THE FONTAN OPERATION HAS BEEN THE STANDARD
of care for the surgical palliation of children
with univentricular congenital heart disease
for nearly four decades.1,2 Although this procedure
can create a stable physiology through early
adulthood, there are inherent limitations related
to the loss of a sub-pulmonary pumping chamber
and the absence of pulsatile flow in the pulmonary
arteries. The direct connection of the inferior and
superior venae cavae to the pulmonary arteries
results in a circulation characterised by chronic low
cardiac output and elevated central venous pressure.
These abnormalities are generally well tolerated for
a number of years, but result in impairment in
exercise capacity starting from a very young age.
Poor exercise performance, objectively measured
by decreased maximal minute oxygen consumption
(VO2), is well documented in Fontan patients.3–8
Studies have consistently demonstrated that average
maximal VO2 in Fontan patients is 60–65% of
predicted value for age and gender.8,9 In a landmark
cross-sectional analysis of 411 Fontan patients, the
Pediatric Heart Network Investigators demonstrated
a population mean maximal VO2 of
26.3 6 6.9 ml/kg/min, about 65% predicated value
for age and gender. However, there was considerable
population variability – maximal VO2 ranged from
19 to 112% predicted value for age and gender.
Some patients performed well, even above the norm,
whereas others had severely diminished exercise
capacity.7 Overall, only 28% of the participants
performed within the normal range for age and
gender. Although longitudinal data are not yet
available on this cohort, smaller longitudinal
studies have demonstrated a progressive decline in
the exercise performance of young adults, as
measured by maximal VO2, of ,2.6%/year.9
Other measures of exercise capacity, such as
maximal physical working capacity or maximal
power, are depressed to a similar degree as maximal
VO2. Interestingly, certain other measures of
exercise performance are less impaired. VO2 at the
anaerobic threshold is significantly better as a
percentage of predicted at ,75%. The reason for
this will be discussed below. In addition to
alterations in metabolic measurements, impaired
chronotropic performance is also observed as a
universal finding in all large studies of exercise in
Fontan patients. Maximal heart rates tend to cluster