O2 pulse slope correlates with stroke volume during exercise in patients with a Fontan circulation

Background Peak oxygen pulse (O 2 pulse=oxygen consumption/heart rate) is calculated by the product of stroke volume (SV) and oxygen extraction. It has been shown to be reduced in patients with a Fontan circulation. However, in the Fontan population, it may be a poor marker of SV. We propose that the slope of the O 2 pulse curve may be more reflective of SV during exercise. Methods We analysed cardiopulmonary exercise test data in 22 subjects with a Fontan circulation (cohort A) and examined the association between peak SV during exercise (aortic flow measured on exercise cardiac MRI), and O 2 pulse parameters (absolute O 2 pulse and O 2 pulse slopes up to anaerobic threshold (AT) and peak exercise). In a separate Fontan cohort (cohort B, n=131), associations between clinical characteristics and O 2 pulse kinetics were examined. Results In cohort A, peak aortic flow was moderately and significantly associated with O 2 pulseslope PEAK (r=0.47, p=0.02). However, neither absolute O 2 pulse AT nor O 2 pulse PEAK was significantly associated with peak aortic flow. In cohort B, O 2 pulseslope PEAK and O 2 pulseslope AT were not significantly associated with clinical parameters, apart from a weak association with forced vital capacity. Conclusion The slope of the O 2 pulse curve to peak exercise may be more reflective of peak SV in the Fontan population than a single peak O 2 pulse value.