Comparison of Four Methods for Calculating Diffusing Capacity by the Single Breath Method
The diffusing capacity of the lungs for carbon monoxide (Deo) can be estimated by a number of techniques, including steady state, single breath, and rebreathing. The single-breath method is most commonly used in the United States, probably because of better standardization and because it does not require a rapid CO analyzer. In the singlebreath method, patients are required to take a full vital capacity breath of the test gas mixture, hold their breath for about 10 s, then exhale. The equation used for calculation of diffusing capacity has breath-hold time in the denominator, where Deo is the diffusing capacity of the lungs for carbon monoxide, Va is the alveolar volume accessible to the inhaled test gas mixture, Pb is barometric pressure in mm Hg, Fa,x and Fi,x are the alveolar and inhaled concentrations of gases (carbon monoxide, CO, or Neon, Ne), IVC is the volume of inspired gas, and Vds is the sum of system (35 ml) and anatomic (150 ml) dead spaces. The initial alveolar CO concentration (Fa,co[0]) is calculated from Fi,co times a dilution factor (equation 3). All calculations are performed with gas volumes expressed STPD, though Va is reported in BTPS for comparison with plethysmographic TLC.
Implicit assumptions underlying the calculations are (1) that the alveolar surface area available for diffusion is exposed to the inhaled gas mixture for the entire time of breath hold, and (2) there is both homogeneity in delivery of inspired gas to the entire Va and homogeneity of ratios of regional Deo to Va.2 These assumptions are not strictly valid and breath-hold time becomes an approximation of the time that the test gas mixture is exposed to the exchanging surface area. canadian neighborhood pharmacy
There have been a number of algorithms proposed to define the breath-hold time, including those of Jones-Meade (JM), Ogilvie (Og) et al, and the Epidemiologic Standardization Project (ESP). In addition, a more physiologic precise algorithm has been proposed that uses three separate diffusion equations in inspiration, breath hold at TLC, and expiration. The three-equation method in its complete form requires continuous sampling of expired gas CO concentration throughout a full expiration following breath hold. Implementation of a limited form of the three-equation method is possible using only one alveolar gas sample. We refer to this method as the three-phase iterative technique (3PIT). The 3PIT method has potential advantages over the Og, JM, and ESP methods in that it takes into account the exact pattern of inspiratory and expiratory flow during the maneuver, whereas the JM, Og, and ESP techniques assume that inspiration and gas collection times are rapid. The purpose of this report was to compare the three traditional timing methods and the 3PIT method in a clinical pulmonary function laboratory setting while keeping sample volume constant.
We addressed two issues. First, what are the differences in average Deo produced by each method, and are such differences related to pulmonary function abnormalities? Second, is the maneuver-to-maneuver reproducibility of the Deo similar among the methods? We calculated diffusing capacities using the four methods in each maneuver performed by a group of 334 patients with a wide range of pulmonary function abnormalities.