Relationship between sensitivity to dyspnea and fluctuating peak expiratory flow rate in the absence of asthma symptoms.

Background: Exacerbation of asthma has a negative impact on quality of life and increases the risk of fatal asthma. One of the known risk factors for patients with a history of near-fatal asthma is reduced sensitivity to dyspnea.
Objective: We aimed to identify patients with such risk before they experienced severe exacerbation of asthma.
Methods: We analyzed asthma symptoms and peak expiratory flow rate (PEFR) values of 53 patients recorded daily in a diary over a mean period of 274 days. Patients matched their symptoms to one of eight categories ranging in severity from 'absent' to 'severe attack'. We then analyzed the relationship between PEFR and asthma symptoms by dividing the PEFR value by the values of clinical parameters, including asthma symptom level.
Results: Average PEFR was 75.2% (50.5-100%) in the 'absent' symptom category, 64.5% (36.6-92.6%) in 'wheeze', 57.3% (25.0-94.7%) in 'mild attack' and 43.6% (20.4-83.1%) in 'moderate attack', with the personal best reading taken as 100%. Thus, differences in PEFR in patients in the same symptom category varied widely. PEFR in wheeze, mild attack and moderate attack did not correlate significantly with duration of asthma, forced expiratory volume in one second or proportion of personal best to standard predicted PEFR values. These PEFRs showed no significant difference in groups divided by type of regular treatment, but showed a significant negative correlation with the coefficient of variation (CV) of PEFR when asthma symptoms were absent. CV for absent symptoms should be between +4.0 and -4.0% when using regression analysis to measure PEFR if the decreased PEFR is in agreement with guidelines.
Conclusion: To determine which patients have reduced sensitivity to dyspnea, CV of PEFR should be considered when asthma symptoms are reported as absent. When patients present with more than 8% fluctuation in PEFR, we should intervene in their treatment, even when they claim to be stable.