Comparison of Four Methods for Calculating Diffusing Capacity by the Single Breath Method: Conclusion
Our results with the 3PIT method can be only loosely compared with results of Graham et al who documented the differences in Deo using Og, JM, ESP, and three-equation methods in patients with emphysema, asthma, and cystic fibrosis. For both Og and JM methods, Graham et al allowed a 1-L dead space washout volume and collected a 1-L gas sample, whereas we used either a 750-ml (VC > 2.0 L) or 500-ml (VC < 2.0 L) dead space washout volume and a 500-ml gas sample. In the three-equation method, Graham et al used the entire expired volume in the computation whereas we were limited to the 500-ml gas sample that was collected by the automated equipment. They found that the ESP method gave the largest calculated Deo compared with Og, JM, and three-equation Deo in patients with emphysema or cystic fibrosis, and 4 percent larger than three-equation Deo in patients with asthma. The pulmonary function abnormalities in our 46 patients with Og > 3PIT included restriction, obstruction, isolated low Deo and reduced FVC and FEVr Graham et al did not state the severity of obstruction in their patient groups, nor did they document differences in Deo by the various methods in patients with pulmonary function abnormalities other than airflow limitation.
The dependence of the ratio of Deo using 3PIT to Og, JM, or ESP methods on TLC, MI, and Deo (Table 4) is difficult to interpret. Apparently, the variations in inspiratory and expiratory flow profiles that the 3PIT calculation takes into account have different influences in patients with abnormal TLC, inefficient gas mixing, and low Deo. This finding needs further experimental support in controlled experiments or modeling studies to draw firm conclusions. www.canadian-familypharmacy.com
The procedure chosen for performing and calculating Deo in the clinical laboratory should be the one that is easiest to perform for both patients and technical personnel (ease of use), give the most reproducible answer (precision), and produces a Deo value that is correct (accuracy). Among the methods described herein, the JM, Og, and ESP methods were equal in ease of operation, since the maneuver was the same and calculation algorithms very similar. Though the 3PIT technique uses the same maneuver, the calculation of Deo involves a complicated iterative algorithm that takes up to 3 min per test, depending on computer hardware. Though the 3PIT technique may have theoretical advantages, in practice the technique was not more reproducible than the other methods, and the Deo answers were not substantially different compared with Og and JM Dcos in the patients with moderate to severe obstructive or restrictive patterns of pulmonary function. Therefore, the 3PIT offers little advantage compared with Og and JM methods when performing Deo using single-breath technique with a fixed alveolar sample volume of about 500 ml.
Either the Og or JM timing method is recommended because both are simple to perform and to calculate, and appear to be as reproducible and accurate as any other technique for calculating single-breath Deo using a single alveolar gas sample of about 500 = ml volume in accordance with the ATS recommendations. The Og Deo will average 3 to 8 percent (0.8 to 2.2 ml/min/mm Hg) less than JM Deo in normal individuals, though average differences should be less in patients with lung disease.