Heart Rate Complexity in Response to Upright Tilt in Individuals with Down Syndrome

Abstract

Individuals with Down syndrome (DS) show less vagal withdrawal in response to sympatho-excitation than individuals without DS. Vagal heart rate (HR) modulation is also reflected in measures of HR complexity. People with DS may therefore show smaller HR complexity decrease in response to upright tilt, a sympatho-excitatory task. Higher HR complexity is associated with lower cardiovascular risk and may be clinically important. PURPOSE: To examine whether HR complexity at rest and during upright tilt differs between persons with and without DS. METHODS: Beat-to-beat R-R interval was recorded in 16 individuals with DS (8 men, 8 women; 26 ± 8 yrs) and 16 individuals without DS (8 men, 8 women; 26 ± 7 yrs) during 10 minutes of rest and 10 minutes of upright tilt at an angle of 80 degrees. Dependent variables at rest or during tilt were obtained from 550 steady-state ectopy- and arrhythmia-free R-R intervals for each participant. These included mean R-R interval and a set of HR complexity indexes: Approximate Entropy, Sample Entropy, Lempel-Ziv Entropy, Correlation Dimension, and Fractal Dimension. Between-group differences in each dependent variable were evaluated using 2 × 2 (group × condition) ANOVA. RESULTS: There were no differences between groups for any of the variables at rest. All variables of HR complexity decreased in response to tilt (p<0.001). In response to tilt, R-R interval, Sample Entropy, Lempel-Ziv Entropy, and Fractal Dimension decreased similarly in participants with and without DS. Approximate Entropy showed a smaller decrease in response to tilt in participants with than without DS (p<0.05; DS: 1.30±0.11 to 1.11±0.20; Non-DS: 1.28±0.13 to 0.94±0.20). Correlation Dimension also showed a smaller decrease in participants with DS (p<0.05; DS: 6.64±1.24 to 5.02±1.27; Non-DS: 7.03±0.63 to 4.38±1.10). CONCLUSION: Upright tilt reduces HR complexity in people with and without DS. Individuals with DS, however, show smaller decreases in some measures of HR complexity in response to tilt. Resting HR complexity does not appear to differ between individuals with and without DS. These results may have implications for cardiovascular risk in people with DS.