Examination of the Carbon Monoxide Diffusing Capacity (DLCO) in Relation to Its Kco and VA Components

The single-breath carbon monoxide diffusing capacity (DLCO) is the product of two measurements during breath holding at full inflation: (1) the rate constant for carbon monoxide uptake from alveolar gas (kco [minute(-1)]) and (2) the accessible alveolar volume (VA). kco expressed per mm Hg alveolar dry gas pressure (Pb*) as kco/Pb*, and then multiplied by VA, equals DLCO; thus, DLCO divided by VA (DLCO/VA, also called Kco) is only kco/Pb* in different units, remaining, essentially, a rate constant. The notion that DLCO/VA corrects DLCO for reduced VA is physiologically incorrect, because DLCO/VA is not constant as VA changes; thus, the term Kco reflects the physiology more appropriately. Crucially, the same DLCO may occur with various combinations of Kco and VA, each suggesting different pathologies. Decreased Kco occurs in alveolar-capillary damage, microvascular pathology, or anemia. Increased Kco occurs with (1) failure to expand normal lungs to predicted full inflation (extrapulmonary restriction); or (2) increased capillary volume and flow, either globally (left-to-right intracardiac shunting) or from flow and volume diversion from lost or damaged units to surviving normal units (e.g., pneumonectomy). Decreased VA occurs in (1) reduced alveolar expansion, (2) alveolar damage or loss, or (3) maldistribution of inspired gases with airflow obstruction. Kco will be greater than 120% predicted incase 1, 100-120% in case 2, and 40-120% in case 3, depending on pathology. Kco and VA values should be available to clinicians, as fundamental to understanding the clinical implications of DLCO. The diffusing capacity for nitric oxide (DLNO), and the DLNO/DLCO ratio, provide additional insights.