Reliability of heart rate as neuroadrenergic marker in the metabolic syndrome

Background: Metabolic syndrome is characterized by a pronounced sympathetic overactivity as documented by the marked increase in muscle sympathetic nerve traffic (MSNA) as well as in plasma norepinephrine values reported in this condition. Whether and to what extent heart rate (HR) reflects the abovementioned adrenergic alterations in metabolic syndrome remains largely undefined. It is also undefined the validity of the abovementioned adrenergic markers in reflecting the main features of the metabolic syndrome. Methods: In 65 metabolic syndrome patients, aged 56.5 ± 1.3 years (mean ± SEM), we measured over a 30-min resting period blood pressure, HR (ECG), venous plasma norepinephrine (HPLC) and MSNA (microneurography). We also evaluated anthropometric and metabolic variables including HOMA index, correlating them with the adrenergic markers. The same measurements were also made in 48 age-matched healthy controls. Results: HR was significantly greater in the metabolic syndrome patients than in controls (74.6 ± 1.5 versus 67.5 ± 1.5 bpm, P < 0.001) and significantly and directly correlated with the elevated norepinephrine and MSNA values (r = 0.25 and 0.33, P < 0.05 and 0.01, respectively). MSNA was significantly and directly related to blood pressure (r = 0.27 and 0.31 SBP and DBP, respectively, P < 0.05 for both), BMI (r = 0.36, P < 0.01), waist circumference (r = 0.34, P < 0.01), waist-to-hip ratio (r = 0.49, P < 0.01) and plasma insulin (r = 0.57, P < 0.01). In contrast, no significant correlation was detectable between HR or norepinephrine and the abovementioned anthropometric and metabolic variables. Conclusion: Our data show that in the metabolic syndrome not only peripheral but also cardiac sympathetic drive is markedly potentiated and HR can be regarded as a marker of adrenergic overdrive characterizing this clinical condition. The reliability of HR (and of plasma norepinephrine) as sympathetic marker appears to be limited, however, this variable being unable to reflect, at variance from MSNA, the main metabolic and anthropometric abnormalities characterizing the metabolic syndrome.