Baroreflex device therapy in the treatment of hypertension

Baroreflex and cerebral autoregulation are inversely correlated

BACKGROUND

The relative stability of cerebral blood flow is maintained by the baroreflex and cerebral autoregulation (CA). We assessed the relationship between baroreflex sensitivity (BRS) and CA in patients with atherosclerotic carotid stenosis or occlusion.

METHODS AND RESULTS

Patients referred for assessment of atherosclerotic unilateral >50% carotid stenosis or occlusion were included. Ten healthy volunteers served as a reference group. BRS was measured using the sequence method. CA was quantified by the correlation coefficient (Mx) between slow oscillations in mean arterial blood pressure and mean cerebral blood flow velocities from transcranial Doppler. Forty-five patients (M/F: 36/9), with a median age of 68 years (IQR:17) were included. Thirty-four patients had carotid stenosis, and 11 patients had carotid occlusion (asymptomatic: 31 patients; symptomatic: 14 patients). The median degree of carotid steno-occlusive disease was 90% (IQR:18). Both CA (P=0.02) and BRS (P<0.001) were impaired in patients as compared with healthy volunteers. CA and BRS were inversely and strongly correlated with each other in patients (rho=0.58, P<0.001) and in healthy volunteers (rho=0.939; P<0.001). Increasing BRS remained strongly associated with impaired CA on multivariate analysis (P=0.004).

CONCLUSIONS

There was an inverse correlation between CA and BRS in healthy volunteers and in patients with carotid stenosis or occlusion. This might be due to a relative increase in sympathetic drive associated with weak baroreflex enhancing cerebral vasomotor tone and CA.

Detection, evaluation, and treatment of severe and resistant hypertension: proceedings from an American Society of Hypertension Interactive forum held in Bethesda, MD, U.S.A., October 10th 2013

The epidemiology, evaluation, and management of severe and resistant hypertension in the United States (US) are evolving. The American Society of Hypertension held a multi-disciplinary forum in October 2013 to review the available evidence related to the management of resistant hypertension with both drug and device therapies. There is strong evidence that resistant hypertension is an important clinical problem in the US and many other regions of the world. Complex drug therapy is effective in most of the patients with severe and resistant hypertension, but there are certain individuals who may be refractory to multiple-drug regimens or have adverse effects that make adherence to the regimen difficult. When secondary forms of hypertension and pseudo-resistance, such as medication nonadherence, or white-coat hypertension based on marked differences between clinic and 24-hour ambulatory blood pressure monitoring, have been excluded, the impact of device therapy is under evaluation through clinical trials in the US and from clinical practice registries in Europe and Australia. Clinical trial data have been obtained primarily in patients whose resistant hypertension is defined as systolic clinic blood pressures of ≥160 mm Hg (or ≥ 150 mm Hg in type 2 diabetes) despite pharmacologic treatment with at least three antihypertensive drugs (one of which is a thiazide or loop diuretic). Baroreceptor stimulation therapy has shown modest benefit in a moderately sized sham-controlled study in drug-resistant hypertension. Patients selected for renal denervation have typically been restricted to those with preserved kidney function (estimated glomerular filtration rate ≥ 45 mL/min/1.73 m2). The first sham-controlled safety and efficacy trial for renal denervation (SYMPLICITY HTN-3) did not show benefit in this population when used in addition to an average of five antihypertensive medications. Analyses of controlled clinical trial data from future trials with novel designs will be of critical importance to determine the effectiveness of device therapy for patients with severe and resistant hypertension and will allow for proper determination of patient selection and whether it will be acceptable for clinical practice. At present, the focus on the management of severe and resistant hypertension will be through careful evaluation for pseudo-resistance and secondary forms of hypertension, appropriate use of combination pharmacologic therapy, and greater utility of specialists in hypertension.

Carotid baroreceptor activation for the treatment of resistant hypertension and heart failure

Carotid baroreceptors play an important role in blood pressure regulation through modification of sympathetic nervous activity. Conditions associated with increased sympathetic activity, such as resistant hypertension and heart failure, represent potential targets for carotid baroreceptor activation. Recent technological advances made available a small device, like a pacemaker, that constantly activates carotid baroreceptors. Primary experimental and clinical data obtained from use of this device point toward significant blood pressure reduction in patients with resistant hypertension, as well as beneficial effects on cardiac structure and function. A large feasibility trial revealed promising results; however, the first randomized study in patients with resistant hypertension raised several concerns regarding the efficacy and safety of baroreceptor activation with the device. This review critically evaluates available data obtained with carotid baroreceptor activation, emphasizing data acquired during the past year, and discusses the advantages and disadvantages as well as the future prospects of this intervention.

