Robert S. Kieval

Prolonged Activation of the Baroreflex Produces Sustained Hypotension

Abstract—The role of baroreflexes in long-term control of arterial pressure is unresolved. To determine whether chronic activation of the baroreflex produces sustained hypotension, we developed a method for prolonged activation of the carotid baroreflex in conscious dogs. This was achieved by chronically implanting electrodes around both carotid sinuses and using an externally adjustable pulse generator to electrically activate the carotid baroreflex. Control values for mean arterial pressure (MAP) and heart rate were 93±3 mm Hg and 64±4 bpm, respectively. After control measurements, the carotid baroreflex was activated bilaterally for 7 days at a level that produced a prompt and substantial reduction in MAP, and for day 1 MAP was reduced to 75±4 mm Hg. Moreover, this hypotensive response was sustained throughout the entire 7 days of baroreflex activation (day 7, MAP=72±5 mm Hg). During prolonged baroreflex activation, heart rate decreased in parallel with MAP, although the changes were not as pronounced (day 7, heart rate=51±3 bpm). Prolonged baroreflex activation was also associated with ≈35% reduction in plasma norepinephrine concentration (control=87±15 pg/mL). After baroreflex activation, hemodynamic measures and plasma levels of norepinephrine returned to control levels. Interestingly, despite the pronounced fall in MAP, plasma renin activity did not increase during prolonged baroreflex activation. These data indicate that prolonged baroreflex activation can lead to substantial reductions in MAP by suppressing the sympathetic nervous system. Furthermore, sustained sympathoinhibitory effects on renin secretion may play an important role in mediating the long-term hypotensive response.

Chronic Baroreceptor Activation Enhances Survival in Dogs With Pacing-Induced Heart Failure

Much of the current pharmacological therapy for chronic heart failure targets neurohormonal activation. In spite of recent advances in drug therapy, the mortality rate for chronic heart failure remains high. Activation of the carotid baroreceptor (BR) reduces sympathetic outflow and augments vagal tone. We investigated the effect of chronic activation of the carotid BR on hemodynamic and neurohormonal parameters and on mortality in dogs with chronic heart failure. Fifteen dogs were instrumented to record hemodynamics. Electrodes were applied around the carotid sinuses to allow for activation of the BR. After 2 weeks of pacing (250 bpm), electrical carotid BR activation was initiated in 7 dogs and continued for the remainder of the study. The start of BR activation was used as a time reference point for the remaining 8 control dogs that did not receive BR activation. Survival was significantly greater for dogs undergoing carotid BR activation compared with control dogs (68.1±7.4 versus 37.3±3.2 days, respectively; P<0.01), although arterial pressure, resting heart rate, and left ventricular pressure were not different over time in BR-activated versus control dogs. Plasma norepinephrine was lower in dogs receiving BR activation therapy 31 days after the start of BR activation (401.9±151.5 versus 1121.9±389.1 pg/mL in dogs not receiving activation therapy; P<0.05). Plasma angiotensin II increased less in dogs receiving activation therapy (plasma angiotensin II increased by 157.4±58.6 pg/mL in control dogs versus 10.1±14.0 pg/mL in dogs receiving activation therapy; P<0.02). We conclude that chronic activation of the carotid BR improves survival and suppresses neurohormonal activation in chronic heart failure.

Prolonged Activation of the Baroreflex Abolishes Obesity-Induced Hypertension

Prolonged electrical activation of the carotid baroreflex produces sustained reductions in sympathetic activity and arterial pressure in normotensive dogs. The main goal of this study was to assess the influence of prolonged baroreflex activation on arterial pressure and neurohormonal responses in 6 dogs with obesity-induced hypertension. After control measurements, the diet was supplemented with cooked beef fat for 6 weeks, whereas sodium intake was held constant. After 4 weeks of the high-fat diet, there were increments in body weight from 25.8±0.7 to 38.6±1.0 kg, mean arterial pressure from 97±2 to 110±3 mm Hg, heart rate from 67±3 to 91±4 bpm, and plasma norepinephrine concentration from 141±35 to 280±52 pg/mL. Plasma glucose and insulin concentrations were elevated, but increases in plasma renin activity during the initial weeks of the high-fat diet were not sustained. During week 5, baroreflex activation resulted in sustained reductions in mean arterial pressure, heart rate, and plasma norepinephrine concentration; at the end of week 5, these values were 87±2 mm Hg, 77±4 bpm, and 166±45 pg/mL, respectively. These suppressed values returned to week 4 levels during a 7-day recovery period after baroreflex activation. There were no changes in plasma glucose or insulin concentrations, or plasma renin activity during prolonged baroreflex activation. These findings indicate that baroreflex activation can chronically suppress the sympathoexcitation associated with obesity and abolish the attendant hypertension while having no effect on hyperinsulinemia or hyperglycemia.

