Baroreflex activation therapy for the treatment of drug-resistant hypertension: new developments

Key challenges in exploring the rat as a preclinical neurostimulation model for aortic baroreflex modulation in hypertension

Electrode-based electrophysiological interfaces with peripheral nerves have come a long way since the 1960s, with several neurostimulation applications witnessing widespread clinical implementation since then. In resistant hypertension, previous clinical trials have shown that "carotid" baroreflex stimulation using device-based baroreflex activation therapy (BAT) can effectively lower blood pressure (BP). However, device-based "aortic" baroreflex stimulation remains untouched for clinical translation. The rat is a remarkable animal model that facilitates exploration of mechanisms pertaining to the baroreceptor reflex and preclinical development of novel therapeutic strategies for BP modulation and hypertension treatment. Specifically, the aortic depressor nerve (ADN) in rats carries a relatively pure population of barosensitive afferent neurons, which enable selective investigation of the aortic baroreflex function. In a rat model of essential hypertension, the spontaneously hypertensive rat (SHR), we have recently investigated the aortic baroreceptor afferents as an alternate target for BP modulation, and showed that "low intensity" stimulation is able to evoke clinically meaningful reductions in BP. Deriving high quality short-term and long-term data on aortic baroreflex modulation in rats is currently hampered by a number of unresolved experimental challenges, including anatomical variations across rats which complicates identification of the ADN, the use of unrefined neurostimulation tools or paradigms, and issues arising from anesthetized and conscious surgical preparations. With the goal of refining existing experimental protocols designed for preclinical investigation of the baroreflex, this review seeks to outline current challenges hindering further progress in aortic baroreflex modulation studies in rats and present some practical considerations and recently emerging ideas to overcome them. Aortic baroreflex modulation.

Benefits of pharmacological and electrical cholinergic stimulation in hypertension and heart failure

Systemic arterial hypertension and heart failure are cardiovascular diseases that affect millions of individuals worldwide. They are characterized by a change in the autonomic nervous system balance, highlighted by an increase in sympathetic activity associated with a decrease in parasympathetic activity. Most therapeutic approaches seek to treat these diseases by medications that attenuate sympathetic activity. However, there is a growing number of studies demonstrating that the improvement of parasympathetic function, by means of pharmacological or electrical stimulation, can be an effective tool for the treatment of these cardiovascular diseases. Therefore, this review aims to describe the advances reported by experimental and clinical studies that addressed the potential of cholinergic stimulation to prevent autonomic and cardiovascular imbalance in hypertension and heart failure. Overall, the published data reviewed demonstrate that the use of central or peripheral acetylcholinesterase inhibitors is efficient to improve the autonomic imbalance and hemodynamic changes observed in heart failure and hypertension. Of note, the baroreflex and the vagus nerve activation have been shown to be safe and effective approaches to be used as an alternative treatment for these cardiovascular diseases. In conclusion, pharmacological and electrical stimulation of the parasympathetic nervous system has the potential to be used as a therapeutic tool for the treatment of hypertension and heart failure, deserving to be more explored in the clinical setting.

Clinical Applicability of Monitoring Antihypertensive Drug Levels in Blood

Dried blood spot (DBS) analysis is a novel analytical method for therapeutic drug monitoring to identify nonadherence to antihypertensive drugs. This study was conducted to evaluate the clinical applicability of measuring drug concentrations of 8 antihypertensive drugs, using DBS and venipuncture. Furthermore, this study aimed to provide more insight into the between-patient variability in drug concentrations. False-negative values from DBS compared with a venipuncture were determined to assess drug adherence. A generalized estimating equation was used to estimate the model parameters, including sex, dose, age, weight, and the time interval, between drug intake and sampling, on the C_plasma (drug concentration in plasma). No false-negative values were found when measuring nonadherence using DBS compared with venipuncture. A high variability in C_plasma between patients was observed, especially at peak concentrations with a fold change reaching from 2.3 to 35.2. The time of intake was significantly related to the height of the C_plasma in 7 of the 8 measured drugs with a P<0.05, but the influence of dose, weight, age, and sex on drug levels differed largely between the measured drugs. DBS is a reliable and convenient method to assess nonadherence to antihypertensive drugs in clinical practice. The C_plasma of the 8 antihypertensive drugs in this study show a large interindividual difference, and therefore, low plasma concentrations do not necessarily mean nonadherence. Nonadherence can only be confirmed if drug levels are undetectable, that is, values below the lower limit of detection.

