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Open Access Research article

Random variation of inspiratory lung function parameters in patients with COPD: a diagnostic accuracy study

Frank J Visser1*, Sunil Ramlal1, Ben Pelzer3, PN Richard Dekhuijzen2 and Yvonne F Heijdra2

Author Affiliations

1 Dept. of Pulmonology, Canisius Wilhelmina Hospital, Nijmegen, 6532 SZ, the Netherlands

2 Dept. of Pulmonology, Radboud University Medical Centre, Nijmegen, 6525 GA, the Netherlands

3 Dept. of Social Science Research Methods, Radboud University, Nijmegen, 6525 GA, the Netherlands

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BMC Pulmonary Medicine 2010, 10:28  doi:10.1186/1471-2466-10-28

Published: 14 May 2010

Abstract

Background

In chronic obstructive pulmonary disease (COPD), the response of the forced expiratory volume in 1 second (FEV1) after bronchodilator application is weak. Inspiratory parameters like the forced inspiratory volume in 1 second (FIV1) and inspiratory capacity (IC) can be responsive to bronchodilators. In an individual patient with COPD, a significant bronchodilator response must at least exceed the random variation for that parameter. Therefore, it is important that the type of scatter is homoscedastic, as the chance of underestimating or overestimating the random variation for low or high parameter values is minimized. The aim of this study is to investigate the random variation (type and quantity) of inspiratory parameters.

Methods

In 79 stable COPD patients, spirometry was performed.

The forced inspiratory volume in 1 second (FIV1), inspiratory capacity (IC), maximal inspiratory flow at 50% (MIF50) and peak inspiratory flow (PIF) were measured five times in one day and again within two weeks of the first measurement. The values of these parameters, taken within one hour, within one day and between two different days, were compared. The coefficient of repeatability (CR) was calculated, and, in addition, linear regression was performed to investigate the type of scatter (homo- or heteroscedastic) of the measured parameters.

Results

The type of scatter was heteroscedastic for all of the parameters when the differences were expressed as absolute values; however, when the differences were expressed as the percent change from the initial values, we found a more homoscedastic scatter. The CR within one hour of each parameter expressed as the percent change from the initial value was: IC, 19%; FIV1, 14%; PIF, 18%; MEF50, 21%.

Conclusions

To obtain a more homoscedastic scatter, percentage changes in FIV1, IC and MIF50 are more appropriate than absolute changes.

In an individual patient with COPD, a significant improvement for a particular parameter must at least exceed the above-mentioned CR.