Background: Aerobic capacity (VO2 max) of patients with Fontan circulation (FC) is lowest within patients with congenital heart disease. The reasons have not been completely elucidated.
Methods: Twenty five young patients with non-failing FC underwent a cardiopulmonary test during an upright ramp cycling. By using a signal morphology impedance cardiography device (physioflow®), stroke volume (SV) was evaluated along with effort. The results were compared with paired healthy controls.
Results: FC patients had lower VO2 max (24 vs 32ml/Kg/min) and maximal cardiac index (CI) (6.4 vs 9.9l/min/m2) than controls, due to impaired maximal SV (42 vs 54ml/m2) and maximal Heart Rate (HR) (154 vs 184/min) (p<0.001). No correlation between SV and HR at peak was found. At ventilatory threshold, SV continued to rise in a part of FC patients. Other FC patients showed an almost a "plateau" as in controls. The more maximal CI was impaired, the more was maximal arterio-venous difference (r=-0.6, p=0.001). Compared to controls, stroke work was lower in FC patients (p<0.01) even though maximal vascular resistance was higher in them (p<0.001).
Conclusion: Impaired SV and chronotropic incompetence are both independently responsible for impaired CI at peak. The increase in arteriovenous difference appeared to be an adaptive response. As the stroke work was low among FC patients, high systemic vascular resistance does not appear to be the cause of SV impairment but rather a consequence. SV monitoring at effort evidences heterogeneous SV profiles among FC patients that could be considered for the management of patients.
Keywords: Aerobic capacity; Congenital heart disease; Fontan circulation; Signal morphology impedance cardiography; Stroke volume.
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