Heart rate thresholds for cardiovascular risk and sympathetic activation in the metabolic syndrome

Aims: We examined whether to what extent resting heart rate (HR) values are capable to reflect in the metabolic syndrome (MS) a different degree of sympathetic activation. We also thought to determine at which HR cutoff values the sympathetic nervous system becomes more activated in the MS. Methods: In 70 MS patients aged 55.5 ± 1.8 (mean ± SEM) years we evaluated muscle sympathetic nerve traffic (MSNA, microneurography) and venous plasma norepinephrine (NE, HPLC assay), subdividing the study population in three different subgroups according to resting clinic and 24-h HR values (< 70, 70–79 and ≥ 80 beats/min). Results: MS patients with clinic HR values ≥ 80 beats/min displayed MSNA and NE values significantly increased when compared to those found in MS with HR between 70 and 79 beats/min or below 70 beats/min (MSNA: 55.2 ± 0.9 vs 44.6 ± 0.6 and 39.2 ± 0.6 bursts/min, P < 0.01, NE: 403.9 ± 6.9 vs 330.1 ± 4.3 and 258.3 ± 6.8 pg/ml, respectively, P < 0.01). A similar behavior was observed for 24-h HR. In the group as a whole both MSNA and plasma NE showed highly significant direct relationships with clinic HR, the correlation being similar for MSNA and NE (r = 0.89 and r = 0.91, P < 0.01 for both) Similar significant relationships were also found between 24-h HR values and MSNA or NE. Conclusions: In the MS HR values ≥ 80 beats/min are associated with an increased sympathetic activation, both when assessed by direct recording of MSNA and when evaluated as plasma NE. The sympathetic overdrive parallels for magnitude the HR elevations, this being the case for both clinic and 24-h HR.