Abstract 4137186: No Room for Chronotropic Incompetence in Living Fontan Patients: Enhanced Heart Rate Increase in Response to Exercise to Compensate Low Stroke Volume Reserve

Background: Fontan patients reveal decreased peak exercise performance in combination with low peak heart rate (HR), which is frequently interpreted as chronological incompetence. This is counterproductive as HR increase has to be enhanced to compensate limited stroke volume increase in Fontan physiology. Methods: Cardiopulmonary exercise testing (CPET) data in asymptomatic Fontan patients and age-matched control subjects were retrospectively studied. Peak and submaximal CPET parameters were obtained by cycle ergometer. Chronotropic capacity was assessed by peak HR/peak [WR/kg] and submaximal slope of HR/[work rate (WR)/kg] (ΔHR/Δ[WR/kg]) for males and females, separately. Results: Table 1 presents CPET data of Fontan patients (51) and controls (65). Fontan patients showed significantly lower peak HR, peak WR, peak oxygen consumption (VO2)/kg, %predicted maximum VO2, and peak oxygen pulse (pOP)/kg than controls in both sexes whereas peak HR/peak [WR/kg] was significantly higher in Fontan patients, indicating higher HR at the maximum effort in Fontan patients than in controls. Fontan patients showed higher ΔHR/Δ[WR/kg], a marker for HR-dependency, and lower Δ[VO2/kg]/ΔHR, a surrogate of stroke volume, suggesting higher HR was compensating lower stroke volume during submaximal phase of exercise in Fontan patients. Figure 1 demonstrates a linear relationship between ΔHR/ΔWR and peak HR/peak WR in both sexes. Male Fontan patients revealed a comparable trend with controls, whereas female Fontan patients showed higher HR increase at the peak exercise effort than controls with the same ΔHR/ΔWR, suggesting more limited stroke volume reserve than male Fontan patients. Conclusions: Fontan patients showed higher HR-response to exercise than controls in both sexes due to limitation in stroke volume increase. Our data indicate that lower peak HR in Fontan patients results from early exhaustion due to limited stroke volume reserve rather than primary chronotropic incompetence.