Modulation of Sympathetic Overactivity to Treat Resistant Hypertension

PURPOSE OF REVIEW

To review the role and evidence for sympathetic overactivity in resistant hypertension and review the therapies that have been studied to modulate the sympathetic nervous system to treat resistant hypertension, with a focus on non-pharmacologic therapies such as renal denervation, baroreflex activation therapy, and carotid body ablation.

RECENT FINDINGS

Based on the two best current techniques available for assessing sympathetic nerve activity, resistant hypertension is characterized by increased sympathetic nerve activity. Several device therapies, including renal denervation baroreflex activation therapy and carotid body ablation, have been developed as non-pharmacologic means of reducing blood pressure in resistant hypertension. With respect to renal denervation, the technologies for renal denervation have evolved since the unfavorable results from the HTN-3 study, and the revised technologies are being actively studied. Data from the first phase of the SPYRAL HTN Clinical Trial Program have been published. Results from the SPYRAL HTN-OFF MED trial suggest that ablating renal nerves can reduce blood pressure in patients with untreated mild-to-moderate hypertension. The SPYRAL HTN-ON MED trial demonstrated the safety and efficacy of catheter-based renal denervation in patients with uncontrolled hypertension on antihypertensive treatment. Interestingly, there was a high rate of medication non-adherence among patients with hypertension in this study. One attractive alternative to radiofrequency ablation is the use of ultrasound for renal denervation. Proof of concept data for the Paradise endovascular ultrasound renal denervation system was recently published in the RADIANCE-HTN SOLO trial. The results of this trial indicate that, among patients with mild to moderate hypertension on no medications, renal denervation with the Paradise system results in a greater reduction in both SBP and DBP at 2months compared with a sham procedure. Overall reductions were similar in magnitude to those noted in the SPYRAL HTN-OFF MED study. With respect to carotid body ablation, there is an ongoing proof of concept study that is investigating the safety and feasibility of ultrasound-based endovascular carotid body ablation in 30 subjects with treatment-resistant hypertension outside of the USA. The sympathetic nervous system is an important contributor to resistant hypertension. Modulation of sympathetic overactivity should be an important goal of treatment. Innovative therapies using non-pharmacologic means to suppress the sympathetic nervous system are actively being studied to treat resistant hypertension.

Efficacy and safety of baroreflex activation therapy for treatment of resistant hypertension: a systematic review and meta-analysis

To provide definite evidence for the anti-hypertensive benefit of Baroreflex Activation Therapy (BAT) for resistant hypertension, we performed a systematic review and meta-analysis to evaluate the efficacy and safety of BAT. Electronic searches were conducted in PubMed, EMBASE, The Cochrane Library and Web of Science. Two reviewers independently determined the eligibility of studies and extracted the data. The quality of all included studies was evaluated by the use of the Newcastle Ottawa Scale (NOS). Disagreements were settled through discussion. Twelve studies, included one randomized clinical trials (RCTs) and eleven prospective studies were eligible for qualitative analysis and five prospective studies were selected for meta-analysis. The data of analysis showed office systolic blood pressure (SBP)(WMD = -24.01, 95% CI = -28.65 to -19.36, P= 0.753I² = 0.0%) and diastolic blood pressure (DBP)(WMD = -12.53, 95% CI = -15.82 to -9.24,P = 0.893,I² = 0.893) decreased by BAT treatment.The effect on SBP was both significant in the Barostim neo TM device (WMD = -22.49, 95% CI = -29.13 to 15.84, P= 0.443; I² = 0.0%) and Rheos System (WMD = 25.46, 95% CI = -31.96 to -18.96, P= 0.703; I² = 0.0%). Our study found office BP were significantly decreased by BAT treatment, but available evidence is limited by risk of bias, small sample size, and few RCTs. Thus, there is presently insufficient evidence to fully evaluate the efficacy and safety of BAT for Patients with Resistant Hypertension. Additional high-quality RCT research with long-term follow-up is required.

