Heart rate and muscle sympathetic nerve variability during reflex changes of autonomic activity

Abstract

Low-frequency (less than 0.15 Hz) fluctuations of heart rate are increased by maneuvers, such as standing or hemorrhage, that increase sympathetic outflow to the heart and vasculature. To test the hypothesis that low-frequency heart rate fluctuations provide an index of sympathetic efferent activity, we compared power spectral measures of heart rate variability with two measures of sympathetic outflow, peroneal nerve sympathetic activity and antecubital vein plasma norepinephrine concentrations. Autonomic outflow was varied with graded stepwise infusions of nitroprusside and phenylephrine, which lowered or raised average diastolic pressures by approximately 15 mmHg. Before vasoactive drug infusions, no spectral measure of heart rate variability correlated significantly with muscle sympathetic activity, plasma norepinephrine concentration, average heart rate, or arterial pressure. During increases of muscle sympathetic activity and probable reductions of cardiac vagal activity induced by nitroprusside, the fraction of heart rate spectral power at low frequencies, but not the absolute value, correlated significantly with muscle sympathetic activity and plasma norepinephrine. However, during reductions of muscle sympathetic activity and probable elevations of cardiac vagal activity induced by phenylephrine, no measure of heart rate variability correlated significantly with muscle sympathetic activity. These findings can be explained by a model of heart rate control in which low-frequency heart rate fluctuations result from changing levels of both the sympathetic and parasympathetic inputs to the sinoatrial node.

Whole-body vibration training reduces arterial stiffness, blood pressure and sympathovagal balance in young overweight/obese women

“…A decrease in sympathovagal balance may reduce the cardiovascular risk in normotensive obese women. 4 Saul et al 40 reported that LFnu power of heart rate variability is associated with muscle sympathetic nerve activity. Recent work has shown that the association between sympathetic activity and AIx is positive in men and negative in women.…”

Comparing low frequency heart rate variability and preejection period: Two sides of a different coin

“…In defence of the LF/HF power it must be noted that these studies were often performed in nonecological physiological contexts; for example, within-subject variance in sympathetic activity was usually induced by infusion of nitroprusside or phenylephrine (Pagani et al, 1997;Saul, Rea, Eckberg, Berger, & Cohen, 1990). Secondly, they were mostly performed on small sample sizes that required the correlations to be in the .60-.80 range to be considered ''significant.''…”

Effects of physical training on autonomic cardiac modulation in hypertension: assessment by heart rate variability analysis

“…[1][2][3] An indirect assessment of the alterations in cardiovascular autonomic modulation accompanying essential hypertension is also possible through the analysis of BP and heart rate (HR) variability (BPV, HRV), which represent a simple, non-invasive means to quantify the amplitude of BP and HR fluctuations occurring at specific frequency regions, which are known to reflect BP and HR modulation by neural autonomic influences. [4][5][6][7][8][9] Most studies have focused on HRV only, mainly because of its ability to reflect both sympathetic and parasympathetic cardiac modulation and because its beat-by-beat fluctuations are easily recorded though the use of one electrocardiographic lead. In the frequency domain, current spectral analysis techniques allow for quantification of the power of HR variations in the very-low (VLF, 0.025-0.05 Hz), low (LF, 0.05-0.15 Hz) and high frequency (HF, 0.15-0.5 Hz) regions of the HR spectra, as well as calculation of other autonomic indices by normalizing these powers vs. total variance or by computing the ratio between the powers reflecting sympathetic and vagal drive (that is, LF/HF ratio).…”

“…In the frequency domain, current spectral analysis techniques allow for quantification of the power of HR variations in the very-low (VLF, 0.025-0.05 Hz), low (LF, 0.05-0.15 Hz) and high frequency (HF, 0.15-0.5 Hz) regions of the HR spectra, as well as calculation of other autonomic indices by normalizing these powers vs. total variance or by computing the ratio between the powers reflecting sympathetic and vagal drive (that is, LF/HF ratio). [4][5][6][7][8] These indices have proven to be useful in characterizing the alterations in cardiac autonomic modulation occurring in essential hypertension and even in pre-hypertensive states, 10 and have also demonstrated their prognostic value in predicting the development of CV disease and events in large population-based studies 11 or in specific clinical settings (that is, post-myocardial infarction and heart failure patients). 12 The spectral analysis of HRV has also been proposed as a tool to quantify changes in autonomic cardiac modulation in response to treatment.…”

“…6,7,19 Although changes in the HF (0.15-0.5 Hz) spectral power of HRV seem to be primarily caused by the modulation of cardiac parasympathetic efferent activity, 5,[19][20][21] at respiratory frequencies below nine breaths per minute sympathetic modulation of respiratoryinduced HR changes also occurs, together with some degree of mechanical stimulation of the sinus node by ventilation cycles. Thus, HR HF power is considered a satisfactory, but incomplete, measure of parasympathetic cardiac control.…”