Clinical interpretation of DLCO and KCO: From rationale to clinical and research applications.

Single-breath methods for measuring the diffusing capacity of the lung for carbon monoxide (DLCO), Krogh's constant for CO (KCO), and alveolar volume (VA) play clinically vital roles in assessing lung diffusion. While the methodology is valid for the kinetics of normal lungs, appropriate interpretations are necessary for lung diseases involving emphysema and/or ventilation heterogeneity. Severe airflow limitations and ventilation heterogeneities lead to the underestimation of lung volume, calculated as VA, relative to the total lung capacity assessed using the helium closed-circuit method. Notably, the relative increase in KCO (DLCO/VA) compared to DLCO-resulting from increased blood flow per alveolar-capillary unit in small lungs with fibrosis-is a distinct feature of interstitial lung disease. Therefore, the combined assessment of DLCO and KCO may help elucidate the pathophysiology of emphysema and/or pulmonary fibrosis. This review aims to explain Krogh's equation, the difference between DLCO and D'LCO, the kinetics, and the clinical application of DLCO (or D'LCO) and KCO. Pulmonary function varies among ethnicities and races; thus, reference equations derived while considering anthropological traits are necessary. Additionally, the link between physiological theory, radiological findings, and the clinical relevance of DLCO and KCO is discussed, mostly based on Japanese studies. In this review, DLCO obtained from the single-breath method is referred to as "D'LCO"; however, for convenience, it is described as "DLCO," with the term "D'LCO" used only where necessary.