Pulse-synchronous sympathetic burst power as a new index of sympathoexcitation in patients with heart failure

Cardiorespiratory synchronisation and systolic blood pressure correlation of peripheral arterial stiffness during endoscopic thoracic sympathectomy

Muscle sympathetic nerve activity (MSNA) is known as an effective measure to evaluate peripheral sympathetic activity; however, it requires invasive measurement with the microneurography method. In contrast, peripheral arterial stiffness affected by MSNA is a measure that allows non-invasive evaluation of mechanical changes of arterial elasticity. This paper aims to clarify the features of peripheral arterial stiffness to determine whether it inherits MSNA features towards non-invasive evaluation of its activity. To this end, we propose a method to estimate peripheral arterial stiffness [Formula: see text] at a high sampling rate. Power spectral analysis of the estimated [Formula: see text] was then performed on data acquired from 15 patients ([Formula: see text] years) who underwent endoscopic thoracic sympathectomy. We examined whether [Formula: see text] exhibited the features of MSNA where its frequency components synchronise with heart and respiration rates and correlates with the low-frequency component of systolic blood pressure. Regression analysis revealed that the local peak frequency in the range of heartbeat frequency highly correlate with the heart rate ([Formula: see text], [Formula: see text]) where the regression slope was approximately 1 and intercept was approximately 0. Frequency analysis then found spectral peaks of [Formula: see text] approximately 0.2 Hz that correspond to the respiratory cycle. Finally, cross power spectral analysis showed a significant magnitude squared coherence between [Formula: see text] and systolic blood pressure in the frequency band from 0.04 to 0.2 Hz. These results indicate that [Formula: see text] inherits the features observed in MSNA that require invasive measurements, and thus [Formula: see text] can be an effective non-invasive substitution for MSNA measure.

Slow and deep respiration suppresses steady-state sympathetic nerve activity in patients with chronic heart failure: from modeling to clinical application

Influences of slow and deep respiration on steady-state sympathetic nerve activity remain controversial in humans and could vary depending on disease conditions and basal sympathetic nerve activity. To elucidate the respiratory modulation of steady-state sympathetic nerve activity, we modeled the dynamic nature of the relationship between lung inflation and muscle sympathetic nerve activity (MSNA) in 11 heart failure patients with exaggerated sympathetic outflow at rest. An autoregressive exogenous input model was utilized to simulate entire responses of MSNA to variable respiratory patterns. In another 18 patients, we determined the influence of increasing tidal volume and slowing respiratory frequency on MSNA; 10 patients underwent a 15-min device-guided slow respiration and the remaining 8 had no respiratory modification. The model predicted that a 1-liter, step increase of lung volume decreased MSNA dynamically; its nadir (−33 ± 22%) occurred at 2.4 s; and steady-state decrease (−15 ± 5%), at 6 s. Actually, in patients with the device-guided slow and deep respiration, respiratory frequency effectively fell from 16.4 ± 3.9 to 6.7 ± 2.8/min ( P < 0.0001) with a concomitant increase in tidal volume from 499 ± 206 to 1,177 ± 497 ml ( P < 0.001). Consequently, steady-state MSNA was decreased by 31% ( P < 0.005). In patients without respiratory modulation, there were no significant changes in respiratory frequency, tidal volume, and steady-state MSNA. Thus slow and deep respiration suppresses steady-state sympathetic nerve activity in patients with high levels of resting sympathetic tone as in heart failure.

Renal insufficiency coexisting with heart failure is related to elevated sympathetic nerve activity

We investigated whether coexisting renal insufficiency (RI) is associated with elevated sympathetic activity in patients with heart failure (HF). Resting muscle sympathetic nerve activity (MSNA) was determined in 101 patients with HF (ejection fraction<0.45) and 8 patients with RI but without HF (RI group). Diagnosis of RI was made of glomerular filtration rates <60ml/min/1.73m(2) estimated using the simplified Modification of Diet in Renal Disease equation. Of 101 patients, 45 had RI (HFRI group) and 56 did not (HF group). HFRI group was older (p<0.05) and given loop diuretics more frequently (p<0.05), and had a lower specific activity scale (p<0.05) than HF group. HFRI group exhibited significantly greater MSNA indices than either HF group or RI group (burst rate, p<0.05; burst incidence, p<0.01). Univariate analysis showed that RI, age, specific activity scale level and dose of furosemide were significant predictors of increased burst incidence of MSNA in patients with HF. Notably, multivariate analysis revealed that RI was the only independent factor for increased MSNA indices. These findings suggest that coexisting RI is associated with elevated sympathetic activity in patients with HF.

Sympathetic neural control of integrated cardiovascular function: Insights from measurement of human sympathetic nerve activity

Sympathetic neural control of cardiovascular function is essential for normal regulation of blood pressure and tissue perfusion. In the present review we discuss sympathetic neural mechanisms in human cardiovascular physiology and pathophysiology, with a focus on evidence from direct recordings of sympathetic nerve activity using microneurography. Measurements of sympathetic nerve activity to skeletal muscle have provided extensive information regarding reflex control of blood pressure and blood flow in conditions ranging from rest to postural changes, exercise, and mental stress in populations ranging from healthy controls to patients with hypertension and heart failure. Measurements of skin sympathetic nerve activity have also provided important insights into neural control, but are often more difficult to interpret since the activity contains several types of nerve impulses with different functions. Although most studies have focused on group mean differences, we provide evidence that individual variability in sympathetic nerve activity is important to the ultimate understanding of these integrated physiological mechanisms.

Differential responses of frequency components of renal sympathetic nerve activity to arterial pressure changes in conscious rats

The present study examined the effects of baroreceptor loading and unloading on the various rhythms present in the renal sympathetic nerve activity (RSNA) of 10 conscious rats. Short-lasting (4–5 min), steady-state decreases (from −10 to −40 mmHg) and increases (from 5 to 30 mmHg) in arterial pressure (AP) were induced by the intravenous infusion of sodium nitroprusside and phenylephrine, respectively. The relationship between changes in AP level and RSNA total power (fast Fourier transform analysis; 0–25 Hz) was characterized by an inverse sigmoid function. Basal AP was located 6.3 mmHg above AP at the midrange of the curve, that is, near the lower plateau. Sigmoid relationships were also observed for spectral powers in the low (LF, 0.030–0.244 Hz), respiratory (0.79–2.5 Hz) and high-frequency (HF, 2.5–25 Hz) bands. In contrast, in the MF band (0.27–0.76 Hz) containing oscillations associated with Mayer waves, the AP-RSNA power relationship showed a bell curve shape with a maximum at 21 mmHg below basal AP. Similarly, changes in RSNA power at the frequency of the heart beat were well characterized by a bell curve reaching a maximum at 22 mmHg below basal AP. Under baseline conditions, LF, MF, respiratory and HF powers contributed ∼3, 10, 18, and 69% of the total RSNA power, respectively. The pulse-synchronous oscillation of RSNA accounted for only 11 ± 1% of HF power. The contribution of HF power to total power did not change consistently with AP changes. Therefore, most of the baroreflex-induced changes in RSNA are mediated by changes in the amplitude of fast, irregular fluctuations.