Oxygen Pulse Kinetics in Fontan Patients During Treadmill Ramp Protocol Cardiopulmonary Exercise Testing

After single-ventricle palliation, patients have variable long-term functional outcomes. Cardiopulmonary exercise testing (CPET) is an assessment tool used to quantify functional outcome. Oxygen pulse kinetics during CPET, which can be an important indicator of dynamic changes in stroke volume reserve, has not been systematically studied in this population. This study aimed to analyze oxygen pulse kinetics during a treadmill ramp protocol among patients with Fontan physiology compared with that of normal subjects and to explore the ability of oxygen pulse kinetics to define functional status further. Peak oxygen pulse and change in oxygen pulse during ramp treadmill CPET were retrospectively collected and compared between 44 Fontan patients and 85 age- and sex-matched control subjects. The peak oxygen pulse was significantly lower in the Fontan group (9.80 ± 4.11 ml/beat) than in the control group (13.62 ± 4.7 ml/beat) ( p ≤ 0.001). The resting oxygen pulse did not differ between the two groups (3.13 ± 1.23 vs. 3.09 ± 1.33 ml/beat; p = 0.88). The oxygen pulse was higher in the patients with chronotropic insufficiency, but the difference was not statistically significant (11.11 ± 4.97 vs. 9.25 ± 3.63 ml/beat; p = 0.17). Regression analysis showed a significant difference in the slope of the oxygen pulse-to-workload relationship. The Fontan group showed no relation between degree of reduction in the oxygen pulse from peak to end of exercise and the underlying defect, peak heart rate, peak oxygen consumption, ventilatory anaerobic threshold (VAT), expired volume (VE)/carbon dioxide output (VCO 2 ) at the VAT, maximum heart rate, or minimum oxygen saturation. Analysis of oxygen pulse kinetics in Fontan patients suggests that there is an early and progressive limitation in stroke volume compared with control subjects. This limitation may be partially masked by increased oxygen extraction. In patients with chronotropic insufficiency, absolute or body surface area-indexed oxygen pulse may be higher than in those with a normal heart rate response. A composite assessment of the oxygen pulse and oxygen pulse kinetics, including the oxygen pulse slope and the percentage of reduction in oxygen pulse from peak to end of exercise, may allow a more comprehensive assessment of the degree of cardiac limitation in this group of patients.