Influence of Prolonged Baroreflex Activation on Arterial Pressure in Angiotensin Hypertension

Despite recent evidence indicating sustained activation of the baroreflex during chronic infusion of angiotensin II (Ang II), sinoaortic denervation does not exacerbate the severity of the hypertension. Therefore, to determine whether Ang II hypertension is relatively resistant to the blood pressure-lowering effects of the baroreflex, the carotid baroreflex was electrically activated bilaterally for 7 days in 5 dogs both in the presence and absence of a continuous infusion of Ang II (5 ng/kg per minute) producing high physiological plasma levels of the peptide. Under control conditions, basal values for mean arterial pressure (MAP) and plasma norepinephrine concentration (NE) were 93±1 mm Hg and 99±25 pg/mL, respectively. By day 7 of baroreflex activation, MAP and NE were reduced to 72±4 mm Hg (−21±3 mm Hg) and 56±15 pg/mL, respectively, but PRA was unchanged (control=0.41±0.06 ng ANG I/mL per hour). All values returned to basal levels by the end of a 7-day recovery period. After 7 days of Ang II infusion, MAP increased from 93±3 to 129±3 mm Hg, whereas NE fell from 117±15 to 86±23 pg/mL. During the next 7 days of baroreflex activation/Ang II infusion, further reductions in NE were not statistically significant, and on the final day of baroreflex activation, the reduction in MAP was only 5±1 mm Hg, compared with 21±3 mm Hg in the control normotensive state. These findings indicate that long-term baroreflex-mediated reductions in arterial pressure are markedly diminished, but not totally eliminated, in the presence of hypertension produced by chronic infusion of Ang II.

Chronic Electrical Stimulation of the Carotid Sinus Baroreflex Improves Left Ventricular Function and Promotes Reversal of Ventricular Remodeling in Dogs With Advanced Heart Failure

Background—Autonomic abnormalities exist in heart failure and contribute to disease progression. Activation of the carotid sinus baroreflex (CSB) has been shown to reduce sympathetic outflow and augment parasympathetic vagal tone. This study tested the hypothesis that long-term electric activation of the CSB improves left ventricular (LV) function and attenuates progressive LV remodeling in dogs with advanced chronic heart failure. Methods and Results—Studies were performed in 14 dogs with coronary microembolization-induced heart failure (LV ejection fraction ≈25%). Eight dogs were chronically instrumented for bilateral CSB activation using the Rheos System (CVRx Inc, Minneapolis, Minn) and 6 were not and served as controls. All dogs were followed for 3 months, and none received other background therapy. During follow-up, treatment with CSB increased LV ejection fraction 4.0±2.4% compared with a reduction in control dogs of −2.8±1.0% (P<0.05). Similarly, treatment with CSB decreased LV end-systolic volume −2.5±2.7 mL compared with an increase in control dogs of 6.7±2.9 mL (P<0.05). Compared with control, CSB activation significantly decreased LV end-diastolic pressure and circulating plasma norepinephrine, normalized expression of cardiac &bgr;1-adrenergic receptors, &bgr;-adrenergic receptor kinase, and nitric oxide synthase and reduced interstitial fibrosis and cardiomyocyte hypertrophy. Conclusions—In dogs with advanced heart failure, CSB activation improves global LV function and partially reverses LV remodeling both globally and at cellular and molecular levels.

Renal Denervation Does Not Abolish Sustained Baroreflex-Mediated Reductions in Arterial Pressure

Recent studies indicate that suppression of renal sympathetic nerve activity and attendant increments in renal excretory function are sustained baroreflex-mediated responses in hypertensive animals. Given the central role of the kidneys in long-term regulation of arterial pressure, we hypothesized that the chronic blood pressure–lowering effects of the baroreflex are critically dependent on intact renal innervation. This hypothesis was tested in 6 dogs by bilaterally activating the carotid baroreflex electrically for 7 days before and after bilateral renal denervation. Before renal denervation, control values for mean arterial pressure and plasma norepinephrine concentration were 95±2 mm Hg and 96±12 pg/mL, respectively. During day 1 of baroreflex activation, mean arterial pressure decreased 13±1 mm Hg, and there was modest sodium retention. Daily sodium balance was subsequently restored, but reductions in mean arterial pressure were sustained throughout the 7 days of baroreflex activation. Activation of the baroreflex was associated with sustained decreases in plasma norepinephrine concentration (≈50%) and plasma renin activity (30% to 40%). All of the values returned to control levels during a 7-day recovery period. Two weeks after renal denervation, control values for mean arterial pressure, plasma norepinephrine concentration, plasma renin activity, and sodium excretion were comparable to those measured when the renal nerves were intact. Moreover, after renal denervation, all of the responses to activation of the baroreflex were similar to those observed before renal denervation. These findings demonstrate that the presence of the renal nerves is not an obligate requirement for achieving long-term reductions in arterial pressure during prolonged activation of the baroreflex.

