Comparison of Four Methods for Calculating Diffusing Capacity by the Single Breath Method: Pulmonary Function Abnormalities
For the 3PIT method, we used the iterative search procedure as described by Graham et al. The process started with a calculation of residual volume, RV = VA — IVC + VDS. Equations 6 (inspiration), 7 (breath hold), and 9 (expiration) of Graham et al were then used to iteratively calculate Deo. In equation 6, every 30 ms the measured incremental inspired volume (obtained from the recorded spirogram) was added to the lung volume, and the added CO and Ne were used to recalculate an instantaneous value for alveolar CO and Ne concentrations using gas dilution and diffusion equations. Inspired gas concentrations were assumed to equal concentrations in the inspired gas tank, obtained by chromatography prior to the first Deo maneuver in each patient. Equation 7 calculated the change in alveolar CO concentration during breath hold, assuming constant lung volume and uniform dilution of the inspired gas into the alveolar volume. Equation 9 calculated mean collected sample concentrations of Ne and CO using the recorded spirogram and gas diffusion equations to update lung volume and alveolar CO concentration every 30 ms. If the calculated CO concentration in the 500-ml alveolar sample (С О c) differed from the actual measured value ( С О m), then the Deo used in the diffusion equations was changed to Dco^ = Dcoo|d X [ С О J С О m + 1] X 2 where Dco^ and Dco^ are the values for Deo used in the previous and next iteration, respectively. The iterative calculation procedure started with a Deo value of 20 and iteration continued until Deo new differed from Dco(4d by less than 0.1 percent. Because gas chromatography measures wet gas concentrations, we adjusted the exhaled gas concentration to convert it to dry gas fraction, and during the calculation, all gas volumes were expressed as standard temperature and pressure, dry (STPD).
The TLC was determined in a body plethysmograph (Medical Graphics Corporation, St. Paul, Minn) by adding thoracic gas volume determined during respiratory efforts against a closed shutter plus inspiratory capacity determined immediately following the closed-shut-ter maneuvers. The average TLC of at least two technically acceptable maneuvers was reported. Forced vital capacity (FVC) and forced expiratory volume in 1 s (FVE,) were determined with a pneumotachograph-based pulmonary function analyzer (Medical Graphics Corporation, St. Paul, Minn) that was calibrated daily. At least three acceptable maneuvers were performed and the FEV, and FVC were both taken from the acceptable maneuver with the largest sum of FVC, FEV, and one third of peak flow.
Data are presented as mean values within categories of pulmonary function abnormalities, defined as follows:
Normal: TLC, FVC, and FEV, all > 85 percent predicted, Deo>80 percent predicted, and FEV,/FVC > 70 percent.
Mild obstruction: FEV,/FVC ratio < 70 percent, with FEV, between 85 percent and 60 percent predicted and TLC > 85 percent predicted.
Moderate obstruction: FEV,/FVC ratio < 70 percent, with FEV, between 60 percent and 40 percent predicted and TLC > 85 percent predicted.
Severe obstruction: FEV,/FVC ratio < 70 percent, with FEV, 40 percent predicted and TLC > 85 percent predicted.
Mild restriction: TLC between 85 percent and 60 percent predicted.
Moderate restriction: TLC < 60 percent predicted.
Reduced Deo: TLC, FVC, and FEV, > 85 percent predicted with FEV,/FVC > 70 percent and Deo < 80 percent predicted.
Reduced FVC, FEV,: TLC > 85 percent predicted, FEV,/FVC>70 and FVC or FEV, < 85 percent predicted.