Introduction: A slow recovery in heart rate (HR) following exercise is due to autonomic dysfunction and is associated with increased cardiovascular event risk. Children with congenital heart defects (CHD) display autonomic dysfunction as shown by a reduction in HR variability (HRV). Whether children with CHD have impairment in post-exercise HR recovery is unknown.
Hypothesis: We tested the hypothesis that children with CHD would have reduced HRV at rest and slower HR recovery kinetics following 6-minute walk testing (6MWT) compared to healthy controls.
Methods: Twenty-five children with CHD (11±2 years; males=14; females=11) and 21 age- and sex-matched controls (11±3 years; males=10; females=11) were studied. CHD diagnoses included Tetralogy of Fallot (n=7), pulmonary or aortic stenosis (n=3), hypoplastic left or right heart syndrome (n=5), Ebstien’s anomaly (n=1), atrial or ventricular septal defect (n=3), transposition of the great arteries (n=1), double inlet left ventricle (n=1), tricuspid atresia (n=1), coarctation of the aorta (n=2), single ventricle (n=1), and dilated cardiomyopathy (n=1). HRV was determined following 10 min supine rest using a 5 min surface ECG recorded epoch. Post-exercise HR kinetics were determined over a 4-min period following the 6MWT using telemetry-based HR. Mono-exponential modeling was used to derive a HR recovery time constant, tau (time to reach 63% change). Analyses were completed using unpaired t-tests with P < 0.05 being significant. Data are mean ± SD.
Results: Children with CHD had a lower 6MWT distance (513±75 vs. 599±81 m; P < 0.001) and lower average exercise HR (122±15 vs. 139±18 beats/min; P = 0.001) compared to controls. Time domain HRV parameters revealed a reduction in the standard deviation of normal R-R intervals (55.9±40.4 vs. 92.1±24.5 ms), the root mean square of successive R-R interval differences (57.4±53.0 vs. 101.1±40.0 ms), and the percentage of consecutive normal R-R intervals that differ by more than 50 ms (24.6±28.7 vs. 53.2±14.0 %) in children with CHD vs. controls, respectively (all P < 0.01). Power spectral HRV analyses revealed no difference in low frequency (LF) power (30±16 vs. 25±16 %; P > 0.05), but significant differences in high frequency (HF) power (34±19 vs. 49±14 %) and the LF/HF ratio (1.4±1.5 vs. 0.6±0.3 %) in children with CHD vs. controls, respectively (all P < 0.05). Post-exercise HR kinetics were slower in children with CHD (tau = 34±15 s) compared to controls (tau = 24±8 s; P = 0.01), indicating a longer recovery time for HR.
Conclusions: Children with CHD have autonomic dysfunction as measured by a reduction in HRV and greater LF/HF ratio. Slower HR kinetics in children with CHD may be due to a reduction in parasympathetic activation during the post-exercise recovery period. Our findings suggest that autonomic dysfunction in children with CHD functionally alters HR control during or immediately following exercise.