Differences of cardiac output measurements by open-circuit acetylene uptake in pulmonary arterial hypertension and chronic thromboembolic pulmonary hypertension: a cohort study

Single-breath open-circuit C2H2 uptake, thermodilution, and cardiopulmonary exercise testing were performed in 72 PAH and 32 CTEPH patients.In PAH patients the results for cardiac output obtained by the two methods showed an acceptable agreement with a mean difference of -0.16 L/min (95% CI -2.64 to 2.32 L/min). In contrast, the agreement was poorer in the CTEPH group with the difference being -0.56 L/min (95% CI -4.96 to 3.84 L/min). Functional dead space ventilation (44.5 ± 1.6 vs. 32.2 ± 1.4%, p < 0.001) and the mean arterial to end-tidal CO2 gradient (9.9 ± 0.8 vs. 4.1 ± 0.5 mmHg, p < 0.001) were significantly elevated among CTEPH patients.Cardiac output evaluation by the C2H2 technique should be interpreted with caution in CTEPH, as ventilation to perfusion mismatching might be more relevant than in PAH.The assessment of cardiac output is a crucial factor in the risk stratification and management of patients with pulmonary hypertension (PH) as it is directly related to the clinical severity of the disease as well as being one of the most important prognostic factors [1].Several methods have been introduced to measure cardiac output in humans. The thermodilution technique has been validated against the direct Fick method, which represents the "gold standard" when evaluating cardiac output [2]. It is used routinely to assess cardiac output in PH patients. However, the thermodilution method is an invasive technique requiring right heart catheterization of the patient. A reliable non-invasive method to determine the cardiac output would allow serial measurements and, thus, would facilitate the follow-up management of PH patients. Among the non-invasive techniques the acetylene (C2H2) rebreathing method has been validated against different other techniques and has gained wide acceptance [2-4]. A drawback to the method, however, is the build-up of carbon dioxide (CO2) as a result of rebreathing. Therefore, open-circuit methods have been developed [5,6]. C2H2 is a non