Congenital heart disease
Relation of Systemic Venous Return, Pulmonary Vascular Resistance, and Diastolic Dysfunction to Exercise Capacity in Patients With Single Ventricle Receiving Fontan Palliation

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Fontan patients have a reduced exercise capacity, primarily owing to limitations in the ability to augment pulmonary blood flow and stroke volume. To date, the mechanism of peak exercise pulmonary blood flow restriction has not been elucidated. We performed a single-center, prospective, crossover trial of supine and upright exercise in Fontan patients and healthy controls to determine the mechanisms of exercise limitation in the Fontan-palliated patient. A total of 29 Fontan patients and 16 control subjects completed the protocol. The duration of exercise, percentage of predicted peak oxygen consumption (VO2) and peak work were reduced in the Fontan group, regardless of posture (p ≤0.03). The percentage of predicted oxygen pulse, a surrogate for pulmonary stroke volume, was not increased with supine posture in the Fontan cohort (upright, 82.3 ± 18.8% vs supine, 82.4 ± 19.7%; p = 0.6). In both groups, the percentage of predicted peak VO2 was lower with supine exercise than with upright exercise (p ≤0.002). Diastolic dysfunction was present in 57% of the Fontan patients and was associated with a reduced percentage of predicted peak VO2 (p = 0.04) and supine peak work (p = 0.008). Six Fontan patients who underwent supine exercise with indwelling catheters failed to demonstrate the expected decrease in pulmonary vascular resistance characteristically seen with peak exercise (at rest, 2.8 ± 0.7 mm Hg/L/min/m2 vs at peak, 2.8 ± 0.9 mm Hg/L/min/m2; p = 0.9). In conclusion, supine exercise in Fontan patients does not result in an increased VO2 or oxygen pulse, suggesting that inadequate venous return might not be the primary limitation of exercise capacity in this population. Diastolic dysfunction and relatively excessive peak exercise pulmonary vascular resistance might be more important factors in Fontan exercise limitation.

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Methods

The University of Michigan institutional review board approved the study. We prospectively recruited Fontan patients aged 8 to 30 years living within 200 miles of the hospital or scheduled for upcoming appointments at our institution. Control subjects of similar age and gender were recruited from the local population. To reduce the risk to the study patients and to avoid possible confounding explanations for exercise intolerance, the exclusion criteria listed in Table 1 were applied. All

Results

The demographic and medical characteristics for the entire study population are summarized in Table 2. No differences between the groups were significant.

The exercise performance measures for both cohorts are listed in Table 3.

Discussion

Reduced exercise capacity in the postoperative Fontan patient has been well documented.12, 13, 14, 15, 16, 17, 18, 19, 20, 21 The etiology of this exercise limitation is multifactorial, although recent data have suggested a reduction in SV reserve as the primary determinant of variation in exercise performance across a large Fontan population.19 Chronotropic incompetence and arterial desaturation, although widely seen in this population and previously thought to explain a significant portion of

Acknowledgment

We acknowledge Robert Gajarski, MD, for his content review and editorial contributions, Caren Goldberg, MD, MS, for her statistical assistance, and Matthew Stephens, MS, and Nicole Warnke, MS, for their tireless efforts in the administration of the exercise testing in our study and on a daily basis.

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