Resp Care
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All pulmonologists, including those recently completing training, should be competent in critically evaluating and interpreting pulmonary function tests (PFTs). In addition, some authorities recommend that respiratory therapists learn to provide preliminary PFT interpretations for the medical directors of PFT labs. The 2005 American Thoracic Society/European Respiratory Society guidelines for interpreting PFTs lack recommendations for the best reference equations for lung volumes and diffusing capacity of the lung for carbon monoxide (D(LCO)), and lack reference equations for non-whites. ⋯ The "nonspecific pattern" occurs in about 15% of patients referred to a PFT lab, but it has many clinical correlates and the course is usually benign. Less common PFT patterns and those resulting from comorbid conditions (such as obesity, respiratory muscle weakness, or heart failure) are not discussed by the guidelines. More than half of patients with interstitial lung disease have a normal ratio of D(LCO)/V(A) (alveolar volume), and many have a normal total lung capacity.
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Single-breath diffusing capacity of the lung for carbon monoxide (D(LCO)) is a common pulmonary function test that measures the ability of the lung to exchange gas across the alveolar-capillary interface. D(LCO) testing is used to narrow the differential diagnosis of obstructive and restrictive lung disease, to aid in disability and transplant assessment, and to monitor medication toxicity. ⋯ Variability is attributable to differences in equipment, testing conditions, patient factors, and reference equations. Laboratories can minimize variability by ensuring that equipment meets recommended standards, implementing effective quality control programs, standardizing testing conditions and testing procedures, and accounting for pertinent patient characteristics.
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We are still at the early phase of finding useful phenotypes in COPD that can guide therapy. However, in a simple sense, "sick patients die." Many phenotypic measurements of severity correlate with mortality in COPD: FEV(1), the ratio of inspiratory capacity to total lung capacity (IC/TLC), diffusing capacity of the lung for carbon monoxide (D(LCO)), 6-min walk distance, and maximum oxygen (O(2)) consumption or maximum watts on exercise testing. However, composite parameters, such as the BODE index (body mass index, air flow obstruction, dyspnea, exercise capacity), perform better, likely because they capture different aspects of severity that affect functional impairment and risk of death. ⋯ The best promise for the future seems to be in composite phenotypes or scores, both for distinguishing asthma from COPD, and for guiding therapeutic options. It may be better to throw out the old, limiting diagnostic concepts. If, instead, we start from outcomes of interest, perhaps we can work back to predictors of these outcomes, and organize new diagnostic entities that have predictive relevance for treatment choices, functional outcomes, and mortality.
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Lung volumes are considered part of a complete pulmonary function test, but their value for enhancing clinical decision making is unknown. Unlike spirometry and diffusing capacity of the lung for carbon monoxide (D(LCO)), which do contribute to confirming or excluding a diagnosis, there are few clear indications when lung volumes are discriminatory. Confirming "restriction" when vital capacity (VC) or FVC is reduced is perhaps the most important. ⋯ Body plethysmography is often considered more accurate than gas dilution methods in the presence of obstruction. However, the differences between techniques are not completely understood. Newer approaches such as computed tomography, although not suitable for routine testing, may help to delineate the true underlying physiology.