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We read with interest the article by Khiabani et al., where the
authors suggested TCPC power loss could affect exercise performance in
Fontan patients. Using indexed Power Loss, "iPL", instead of unadjusted
PL, they report higher iPL correlated with worse exercise. We believe that
this approach is misleading leading to wrong conclusions.
where PL, p, Q, and...
where PL, p, Q, and BSA are the TCPC power loss, blood density, TCPC
flow, and body surface area. The justification for using iPL is that it
"accounts for differences in flow and BSA between different patients".
However, this may not be true. While the authors state iPL is a flow-
independent resistance index , there are no data supporting it is
independent of BSA. This is critically important, because if iPL is
dependent on BSA, then using iPL to correlate against exercise parameters
that are themselves dependent on BSA, would mandate 'self correlation'.
Using another dimensionless parameter, Reynolds number (Re), as an
example, one can propose to correlate Re with exercise. Since Re directly
depends on blood flow velocity, and velocity indeed changes with exercise,
a compulsory correlation would be uncovered without being clinically
The problem with iPL is indexing by BSA^2. Since BSA is not directly
relating to fluid flow, BSA should not be used in dimensional analysis of
flow physics. To illustrate this, while patient age is empirically related
to body size and blood flow, it would be erroneous to use age as a time
dimension in a dimensional analysis of TCPC flow physics. A more
appropriate choice would be using anatomic length measurements such as
diameter of major blood vessels.
Without knowing iPL is independent of BSA, the correlation between
iPL and peak VO2 does not convincingly support the study conclusion.
Another study which did not use the authors' iPL found no correlation
between power loss and exercise capacity. Others have shown TCPC power
loss represents a fraction of total systemic power, making it
questionable whether power loss is physiologically significant. We
encourage the authors to use alternative metrics, and reassess the
correlation between TCPC power loss and exercise capacity.
1 Khiabani RH, Whitehead KK, Han D, Restrepo M, Tang E, Bethel J,
Paridon SM, et al. Exercise capacity in single-ventricle patients after
Fontan correlates with haemodynamic energy loss in TCPC. Heart 2014
2 Dasi LP, Pekkan K, Katajima HD, Yoganathan AP. Functional analysis
of Fontan energy dissipation. J Biomech 2008, 41: 2246-2252
3 Bossers SS, Cibis M, Gijsen FJ, Schokking M, Strengers JL,
Verhaart RF, Moelker A, et al. Computational fluid dynamics in Fontan
patients to evaluate power loss during simulated exercise. Heart 2014,
4 Kung E, Baretta A, Baker C, Arbia G, Biglino G, Corsini C,
Schievano S, et al. Predictive modeling of the virtual Hemi-Fontan
operation for second stage single ventricle palliation: Two patient-
specific cases. Journal of Biomechanics 2013, 46: 423-429