Heart rate variability in chronic heart failure.

The analysis of heart rate variability (HRV) provides information about autonomic cardiovascular control in healthy subjects. In the past 15 years, several articles have been published regarding HRV and chronic heart failure (CHF). The results of these papers substantially demonstrated that HRV is significantly different in CHF patients compared to controls. Moreover, some variables derived from HRV analysis showed significant independent prognostic capacity. In particular, the reduction of variance (expressed as SDNN) and low-frequency spectral component of HRV (ranging from 0.03 to 0.15 Hz) seem related to an increased mortality in CHF. Nevertheless, these variables are not yet considered in clinical practice. A better understanding of the physiopathological basis of the reported alterations of HRV in CHF patients is required in order to permit its use as a clinical tool for prognosis and tailored therapy in individual CHF patients.

Orthostatic intolerance: different abnormalities in the neural sympathetic response to a gravitational stimulus.

In this paper we shall focus on the different abnormalities in the neural sympathetic response to a gravitational stimulus, characterising syndromes with symptoms of orthostatic intolerance. In Vaso vagal Syncope, an increase or a reduction of cardiac and vascular sympathetic modulation have been described in occasional and habitual fainters, respectively. Pure Autonomic Failure (PAF) is characterized by a global cardiovascular denervation. Accordingly, the spectral markers of cardiac and vascular sympathetic modulation are absent or reduced. However, a concomitant vagal diminished activity is present. In Chronic Orthostatic Intolerance (COI), the most common form of dysautonomia in young female, an abnormal regional distribution of sympathetic discharge has been hypothesized during standing. Indeed, an overall increased sympathetic activity is present in recumbent position; during tilt a blunted vascular sympathetic discharge, with a concomitant exaggerated cardiac sympathetic modulation, is evident. Baroreflex Failure is a syndrome that may result from neck surgery or irradiation due to different forms of regional cancer. It is characterized by a volatility of blood pressure and heart rate, without habitual orthostatic hypotension. In the present paper, we describe a case of Baroreflex Failure with marked orthostatic hypotension in spite of a huge muscle sympathetic nerve activity (MSNA) and high levels of plasma cathecolamines. The most relevant finding was the absence of any coordinate rythmicity in blood pressure, heart rate and MSNA, both at rest and during tilt, particularly in the frequency band likely to be related with sympathetic modulation, i.e. at 0.1 Hz. We hypothesize that the absence of 0.1 Hz spontaneous fluctuations might play a role in sustaining orthostatic hypotension.

Detection of low- and high-frequency rhythms in the variability of skin sympathetic nerve activity.

Spectral analysis of skin blood flow has demonstrated low-frequency (LF, 0.03-0.15 Hz) and high-frequency (HF, 0.15-0.40 Hz) oscillations, similar to oscillations in R-R interval, systolic pressure, and muscle sympathetic nerve activity (MSNA). It is not known whether the oscillatory profile of skin blood flow is secondary to oscillations in arterial pressure or to oscillations in skin sympathetic nerve activity (SSNA). MSNA and SSNA differ markedly with regard to control mechanisms and morphology. MSNA contains vasoconstrictor fibers directed to muscle vasculature, closely regulated by baroreceptors. SSNA contains both vasomotor and sudomotor fibers, differentially responding to arousals and thermal stimuli. Nevertheless, MSNA and SSNA share certain common characteristics. We tested the hypothesis that LF and HF oscillatory components are evident in SSNA, similar to the oscillatory components present in MSNA. We studied 18 healthy normal subjects and obtained sequential measurements of MSNA and SSNA from the peroneal nerve during supine rest. Measurements were also obtained of the electrocardiogram, beat-by-beat blood pressure (Finapres), and respiration. Spectral analysis showed LF and HF oscillations in MSNA, coherent with similar oscillations in both R-R interval and systolic pressure. The HF oscillation of MSNA was coherent with respiration. Similarly, LF and HF spectral components were evident in SSNA variability, coherent with corresponding variability components of R-R interval and systolic pressure. HF oscillations of SSNA were coherent with respiration. Thus our data suggest that these oscillations may be fundamental characteristics shared by MSNA and SSNA, possibly reflecting common central mechanisms regulating sympathetic outflows subserving different regions and functions.

Linear and non-linear 24 h heart rate variability in chronic heart failure.

It has recently been demonstrated that SDNN of heart rate variability (HRV) is a useful independent prognostic tool in chronic heart failure (CHF). The purpose of the present study was to evaluate if spectral and non-linear analysis of 24-h HRV, considered markers of autonomic cardiac modulation, contain independent prognostic information in CHF patients. Twenty normal subjects and thirty consecutive outpatients with clinically stable CHF were studied for 2 years. Periods of 300 R-R intervals were analyzed from Holter recordings. The power spectral analysis, the slope of the linear relationship between log-power versus log-frequency (1/f), and the complexity content (using corrected conditional entropy; CCE) of the R-R series were calculated. The normalized power of the low frequency spectral component (LF) and the 1/f slope were significantly lower in patients compared to controls (respectively 30.1 +/- 3.0 vs. 48.6 +/- 3.4 and -1.27 +/- 0.04 vs. -1.08 +/- 0.05; P < 0.05). Moreover, the patients who died during the study presented a reduced LF (20.9 +/- 4.1 vs. 35.5 +/- 3.5 nu; P < 0.05) and a steeper 1/f slope (-1.40 +/- 0.09 vs. -1.21 +/- 0.04 nuts, P < 0.05) compared to survivors. These results remained significant in a logistic model including heart rate and SDNN. The information content present in spectral and non-linear analysis of HRV in CHF patients has prognostic relevance independently from the time domain measures of HRV. In particular, the reduction of LF power seems the best indicator among those considered.