Mechanisms of maintained exercise capacity in adults with repaired tetralogy of Fallot

Background

The mechanisms whereby cardiac output is augmented with exercise in adult repaired tetralogy of Fallot (TOF) are poorly characterised.

Methods

16 repaired TOF patients (25 ± 7 years of age) and 8 age and sex matched controls (25 ± 4 years of age) underwent cardiopulmonary exercise testing and then real-time cardiac MRI (1.5 T) at rest and whilst exercising within the scanner, aiming for 30% heart rate reserve (Level 1) and 60% heart rate reserve (Level 2), using a custom-built MRI compatible foot pedal device.

Results

At rest, TOF patients had severely dilated RVs (indexed RV end-diastolic volume: 149 ± 37 mL/m²), moderate-severe PR (regurgitant fraction 35 ± 12%), normal RV fractional area change (FAC) (52 ± 7%) and very mildly impaired exercise capacity (83 ± 15% of predicted maximal work rate). Heart rate and RV FAC increased significantly in TOF patients (75 ± 10 vs 123 ± 17 beats per minute, p < 0.001; 44 ± 7 vs 51 ± 10%, p = 0.025), and similarly in control subjects (70 ± 11 vs 127 ± 12 beats per minute, p < 0.001; 49 ± 7 vs 61 ± 9%, p = 0.003), when rest was compared to Level 2. PR fraction decreased significantly but only modestly, from rest to Level 2 in TOF patients (37 ± 15 to 31 ± 15%, p = 0.002). Pulmonary artery net forward flow was maintained and did not significantly increase from rest to Level 2 in TOF patients (70 ± 19 vs 69 ± 12 mL/beat, p = 0.854) or controls (93 ± 9 vs 95 ± 21 mL/beat, p = 0.648).

Conclusions

During exercise in repaired TOF subjects with dilated RV and free PR, increased total RV output per minute was facilitated by an increase in heart rate, an increase in RV FAC and a decrease in PR fraction.