Heart Rate Reserve in Fontan Patients: Chronotropic Incompetence or Hemodynamic Limitation?

Abstract

Background Patients with a Fontan circulation achieve lower peak heart rates ( HR ) during exercise. Whether this impaired chronotropic response reflects pathology of the sinoatrial node or is a consequence of altered cardiac hemodynamics is uncertain. We evaluated the adequacy of HR acceleration throughout exercise relative to metabolic demand and cardiac output in patients with a Fontan circulation relative to healthy controls. Methods and Results Thirty subjects (20 healthy controls and 10 Fontan patients) underwent cardiac magnetic resonance imaging with simultaneous invasive pressure recording via a pulmonary and radial artery catheter during supine bicycle exercise to near maximal exertion. Adequacy of cardiac index, stroke volume, and HR reserve was assessed by determining the exercise‐induced increase (∆) in cardiac index, stroke volume, and HR relative to the increase in oxygen consumption ( VO 2 ). HR reserve was lower in Fontan patients compared with controls (71±21 versus 92±15 bpm; P =0.001). In contrast, increases in HR relative to workload and VO 2 were higher than in controls. The change in cardiac index relative to the change in VO 2 (∆cardiac index/∆ VO 2 ) was similar between groups, but Fontan patients had increased ∆ HR /∆ VO 2 and reduced ∆ stroke volume/∆ VO 2 compared with controls. There was an early and marked reduction in stroke volume during exercise in Fontan patients corresponding with a plateau in cardiac output at a low peak HR . Conclusions In Fontan patients, the chronotropic response is appropriate relative to exercise intensity, implying normal sinoatrial function. However, premature reductions in ventricular filling and stroke volume cause an early plateau in cardiac output beyond which further increases in HR would be physiologically implausible. Thus, abnormal cardiac filling rather than sinoatrial node dysfunction explains the diminished HR reserve in Fontan patients.

Resting respiratory lung volumes are “healthier” than exercise respiratory volumes in different types of palliated or corrected congenital heart disease

“…In patients with TCPC, the performance strongly relies on cardiovascular balance and pulmonary perfusion rather than ventilation. In these patients, pulmonary blood flow is known to be heart-rate dependent, which in turn depends on autonomic dysfunction, cardiac arrhythmias, and abnormal cardiac filling 47,48 : as expected, our patients with TCPC had chronotropic incompetence and reduced ventricular function.…”

Appropriate heart rate during exercise in Fontan patients

“…35 Recently, cardiac MRI has been used to measure stroke volume and cardiac output during exercise in Fontan patients. 19 An early reduction in stroke volume and a cardiac output plateau at submaximal heart rates were found. We had no direct measurements of central haemodynamics, but our calculation of oxygen pulse at maximal effort supports the concept of impaired stroke volume in the Fontan patients.…”

“…It has been suggested that Fontan patients perform better at submaximal exercise level and that training should be designed accordingly. 33 In a recent publication by Claessen et al, 19 they reported Fontan patients with higher increase in heart rate relative to workload than in a control group. However, their control group was not matched regarding age or gender, and the exercise test was only submaximal with heart rates of 144-155 beats per minute, more corresponding to our midpoint values.…”

“…Chronotropic incompetence is broadly defined as the inability to increase heart rate commensurate with increased physical activity or demand and often mentioned as a limiting factor for physical capacity in Fontan patients. 19,32 The question is if limited maximal heart rate is a primary sinoatrial dysfunction or a secondary effect of the Fontan haemodynamics. In our study, chronotropic index was lower for patients than controls.…”

“…18 A recent report with measurements of stroke volume during exercise in a small number of adult Fontan patients suggests that the main limiting factor for cardiac output and maximal exercise capacity is impaired diastolic filling and stroke volume by the ventricle rather than heart rate. 19 Gewillig and others have in several publications 15,[20][21][22][23] suggested that a limited increase in heart rate during exercise in Fontan patients may be an autonomous protective measure against a fall in cardiac output rather than a limiting factor of cardiac output per se.…”