In defence of the carbon monoxide transfer coefficientKCO(TL/VA)

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Abstract
The carbon monoxide transfer factor (TL,CO) is the product of the two primary measurements during breath-holding, the CO transfer coefficient (KCO) and the alveolar volume (VA).KCOis essentially the rate constant for alveolar CO uptake (Krogh'skCO), and in healthy subjects, increases whenVAis reduced by submaximal inflation, or when pulmonary blood flow increases. Recently, new reference values were proposed for clinical use which included the observedVAat full inflation; this was claimed to “eliminate the need forKCO”.In this commentary, some mechanismse.g.respiratory muscle weakness, lung resection, diffuse alveolar damage and airflow obstruction, which decrease or increase total lung capacity (TLC) are reviewed.Even when alveolar structure and function are normal, the change inKCOat a givenVAvaries according to the underlying pathophysiological mechanism. The advantages and disadvantages of normalizingKCOandTL,COto predisease predicted TLC or to the patient's actualVA(using lack of expansion or loss of alveolar units models) are considered.Examination of carbon monoxide transfer coefficient and alveolar volume separately provides information on disease pathophysiology which cannot be obtained from their product, the carbon monoxide transfer factor.