FOT better than spirometry for identifying poor asthma control

An alternative method for measuring lung function, oscillometry, is better at identifying patients with poorly controlled asthma when compared to spirometry, according to NSW researchers

Measurement of bronchodilator response (BDR) with the forced oscillation technique (FOT) is feasible in patients with asthma and BDR is associated worse asthma control, according to a study in 52 patients with asthma led by Dr Alice Cottee of the  Department of Respiratory Medicine, Concord Repatriation General Hospital.

In the study, FOT was performed during tidal breathing with a commercial device (Tremoflo C-100) immediately prior to spirometry, both before and after bronchodilator.

Overall, BDR was identified more frequently by FOT than spirometry (54% vs 27% of subjects).

BDR measured by reactance parameters also identified more subjects with poor asthma control than spirometry (69% vs 41%). BDR assessed by area under the reactance curve (AX) had the greatest sensitivity to detect poor asthma control (22 subjects; 69% sensitivity, specificity 75%), whereas spirometry identified it in fewer people but with fewer false positives (13 subjects, 41% sensitivity, specificity 95%). In addition, BDR was associated with spirometric BDR and correlated with asthma control.

Writing in Chest, the researchers said the presence of a bronchodilator response (BDR) is useful in establishing the diagnosis of asthma, and findings from previous studies had shown that a persistent BDR despite antiinflammatory treatment was associated with greater inhaled corticosteroid doses, lower FEV1, worse asthma control, higher exacerbation rates and increased mortality

This suggested that ongoing BDR may be an important but under-recognized ‘treatable trait’ target in asthma, they said.

They also noted that currently the ongoing monitoring of lung function in asthma is often limited to spirometry (FEV1) and the results are commonly not in agreement with reported symptoms

“The results from this study indicate that resistance alone relates poorly to symptom control.  In comparison, BDR assessed by reactance parameters [was] more sensitive than spirometry, identifying 17% more subjects with poor asthma control …” they said

They acknowledged that the improved sensitivity of BDR assessed by FOT came at the expense of reduced specificity compared to spirometry, but said that FOT could be used in conjunction with other methods to assess asthma.

“Since the identification of patients with poor asthma control is a clinical priority in the management of asthma, the tradeoff of improved sensitivity with a higher false positive rate is reasonable and may be offset by the combined use of FOT, spirometry, and clinical assessment,” they suggested.

The speculated that FOT assessment of BDR in reactance may provide a better picture of asthma control than spirometry by reflecting the opening up of small airways rather than airflow through more central larger airways.

“The peripheral airways … contribute very little to bulk flow measured at the mouth partly due to the significantly larger cumulative cross-sectional area when compared to the central airways. As a consequence, spirometry is less sensitive to detect more peripheral airways changes that may only be reflected in parameter such as reactance  and this potentially accounts for the stronger association seen with reactance parameters and asthma control,” they wrote.

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