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Page 4 of 7 Wang-Giuffre et al. Vessel Plus 2022;6:29 https://dx.doi.org/10.20517/2574-1209.2021.98

circulation have problems exercising with anaerobic metabolism. It is difficult to sustain high intensity
exercise in a Fontan circulation, as there is decreased flow through the pulmonary vasculature due to lack of
subpulmonary pump and increased pulmonary vascular resistance, thus diminishing cardiac output with
progressive exercise. For these reasons, submaximal measures can also be used to monitor Fontan function
over time.

VO at anaerobic threshold
2
Anaerobic threshold (AT) is the point during exercise at which the patient changes from aerobic to
anaerobic metabolism. AT is decreased in patients with poor aerobic fitness as well as heart failure patients.
While VO peaks in Fontan patients are generally decreased, VO at the anaerobic threshold is less impaired
2
2
[25]
and is generally ~75% expected as opposed to 65% expected .
There is no data to suggest a clear cut-off for significantly increased risk for mortality; however, the lower
the AT, the more risk of morbidity and mortality. Fernandes et al. suggested that a cut-off of
[26]
9 mL/kg/min was associated with a significantly increased risk of mortality, while Diller et al. showed a
[22]
correlation between a lower VO at AT and hospitalization.
2
Interestingly, some interventions such as pulmonary vasodilators and strength training have shown
improvement in the VO at AT [3,27] . Therefore, despite a lack of clear cut-off, VO at AT is a measure that
2
2
can be trended over time to assess a Fontan’s function and aerobic fitness in Fontan patients.
Oxygen uptake efficiency slope
Oxygen uptake efficiency slope (OUES) is a submaximal relationship of oxygen consumption and minute
ventilation, is calculated from submaximal data, and does not depend on intra/inter observer
variability [28,29] . It is calculated using a logarithmic curve using the equation: VO = A log10 VE + B, where
2
“A” is the OUES, VE is minute ventilation, and it has been well validated in healthy adults and patients with
heart failure .
[28]
The higher OUES or the steeper the slope, the more efficient the oxygen uptake. It has been shown to
strongly predict peak VO in Fontan patients and is significantly lower in Fontan patients compared to
2
[30]
healthy subjects and other congenital heart disease patients, such as Tetralogy of Fallot . Multiple studies
have shown OUES as an excellent predictor of adverse cardiac events in Tetralogy of Fallot patients.

OUES has been validated as an excellent predictor of VO peak in Fontan patients. However,
2
[31]
[30]
Bongers et al. and Giardini et al. noted that in Fontan patients with baseline cyanosis, OUES taken
during the first half of exercise did not correlate with peak OUES, this was not the finding in noncyanotic
Fontan patients. A study from Taiwan showed an OUES ≤ 45% predicted 2-year cardiac morbidity and
hospitalization with a sensitivity of 64% and specificity of 93% .
[32]
OUES is a promising independent predictor of outcomes in Fontan patients, and further studies need to be
performed to validate the usefulness of this measure.

Exercise oscillatory ventilation
Exercise oscillatory ventilation (EOV) is a common measure in adult heart failure patients and has long
been established as a poor prognostic indicator in adult heart failure [33-35] . In a study from Japan, it was noted
that 58% of their young Fontan patients displayed EOV .
[36]