SINGLE-BREATH CARBON MONOXIDE DIFFUSING CAPACITY*,
Section snippets
PATHWAY FOR DIFFUSION
The pathway for diffusion of O2 and CO, schematically illustrated in Figure 1, is helpful in understanding the test and its limitations and can aid in interpretation of the test. Oxygen and CO simultaneously diffuse across the alveolar capillary membrane, across a thin layer of plasma, across a red blood cell membrane, and within the red blood cell until they encounter and compete for the same hemoglobin binding sites. In a landmark paper published in 1957, Roughton and Forster40, 62 described
THE SINGLE-BREATH, BREATH-HOLDING MEASUREMENT OF DIFFUSING CAPACITY
Although several methods of measuring DlCO have been described in the literature, this article addresses only the single-breath with breath-holding method because: (1) it is the method most commonly used for day-to-day clinical testing today. (2) The testing procedure has been standardized by several respiratory and thoracic societies. (3) There are well-defined reference values and lower limits for healthy subjects. (4) Most commercial instruments use this method. (5) Most studies
INTERPRETING DIFFUSING CAPACITY—TEST QUALITY AND REFERENCE VALUES
The ATS published a guideline on interpretive issues for lung function testing, including the selection of reference values.8 The first step in interpretation is to evaluate and comment on test quality, looking at the sources of variability just addressed. Specifically, the tracings should be evaluated for: (1) the adequacy and timing of the inspired volume of test gas (criterion: an inspired volume > 90% of the largest previously measured vital capacity inhaled within 2.5 seconds in
APPROACH TO DISEASE CONDITIONS THAT MAY ALTER DIFFUSING CAPACITY
The Roughton-Forster equation may not be entirely correct conceptually37 but it does provide a nice guide to the effect of most pulmonary diseases on DlCO. Consider each term in the equation: Dm is altered by surface area, alveolar capillary membrane thickness, and physical diffusivity of tissue. Diseases that alter any or all of these elements reduce DlCO. These effects would reduce DlCO in pulmonary fibrosis and emphysema. Theta (θ) is altered by elements that affect the number of binding
SUMMARY
Measurement of DlCO remains a clinically useful way to assess transfer of gases across the lung. It is important, however, to be vigilant in controlling the sources of variation and to be aware of those that remain when interpreting the measured values.
Acknowledgment
The authors thank Janet Embry for editorial assistance in revising the manuscript.
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*STPD = standard temperature pressure dry
Address reprint requests to Robert O. Crapo, MD, Pulmonary Division, LDS Hospital, 8th Ave and C Street, Salt Lake City, UT 84143, e-mail: ldrcrapo@ihc.com