Comparison of Four Methods for Calculating Diffusing Capacity by the Single Breath Method: Standard Deviation
Predicted values were taken from Miller et al for Deo and TLC, and Miller for FVC and FEV,. Standing height (cm) without shoes and weight (kg) were measured immediately prior to pulmonary function evaluation. These categories are used as a framework within which to illustrate the dependence of the differences among Deo values on pulmonary function abnormality. The “normal” category is narrowly defined, with a higher cutoff than the example given by the ATS and probably higher than the cutoff recommended by the Intermountain Thoracic Society (ITS). The “mild” categories of restriction and obstruction are, therefore, also broader than recommended by ATS or ITS, and include “borderline” cases. The severe category of obstruction has a lower cutoff than given in the example by the ATS and includes patients with severe and very severe obstruction. Our definition of restriction includes patients with combined restrictive and obstructive defects. We define for completeness a category that includes isolated reduced Deo in absence of obstruction or restriction, and a category of reduced VC and/or FEV,, both without evidence for obstruction or restriction as defined in this article. canadian health & care mall
Data analysis was performed using SAS. Mean and standard deviation (SD) of Deo values was calculated for each of the 283 patients with two or more acceptable maneuvers. The differences in mean Deo values and within-patient SD (WP-SD) of Deo by the various protocols were determined using paired Student’s t tests. To determine if the differences in Deo were a function of other pulmonary function variables, stepwise multiple linear regression analysis was performed on the ratios of the first acceptable Deo value (eg, Og/JM) as the dependent variable and TLC, FEV,, FVC, FEV,/FVC, mean Deo (Og), mixing index (MI), IT, ET, and LT as independent variables. The ratio of Deo values is equal to the inverse ratio of breath-hold times for the Og, JM, and ESP Deo comparisons, since every term except BHT will cancel from the right side of equation 1 when the equations used to calculate Deo are divided. In a stepwise regression procedure with backward elimination of nonsignificant predictor variables, the dependent variable was tested for significant correlation with the independent variables (predictors) using multiple linear regression. The least significant predictor variable was then removed and the regression analysis was repeated until only predictor variables with an acceptable level of significance remained in the model. We used a high level of significance (p < 0.005) to minimize inclusion of predictor variables that had statistical but little physiologic significance.