Device-Based Therapy for Drug-Resistant Hypertension: An Update

Drug-resistant hypertension (RH) remains a significant and common cardiovascular risk despite the availability of multiple potent antihypertensive medications. Uncontrolled resistant hypertension contributes substantially to excessive cardiovascular and renal morbidity and mortality. Clinical and experimental evidence suggest that sympathetic nervous system over-activity is the main culprit for the development and maintenance of drug-resistant hypertension. Both medical and interventional strategies, targeting the sympathetic over-activation, have been designed in patients with hypertension over the past few decades. Minimally invasive, catheter-based, renal sympathetic denervation (RDN) and carotid baroreceptor activation therapy (BAT) have been extensively evaluated in patients with RH in clinical trials. Current trial outcomes, though at times impressive, have been mostly uncontrolled trials in need of validation. Device-based therapy for drug-resistant hypertension has the potential to provide alternative treatment options to certain groups of patients who are refractory or intolerant to current antihypertensive medications. However, more research is needed to prove its efficacy in both animal models and in humans. In this article, we will review the evidence from recent renal denervation, carotid baroreceptor stimulation therapy, and newly emerged central arteriovenous anastomosis trials to pinpoint the weak links, and speculate on potential alternative approaches.

Effect of Cardiac-Cycle-Synchronized Selective Vagal Stimulation on Heart Rate and Blood Pressure in Rats

INTRODUCTION

Activation of the baroreflex system through the selective vagal nerve stimulation (sVNS) may become a treatment option for therapy-resistant hypertension, which is a frequently observed problem in the antihypertensive therapy. In previous studies, we used continuous sVNS to lower blood pressure (BP) without major side effects in a rat model. As continuous stimulation is energy consuming and sVNS could be implemented in an antihypertensive stimulator, it was the aim of this study to investigate the efficacy of pulsatile, cardiac-cycle-synchronized sVNS (cssVNS) on the reduction of BP.

METHODS

A multichannel cuff electrode was wrapped around the left vagal nerve in six male Wistar rats under Isoflurane anesthesia. BP was recorded in the left carotid artery. An electrocardiogram (ECG) was obtained via subcutaneous needle electrodes. The aortic depressor nerve fibers in the vagal nerve bundle were selectively stimulated with 18 parameter settings within a window of 15-30 ms after the R-peak in the ECG. The stimulation paradigm included every heartbeat, every second heart beat, and every third heart beat. BP and heart rate were initially recorded over 10 min.

RESULTS

Using cssVNS, BP could be significantly reduced over 30 min and maintained at this level. While the highest BP reduction was seen during cssVNS at every heartbeat with minimal bradycardia, less-yet significant-BP reduction was seen during cssVNS at every second or third heartbeat without causing detectable bradycardia.

CONCLUSION

cssVNS can chronically reduce BP in rats avoiding measurable bradycardic side effects. This energy-efficient technique might allow the implementation of sVNS using an implantable device to permanently lower BP in patients.

FUNDING

The study was funded by Bundesministerium fur Bildung und Forschung/German Federal Ministry of Education and Research among the call "Individualisierte Medizintechnik" under the grant number FKZ 13GW0120B.

Interventional therapy for hypertension: Back on track again?

Treatment-resistant hypertension, or resistant hypertension, is defined as blood pressure that remains above target despite concurrent use of at least three antihypertensive agents from different classes at optimal doses, one of which should be a diuretic. Important considerations in the diagnosis of treatment-resistant hypertension include the exclusion of pseudoresistance and the evaluation of potential secondary causes of hypertension and of concomitant conditions that maintain high blood pressure. The ability to diagnose true treatment-resistant hypertension is important for selection of patients who may be appropriately treated with an invasive therapy. Currently, there are three interventional approaches to treat resistant hypertension, namely: (1) reduction of the activity of the sympathetic nervous system by renal nerve ablation, (2) stimulation of baroreceptors and (3) creation of a peripheral arterial venous anastomosis. This review focuses on the rationale behind these invasive approaches and the clinical results.

Cardiovascular responses elicited by continuous versus intermittent electrical stimulation of the aortic depressor nerve in conscious rats

AIMS

Short-term (seconds or minutes) continuous electrical activation of the aortic depressor nerve (ADN) in conscious rats has been successfully used to investigate baroafferent function in experimental hypertension, heart failure, and peripheral inflammation. The aim of this study was to characterize the hemodynamic responses elicited by longer periods (60min) of continuous or intermittent electrical baroreflex activation.

