cardiac output increase with zero-resistance cycling and is unchanged with positive inspiratory pressure With positive expiratory pressure, stroke volume falls to baseline while cardiac output is maintained by increases in heart rate Thus it seems that the skeletal muscle activity acts as a pump, augmenting venous return and cardiac output, whereas negative pressure associated with inspiratory chest wall mechanics plays a minor role FIG 73.30 Stroke volume augmentation with exercise in Fontan patient The largest increase from baseline (rest) in the stroke volume index (yaxis) was seen with zero-resistance cycling (0W) (muscle pump) condition The stroke volume index returned to near baseline values with the expiratory load (EL) IL, Inspiratory load *P = 05 compared with baseline [rest] within groups; †P = 05 between groups within condition (From Shafer KM., Garcia JA, Babb TG, et al The importance of the muscle and ventilatory blood pumps during exercise in patients without a subpulmonary ventricle [Fontan operation] J Am Coll Cardiol 2012;60:2115–2121.) Although children with a Fontan circulation may have strength and body composition similar to controls, adult patients with a Fontan have reduced skeletal muscle mass and impaired aerobic capacity (Figs 73.31 and 73.32).328 Encouraging information comes from several small studies involving patients with a Fontan circulation One used an intensive muscle resistance training program With training, there was a significant increase in lean body mass and in peak oxygen uptake (Fig 73.33).336 In addition, resting cardiac output increased In the other study there was significant increase in peak VO2 and exercise duration following a 3-month controlled respiratory training program.411 Interestingly the improvement in exercise capacity was greater than that seen in short-term pulmonary vasodilator trials Clearly more work is required in this area FIG 73.31 Skeletal muscle mass in Fontan subjects A t score represents the number of standard deviations from the young normal reference mean A value less than −2.0 represents marked muscle wasting, defined as in the sarcopenic range (From Cordina R, O'Meagher S, Gould H, et al Skeletal muscle abnormalities and exercise capacity in adults with a Fontan circulation Heart 2013;1530–1534.) FIG 73.32 Muscle aerobic capacity as measured by the rate constant (k) of postexercise phosphocreatine resynthesis during calf muscle 31P magnetic resonance spectroscopy (From Cordina R, O'Meagher S, Gould H, et al Skeletal muscle abnormalities and exercise capacity in adults with a Fontan circulation Heart 2013;99[20]:1530–1534.)