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Interrelationship of airflow limitation and ventilation inhomogeneity in obstructive lung disease Ross, Jocelyn Susan


The purpose of this study was to investigate the interrelationship between airflow limitation and ventilation inhomogeneity in individuals with asthma and cystic fibrosis. Tests of forced expiration [the forced expiratory volume in one second (FEV₁) and the forced expiratory flow during 25-75% of the vital capacity (FEF[sub 25-75%])] and the single-breath nitrogen test [slope of Phase III (SBN₂/L%) ] were conducted in fourteen control subjects [mean age 24 years], fourteen individuals with asthma [mean age 23 years] and seven individuals with cystic fibrosis [mean age 20 years]. The tests were conducted according to the standards and procedures of the American Thoracic Society and the National Heart and Lung Institute. A questionnaire assessing activity level and subjective rating of symptoms was also administered. All subjects were tested on two occasions within a seven month period, from July to February. The subjects were able to complete all of the test procedures without difficulty. The control group had the highest mean values of 102% and 95% predicted for FEV₁ and FEF[sub 25-75%] respectively. The asthmatic group fell between the control and cystic fibrosis groups with mean FEV₁ and FEF[sub 25-75%] values of 80% and 59% predicted respectively. In the cystic fibrosis group, the mean FEV₁ was 44% predicted and the mean FEF[sub 25-75%] was 24% predicted. The SBN₂/L% mean values followed a similar trend with the lowest percent predicted values in the control group [115%] followed by the asthmatic group [153%] and the cystic fibrosis group with the highest mean [937%]. In the cystic fibrosis group, the SBN₂/L% was disproportionately impaired in comparison to the severity of airflow limitation. Over all groups, the coefficients of variation for the SBN₂/L% averaged less than 10%. Within a given test session, the tests of airflow limitation and ventilation inhomogeneity were correlated, however, the change in these test values between the two test sessions did not correlate. The difference in mean test values between the asthmatic and cystic fibrosis groups could be explained on the basis of the underlying mechanisms of airway obstruction in these diseases. The moderately impaired test performance in the asthmatic group may reflect the variable bronchoconstriction and airway inflammation in this disease, whereas the severely impaired test performance in the cystic fibrosis group is likely due to the greater degree and severity of obstruction from mucus plugging and bronchiectasis. In all groups, the lack of correlation of the change in test values in individuals between test sessions reflects the different pathophysiologic phenomenon that influences each test, and also the different time courses of recovery of airflow rates and ventilation homogeneity towards baseline levels. Thus, on different occasions, ventilation inhomogeneity can be more severe than expected from assessment of airflow limitation. In addition, within a test session in the cystic fibrosis group, the SBN₂/L% was more severely impaired than expected on the basis of spirometry. Since more severe ventilation inhomogeneity may exist in cystic fibrosis and asthma compared to airflow limitation, it may well be that the inclusion of the SBN₂ test [which this study has shown to have a satisfactory coefficient of variation] might be useful in the management of these chronic diseases. Further research is necessary to determine whether the SBN₂/L% can predict the rate of decline in spirometry in these individuals.

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