Mechanisms underlying abnormal cardiorespiratory response in Fontan patients

Abstract

Low peak oxygen consumption (peak VO 2 ) is a strong predictor of mortality in adults with congenital heart diseases. In univentricular hearts, after Fontan procedure, peak VO 2 is known low at 59% of predicted value for the 50th percentile. But the mechanisms underlying this abnormal cardiorespiratory response to exercise are not well known. We aimed to determine which parameters can predict low peak VO 2 (< 59% of predicted value) in Fontan patients. Twenty-one patients (mean age 23 ± 8 years) underwent cardiopulmonary exercise testing (CPX) with arterial blood gas measurements. As expected, peak VO 2 was low (24.4 mL/kg/min). All patients, despite oxygen saturation, had resting shunt with PaO 2 + PaCO 2 <120 mmHg (mean 96 ±8 mmHg). Twelve patients had peak VO 2 < 59% of predicted value, which is considered < 50th percentile in Fontan population. In this group, resting oxygen saturation was lower (94.3 ± 1.8% vs. 96.7 ± 1.4%, P = 0.02), alveolar–arterial gradient was higher at rest (52 ± 9 mmHg vs. 44 ± 5 mmHg, P = 0.03) and at exercise (58 ± 10 mmHg vs. 50 ± 6 mmHg, P = 0.04). Secondly, these patients had lower exercise heart rate (122 ± 38 bpm vs. 169 ± 25 bpm, P = 0.002), lower chronotropic reserve (49 ± 31 bpm vs. 79 ± 17 bpm, P = 0.009) and had more often pacemakers (3 patients versus 0, P = 0.05). Finally, in this group with altered peak VO 2 , we found more Fontan procedures (or modified Fontan) than extracardiac total cavopulmonary connections (5 patients or 42% vs 1 or 11%, P = 0.03), patients were older (27.6 ± 7 years vs. 16.8 ± 6.4 years, P = 0.0009), and converted to extracardiac cavopulmonary connection later (at 21.1 ± 13.9 years old vs. 3.8 ± 2.4 years old, P = 0.002). Severe alteration of aerobic capacity evaluated by peak VO 2 measurement in Fontan patients is correlated to the importance of cyanosis, the limited capacity to raise heart rate at exercise and the type of surgery.