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In defence of the carbon monoxide transfer coefficient K_CO (T_L/V_A)

Division of Respiratory Medicine, National Heart and Lung Institute, Imperial College School of Medicine, Hammersmith Hospital Campus, London, UK

CORRESPONDENCE: J.M.B. Hughes, Division of Respiratory Medicine, Imperial College School of Medicine, Hammersmith Hospital Campus, Ducane Road, London, W12 0NN, UK. Fax: 44 2088789681

Keywords: carbon monoxide, diffusing capacity, gas exchange, pulmonary function, transfer coefficient, transfer factor

Received: February 17, 2000
Accepted June 26, 2000

Abstract

The carbon monoxide transfer factor (T_L,CO) is the product of the two primary measurements during breath-holding, the CO transfer coefficient (K_CO) and the alveolar volume (V_A). K_CO is essentially the rate constant for alveolar CO uptake (Krogh's k_CO), and in healthy subjects, increases when V_A is reduced by submaximal inflation, or when pulmonary blood flow increases. Recently, new reference values were proposed for clinical use which included the observed V_A at full inflation; this was claimed to "eliminate the need for K_CO".

In this commentary, some mechanisms e.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 in K_CO at a given V_A varies according to the underlying pathophysiological mechanism. The advantages and disadvantages of normalizing K_CO and T_L,CO to predisease predicted TLC or to the patient's actual V_A (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.

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