Spectral analysis of HRV, using nu or LF-to-HF ratio, appears to be capable of providing a noninvasive quantitative evaluation of graded changes in the state of the sympathovagal balance.

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In heart failure (HF), the haemodynamic response to exercise in relation to left atrial (LA) dynamics is unexplored. We sought to define whether abnormal LA dynamics during exercise may play a role in cardiac output (CO) limitation and tested its ability to predict outcome.

Background-The possible mechanisms by which ␤-adrenergic antagonists may act on the neural regulation of the cardiovascular system are still elusive. Recent studies reported a marked increase of postganglionic muscle sympathetic nerve activity (MSNA) after acute ␤-blockade associated with unchanged values of arterial blood pressure and baroreflex sensitivity. We tested the hypothesis that acute ␤-blockade might also alter the oscillatory characteristics of MSNA, thus decreasing its effectiveness on peripheral vasoconstriction. Methods and Results-In 11 healthy volunteers, ECG, MSNA, arterial pressure, and respiration were recorded before and after atenolol (0.05 mg/kg IV bolus) administration. The frequency distribution of RR interval, MSNA, systolic arterial pressure (SAP), and respiratory variability was assessed by spectrum and cross-spectrum analysis. Spontaneous baroreflex sensitivity (␣-index) and plasma catecholamines (high-performance liquid chromatography) were measured. Atenolol induced a significant increase in RR interval (14.3Ϯ1.6%) with no changes in systolic and diastolic arterial pressure. MSNA increased (42Ϯ13% from 18Ϯ2 bursts per minute). The low-frequency (LF) component of RR and MSNA variability decreased (Ϫ44Ϯ7% and Ϫ24Ϯ5%, respectively), whereas the high-frequency (HF) component increased (163Ϯ55% and 34Ϯ11%, respectively), expressed in normalized units. Spectral coherence, an index of oscillatory coupling, decreased between LF RR and LF MSNA , whereas it increased between HF MSNA and HF Resp . SAP variability, ␣-index, and plasma catecholamines remained unchanged. Conclusions-Atenolol induced a change in MSNA frequency distribution reflecting a stronger respiratory coupling. This shift toward high frequency, despite an increase in MSNA, may lead to a less efficient sympathetic vasomotor modulation.

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