Effective argon-FRC with a rebreathing technique in children with acute asthma

Abstract

In children with asthma the measurement of functional residual capacity (FRC) with standard dilution techniques requires long rebreathing times of the inert gas, i.e., helium, to reach alveolar units with long time constants. A modification of the argon-freon-22 rebreathing technique enables argon to dilute in the readily accessible lung volume and potentially can give a measurement of FRC. However, the rebreathing tests cannot be prolonged for more than 1 min thus allowing argon to reach only the rapidly accessible lung units which are grossly useful in gas exchange (effective FRC, EFRC). The aim of this study was to measure the EFRC and standard lung function in children with acute severe asthma, assess their response to nebulized salbuta-mol, and evaluate the relations of the EFRC response to baseline spirometric measurements. Twenty-four asthmatic children who were admitted to the hospital with an acute asthma attack had spirometry and triplicate EFRC measurements before and after treatment with nebulized salbutamol. Eighteen patients had repeated the respiratory tests 50 days later when they had fully recovered. A significant proportion (28.9%) of the argon traces obtained at baseline before treatment did not equilibrate. However, in the great majority of these tests the oscillation of the argon traces over the last 3 breaths of the test was < 5% of the simultaneous argon concentration. The EFRC values derived from the argon traces with minimal oscillation (< 5%) were reduced during the acute asthma attack when compared with the paired values obtained after recovery (P = 0.03). The administration of salbutamol caused a fall from the baseline EFRC (P = 0.05). There was a significant correlation between the EFRC and FVC values (expressed as percentage of normal) in all 3 of the test occasions. The changes in the pretreatment and posttreatment EFRC values induced by recovery were correlated to the corresponding changes in the spirometric indices. In conclusion, rebreathing techniques can give quick measurements of EFRC in patients with acute asthma. The changes induced by recovery in baseline and postsalb-utamol EFRC follow the pattern of the corresponding spirometric indices changes and not the expected typical FRC pattern.