MAIN METHODS

Wistar rats were implanted with an electrode around the left ADN and a catheter into a femoral artery. The systolic, diastolic and mean arterial pressure and heart rate were recorded in subjects randomly assigned to continuous or intermittent electrical stimulation. The time-course of cardiovascular responses in conscious rats was examined during longer-term (60min) continuous (n=6) or intermittent (5s ON/3s OFF; n=10) electrical stimulation (0.5mA; 0.25ms; 30Hz) of the ADN.

KEY FINDINGS

The prompt (20s) hypotensive response was greater under continuous stimulation, but no difference was detected in the bradycardic response. The hypotensive response was sustained only by continuous stimulation while no sustained bradycardia was observed in either protocol.

SIGNIFICANCE

These findings indicate that continuous stimulation of the ADN is more effective in reducing arterial pressure over a longer period (60min) of stimulation. Nevertheless, both protocols - continuous or intermittent - were unable to elicit a sustained bradycardia.

Role of Chemoreceptor Activation in Hemodynamic Responses to Electrical Stimulation of the Carotid Sinus in Conscious Rats

Electric carotid baroreflex activation has been used to treat patients with resistant hypertension. It is hypothesized that, in conscious rats, combined activation of carotid baro- and chemoreceptors afferences attenuates the reflex hypotension. Rats were divided into 4 groups: (1) control group, with unilateral denervation of the right carotid chemoreceptors; (2) chemoreceptor denervation group, with bilateral ligation of the carotid body artery; (3) baroreceptor denervation group, with unilateral denervation of the left carotid baroreceptors and right carotid chemoreceptors; and (4) carotid bifurcation denervation group, with denervation of the left carotid baroreceptors and chemoreceptors, plus denervation of the right carotid chemoreceptors. Animals were subjected to 4 rounds of electric stimulation (5 V, 1 ms), with 15, 30, 60, and 90 Hz applied randomly for 20 s. Electric stimulation caused greater hypotensive responses in the chemoreceptor denervation group than in the control group, at 60 Hz (-37 versus -19 mm Hg) and 90 Hz (-33 versus -19 mm Hg). The baroreceptor denervation group showed hypertensive responses at all frequencies of stimulation. In contrast, the carotid sinus denervation group showed no hemodynamic responses. The control group presented no changes in heart rate, whereas the chemoreceptor denervation group and the baroreceptor denervation group showed bradycardic responses. These data demonstrate that carotid chemoreceptor activation attenuates the reflex hypotension caused by combined electric stimulation of the carotid sinus and the carotid sinus nerve in conscious rats. These findings may provide useful insight for clinical studies using baroreflex activation therapy in resistant hypertension and heart failure.

Limited acute influences of electrical baroreceptor activation on insulin sensitivity and glucose delivery: a randomized, double-blind, crossover clinical study

Arterial baroreflexes may regulate resistance vessels supplying glucose to skeletal muscle by modulating efferent sympathetic nervous system activity. We hypothesized that selective manipulation of baroreflex activity through electrical carotid sinus stimulation influences insulin sensitivity by changing muscular glucose delivery. We enrolled 16 hypertensive patients who responded to treatment with an electrical carotid sinus stimulator. Patients were submitted to a frequently sampled intravenous glucose tolerance test (FSIGT) with the stimulator on and with the stimulator off on separate days in a randomized, double-blind, crossover study. We monitored interstitial glucose, lactate, and pyruvate in the vastus lateralis muscle using microdialysis. Glucose and insulin concentrations in arterialized venous blood before and during FSIGT were virtually identical with the stimulator on and with the stimulator off. Insulin sensitivity, the primary end point of this study, was 3.3 ± 1.0 (mU/L)(-1) ⋅ min(-1) and 4.4 ± 2.6 (mU/L)(-1) ⋅ min(-1) (on vs. off; P = 0.7). Interstitial glucose, lactate, and pyruvate increased similarly during FSIGT regardless of the stimulator settings. In conclusion, acute changes in baroreceptor stimulation did not elicit significant changes in muscular glucose delivery and whole-body insulin sensitivity. Baroreflex-mediated changes in sympathetic vasomotor tone may have a limited acute effect on muscle glucose metabolism in patients with treatment-resistant hypertension.

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.