Double tracer gas single-breath washout: promising for clinics or just a toy for research?

Ventilation inhomogeneity reflects the efficiency of gas transport in both central, convection-dependent and peripheral, diffusion-dependent airways. In recent years, there has been a renaissance of studies using inertgas washout to assess ventilation inhomogeneity. This trend is due to promising results of multiple-breath washout (MBW) tests in different populations with various lung diseases, such as cystic fibrosis [1] and asthma [2]. Even though the MBW is becoming more widely used and better standardised, the long duration of the test is a potential impediment in daily clinical practice and in patients with severe lung disease. One alternative is to study the ventilation distribution based on one single breath, by performing the so-called single-breath washout (SBW). The SBW test is not new, having been done for the first time more than 60 years ago, by using 100% oxygen to study the washout of the resident nitrogen (N2) [3], or by using a double tracer gas (DTG) mixture, such as helium (He) and sulfur hexafluoride (SF6) [4]. Usually the SBW test is done during a vital capacity manoeuvre (VC-SBW) and there is a recent European Respiratory Society/American Thoracic Society consensus statement on how to perform the VC-SBW test [5]. A clear disadvantage of the VC-SBW test is the dependence on the vital capacity manoeuvre. This is particularly a problem in children younger than 12 years and, in general, results might be influenced by restricted ability of patients performing the manoeuvre [5]. Furthermore, it has been shown that deep inspiration influences expiratory slopes [6, 7]. In addition, analysis of SF6 and He needs a mass spectrometer, which is not practical for clinical use.