Background: As differences in gas exchange between pulmonary arterial hypertension (PAH) and chronic thromboembolic pulmonary hypertension (CTEPH) have been demonstrated, we asked if cardiac output measurements determined by acetylene (C2H2) uptake significantly differed in these diseases when compared to the thermodilution technique. Method: Single-breath open-circuit C2H2 uptake, thermodilution, and cardiopulmonary exercise testing were performed in 72 PAH and 32 CTEPH patients. Results: 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. Conclusion: 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.
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