Fluid Structure Interaction With Contact Surface Methodology for Evaluation of Endovascular Carotid Implants for Drug-Resistant Hypertension Treatment

Drug-resistant hypertensive patients may be treated by mechanical stimulation of stretch-sensitive baroreceptors located in the sinus of carotid arteries. To evaluate the efficacy of endovascular devices to stretch the carotid sinus such that the induced strain might trigger baroreceptors to increase action potential firing rate and thereby reduce systemic blood pressure, numerical simulations were conducted of devices deployed in subject-specific carotid models. Two models were chosen—a typical physiologic carotid and a diminutive atypical physiologic model representing a clinically worst case scenario—to evaluate the effects of device deployment in normal and extreme cases, respectively. Based on the anatomical dimensions of the carotids, two different device sizes were chosen out of five total device sizes available. A fluid structure interaction (FSI) simulation methodology with contact surface between the device and the arterial wall was implemented for resolving the stresses and strains induced by device deployment. Results indicate that device deployment in the carotid sinus of the physiologic model induces an increase of 2.5% and 7.5% in circumferential and longitudinal wall stretch, respectively, and a maximum of 54% increase in von Mises arterial stress at the sinus wall baroreceptor region. The second device, deployed in the diminutive carotid model, induces an increase of 6% in both circumferential and longitudinal stretch and a 50% maximum increase in von Mises stress at the sinus wall baroreceptor region. Device deployment has a minimal effect on blood-flow patterns, indicating that it does not adversely affect carotid bifurcation hemodynamics in the physiologic model. In the smaller carotid model, deployment of the device lowers wall shear stress at sinus by 16% while accelerating flow entering the external carotid artery branch. Our FSI simulations of carotid arteries with deployed device show that the device induces localized increase in wall stretch at the sinus, suggesting that this will activate baroreceptors and subsequently may control hypertension in drug-resistant hypertensive patients, with no consequential deleterious effects on the carotid sinus hemodynamics.

Morbidity and mortality of orthostatic hypotension: implications for management of cardiovascular disease

Orthostatic hypotension (OH) is the failure of cardiovascular reflexes to maintain blood pressure on standing from a supine or sitting position. Although OH may cause symptoms of dizziness or syncope, asymptomatic OH (AOH) is far more common and is an independent risk factor for mortality and cardiovascular disease (CVD). The prevalence of AOH increases with age, the presence of hypertension or diabetes and the use of antihypertensive or other medications. The implications of AOH for the treatment of CVD and hypertension are not well defined. This review provides an overview of the current information on this topic and recommends the more frequent assessment of OH in clinical practice and in future clinical trials.

Therapeutic strategies for targeting excessive central sympathetic activation in human hypertension

The pathogenesis of hypertension and its mode of progression are complex, multifactoral and incompletely understood. However, there is accumulating evidence from humans and animal models of hypertension indicating that excessive central sympathetic nerve activity (SNA) plays a pathogenic role in triggering and sustaining the essential hypertensive state (the so-called 'neuroadrenergic hypothesis'). Importantly, augmented central sympathetic outflow has also been implicated in the initiation and progression of a plethora of pathophysiological processes independent of any increase in blood pressure, such as left ventricular hypertrophy and cardiac arrhythmias. Thus, the sympathetic nervous system constitutes an important putative drug target in hypertension. However, traditional pharmacological approaches for the management of essential hypertension appear ineffective in reducing central sympathetic outflow. Recently, several new and promising therapeutic strategies targeting neurogenic hypertension have been developed. The present report will provide a brief update of this topic with a particular emphasis on human studies examining the efficacy of novel pharmacological approaches (central sympatholytics and statins), lifestyle modification (aerobic exercise training, weight loss and stress reduction) and surgical intervention (renal denervation, chronic carotid baroreflex stimulation and deep brain stimulation) in reducing excessive central sympathetic activation in hypertension.

An Implanted Closed-loop Chip System for Heart Rate Control: System Design and Effects in Conscious Rats

Objective

To evaluate the efficiency of an implanted chip system for the control of heart rate (HR).

Methods

The HR was recorded in six conscious Sprague-Dawley (SD) rats. An implanted chip system was designed to regulate the HR by stimulating the right cervical vagus nerve according to the feedback of real time HR. Each rat was subjected to 30-min regulation and 30-min recovery. The change of HR during the regulation period was compared with the control. The ECG was recorded during the experiment for 24 h.

Results

The ECG signals were successfully recorded during the experiment. The HR was significantly decreased during the period of regulation compared with control (-79.3 ±34.5, P < 0.01, n = 6) and then recovered to normal after regulation.

Conclusion

The described implanted chip system can regulate the HR to a designated set point.

Baroreflex: a new therapeutic target in human stroke?

BACKGROUND AND PURPOSE

Autonomic dysfunction, including increased sympathetic drive and blunted baroreflex, has repeatedly been observed in acute stroke. Of clinical importance is that the stroke-related autonomic imbalance seems to be linked to worse outcome after stroke. Here, we discuss the role of baroreflex impairment in acute stroke and its possible pathophysiological and therapeutic relevance. Summary of Review- Possible mechanisms linking baroreflex impairment with unfavorable outcome in stroke may include increased cardiovascular morbidity and mortality, promotion of secondary brain injury due to local inflammation, hyperglycemia, or altered cerebral perfusion.

CONCLUSIONS

We suggest therefore that the modifying of autonomic functions may have important therapeutic implications in acute ischemic as well as in hemorrhagic stroke.