Tracer Gas Physicochemical Properties Influence DLCO Measurements in Patients With Ventilation Heterogeneity

Measurement of the lung diffusing capacity for carbon monoxide (DLCO) requires an inert tracer gas-traditionally helium (He)-to estimate alveolar volume (VA). However, alternative gases such as argon (Ar), sulfur hexafluoride (SF₆), and methane (CH₄) differ markedly in diffusivity and molecular weight, potentially influencing results. This physiological pilot study evaluated 27 subjects (healthy subjects; airflow obstruction pattern with preserved DLCO; emphysema and finally patients exhibiting a combined obstructive-restrictive ventilatory pattern, consistent with combined pulmonary fibrosis and emphysema (CPFE) or interstitial lung disease (ILD) with concomitant airflow limitation.) using a multigas DLCO mixture. Measurements were obtained with a Cosmed Quark PFT 4 system and mass spectrometry, analyzing gas concentrations at 25%, 50%, and 75% of expiration to assess ventilation distribution. Differences in tracer gas diffusivity influence VA estimation and DLCO in diseased lungs (maximum group mean difference +26.7%, found in emphysema group, SF₆ vs He), while healthy subjects were unaffected. Standardization of tracer gas use would improve inter-laboratory comparability. When not feasible, sampling later in expiration may reduce variability, pending prospective validation.