severity. Excessive bronchoconstriction, reflected by the maximal percentage fall in forced vital capacity (FVC) at PD₂₀ (FVC %) during BHR testing, is considered to be the most important pathophysiological determinant in fatal asthma. The present study hypothesized that an index which combines both the ease of airway narrowing and excessive bronchoconstriction, FVC %/log(PD₂₀), may be better in assessing asthma severity, especially in those at risk of near-fatal attacks.

The dose-response curves of 46 asthmatics who underwent methacholine challenge testing were studied. Group 1 (n=14) patients had mild disease, Group 2 (n=21) had moderate disease and Group 3 (n=11) had severe disease, as classified according to the Global Initiative for Asthma. Nine patients had prior intubation for near-fatal asthma.

FVC %/log (PD₂₀) was better than FVC % and PD₂₀ in categorizing patients into the three severity groups (p<0.0001), but more importantly, it was able to discriminate patients with previous intubation from those without (p=0.04). It also correlated better with FEV₁ (% predicted), frequency of symptoms and inhaled steroid requirement than either index alone.

It is concluded that the percentage fall in forced vital capacity/log of the provocative dose causing a 20% fall in forced expiratory volume in one second combines information on the ease and excessive degrees of airway narrowing in asthma. This new index may be better at assessing asthma severity and in discriminating those at risk of near-fatal attacks.

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