Power spectral analysis of heart rate variability in obese subjects: Evidence of decreased cardiac sympathetic responsiveness

OBJECTIVE: To investigate changes in sympathetic nervous system function in obesity. DESIGN: Cross-sectional clinical study. SUBJECTS: 18 middle-aged obese patients 143-55 years, BMI>33 kg/m(2)) and 26 age- and sex-matched normal-weight controls (44-56 years, BMI<26 kg/m(2)). MEASUREMENTS: Post-synaptic sympathetic response studied by power spectral analysis of heart rate variability at rest and during sympathetic stimulus obtained through passive head-up tilt. Spectral analysis comprised two frequency domain components: high-frequency power (HF), reflecting parasympathetic activity and low-frequency power (LF), in particular the LF:HF ratio, reflecting sympathetic function. Pre-synaptic sympatho-adrenal function was assessed by measurement of 8.00 am plasma noradrenaline. RESULTS: Obese patients had significantly lower spectral indexes of sympathetic response and higher spectral markers of parasympathetic activity than nonobese subjects both at rest (25.9+/-3.5 vs 38.6+/-1.7 LF NUs, P<0.001) and after tilt (0.98+/-0.40 vs 2.30+/-0.39 LF:HF, P<0.05; 62.7+/-6.9 vs 41.1+/-4.9 HF NUs, P<0.05). By contrast, the obese subjects had higher noradrenaline levels (289.32+/-7.40 vs 159.80+/-19.20 pg/ml, P<0.001). No relation was found between these neuroautonomic indexes and body mass index. CONCLUSION: Obese subjects seem to have increased pre-synaptic sympatho-adrenal function but a depressed endorgan cardiovascular response.