Alterations in Baroreflex Sensitivity Do Not Explain Attenuated Responses to Sympathoexcitation in Down Syndrome

Abstract

1091 Individuals with Down Syndrome (DS) have markedly reduced heart rate (HR) and blood pressure (BP) responses to isometric handgrip exercise (EX) in comparison to nondisabled controls (C). It is possible that impairments in arterial baroreceptor responsiveness contribute to the chronotropic incompetence and BP attenuation seen with sympathetic activation in persons with DS. PURPOSE: This study compared baroreflex sensitivity (BRS) at rest and during EX in individuals with DS and C. METHODS: HR and BP were continuously recorded in 12 individuals with DS (25.0 +/− 8.3 years) and 10 C (27.5 +/− 8.5 years) during epochs of rest (10 minutes supine) and sympathetic activation (2 minutes EX maintained at 30% of maximum voluntary contraction). HR and BP were assessed on a beat-to-beat basis. Spontaneous BRS was derived via the sequence method, using episodes of three or more consecutive heartbeats in which R-R interval and the corresponding systolic blood pressure (SBP) changed in the same direction (either up or down). The slope of the regression line for each episode was calculated and the mean slope was taken as an index of BRS. Data were analyzed using an ANOVA with repeated measures. RESULTS: There were no differences in HR at rest or EX between groups. SBP was significantly lower in the DS group at rest (106.09 +/− 10.2 vs. 116.52 +/− 12.48 mmHg, p<0.05) and during EX (123.86 +/− 16.06 vs. 150.13 +/− 16.73 mmHg, p<0.001). There was a significant group-by-time interaction for both change in HR and change in SBP with EX. When controlling for resting SBP, the DS group had a lower BRS at rest (16.02 +/− 5.88 vs. 21.19 +/− 13.22 ms/mmHg, p<0.05) and during EX (7.81 +/− 3.38 vs. 12.10 +/− 8.16 ms/mmHg, p<0.05) but there was no group-by-time interaction. CONCLUSION: When controlling for resting BP, spontaneous BRS was significantly reduced in persons with DS, both at rest and during EX. Since the change in BRS with EX was similar between groups, whereas the change in HR was greater in C, the attenuated HR response to EX in individuals with DS was not explained by impaired BRS.