Do rebreathing manoeuvres for non-invasive measurement of cardiac output during maximum exercise test alter the main cardiopulmonary parameters?

Background: Inert gas rebreathing has been recently described as an emergent reliable non-invasive method for cardiac output determination during exercise, allowing a relevant improvement of cardiopulmonary exercise test clinical relevance. For cardiac output measurements by inert gas rebreathing, specific respiratory manoeuvres are needed which might affect pivotal cardiopulmonary exercise test parameters, such as exercise tolerance, oxygen uptake and ventilation vs carbon dioxide output (VE/VCO2) relationship slope. Method: We retrospectively analysed cardiopulmonary exercise testing of 181 heart failure patients who underwent both cardiopulmonary exercise testing and cardiopulmonary exercise test+cardiac output within two months (average 16 ± 15 days). All patients were in stable clinical conditions (New York Heart Association I–III) and on optimal medical therapy. Results: The majority of patients were in New York Heart Association Class I and II (78.8%), with a mean left ventricular ejection fraction of 31 ± 10%. No difference was found between the two tests in oxygen uptake at peak exercise (1101 (interquartile range 870–1418) ml/min at cardiopulmonary exercise test vs 1103 (844–1389) at cardiopulmonary exercise test-cardiac output) and at anaerobic threshold. However, anaerobic threshold and peak heart rate, peak workload (75 (58–101) watts and 64 (42–90), p < 0.01) and carbon dioxide output were significantly higher at cardiopulmonary exercise testing than at cardiopulmonary exercise test+cardiac output, whereas VE/VCO2 slope was higher at cardiopulmonary exercise test+cardiac output (30 (27–35) vs 33 (28–37), p < 0.01). Conclusion: The similar anaerobic threshold and peak oxygen uptake in the two tests with a lower peak workload and higher VE/VCO2 slope at cardiopulmonary exercise test+cardiac output suggest a higher respiratory work and consequent demand for respiratory muscle blood flow secondary to the ventilatory manoeuvres. Accordingly, VE/VCO2 slope and peak workload must be evaluated with caution during cardiopulmonary exercise test+cardiac output.