Acute Device-Based Blood Pressure Reduction: Electrical Activation of the Carotid Baroreflex in Patients Undergoing Elective Carotid Surgery

Carotid sinus baroreceptors are involved in controlling blood pressure (BP) by providing input to the cardiovascular regulatory centers of the medulla. The acute effect of temporarily placing an electrode on the carotid sinus wall to electrically activate the baroreflex was investigated. We studied 11 patients undergoing elective carotid surgery. Baseline BP was 146+30/66±17 mm Hg and heart rate (HR) 72±7 bpm (mean ± standard deviation). An electrode was placed upon the carotid sinus and after obtaining a steady state baseline of BP and HR, an electric current was applied and increased in 1-volt increments. A voltage dependent and highly significant reduction in BP was observed which averaged 18±26* and 8.0±12 mm Hg for systolic BP and diastolic BP, respectively. Maximal reductions occurred at 4.4±1.2 V: 23±24 mm Hg*, 16±10 mm Hg* and 7±12 bpm* for systolic BP, diastolic BP and HR, respectively (=p <.05). Thus, electrical stimulation of the carotid sinus activates the carotid baroreflex resulting in a reduction in BP and HR. This presents a proof of concept for device based baroreflex modulation in acute BP regulation and adds to the available data which provide a rationale for evaluating this system in the context of chronic BP reduction in hypertensive patients.

Design Considerations for Clinical Trials of Autonomic Modulation Therapies Targeting Hypertension and Heart Failure

Several device-based approaches to autonomic nervous system modulation are under investigation for the treatment of resistant hypertension and heart failure (Table 1).1 This line of research has evolved from the recognition that these diseases originate or are worsened by excess sympathetic activity and loss of parasympathetic tone.9–13 Drug therapies, including β-blockers, α-blockers, and centrally acting antihypertensive drugs, can modulate these neurohormonal systems, but they are often insufficient to control blood pressure (BP) or are limited by side effects or nonadherence. Technological innovations have produced devices that modulate the autonomic nervous system, including renal denervation, carotid baroreceptor stimulation, vagal nerve stimulation, and spinal cord stimulation. View this table: Table 1. Select Completed and Ongoing Clinical Trials of Autonomic Modulation Therapies in Hypertension and Heart Failure In Europe, several autonomic modulation therapy devices have received the Conformite Europeenne mark.14 US Food and Drug Administration evaluation of these devices is ongoing. The need for adequately powered, randomized, controlled studies with longer follow-up to capture definitive evidence of safety and effectiveness has been noted.14–17 The 9th and 10th Global Cardiovascular Clinical Trialists Forum (Paris, France, December 2012 and December 2013) convened a panel of primary investigators of ongoing trials, along with biostatisticians, National Institutes of Health scientists, European, and United States regulators, and medical device and pharmaceutical industry scientists to discuss the strengths and limitations of current clinical trials, optimal designs for future trials, approvability of new devices, and considerations for integrating these technologies into practice. This article summarizes the key discussion points and identifies knowledge gaps in this field that need to be addressed by additional research. The mechanisms of autonomic modulation are complex, and a comprehensive review of these mechanisms is outside the scope of this article. Briefly, all existing strategies aim to decrease central sympathetic outflow. Renal …

Implantable Carotid Sinus Stimulator for the Treatment of Resistant Hypertension: Local Effects on Carotid Artery Morphology

BACKGROUND The Rheos System is a chronically implanted carotid sinus baroreflex activating system with a pulse generator and bilateral perivascular carotid sinus leads (CSLs) that is being evaluated in prospective clinical trials for the treatment of drug-resistant hypertension. We evaluated carotid artery structural integrity after implantation of the CSLs. METHODS To assess the effect of chronic CSL attachment, 29 CSLs were implanted on the common carotid arteries of eight sheep. The studies were terminated at 3 and 6 months postimplantation to assess anatomic and histologic changes. Additionally, 10 patients with resistant hypertension were enrolled in the Rheos Multicenter Feasibility Trial. Duplex ultrasound (DUS) was performed before device implantation and at 1 and 4 months postimplantation in this patient cohort. An independent core laboratory assessed all DUSs. RESULTS Ovine carotid angiography revealed no significant stenoses, while anatomic and histologic evaluations demonstrated electrode encapsulation in a thin layer of connective tissue with no evidence of stenosis, erosion, or inflammation. DUS evaluation revealed no significant increase in peak systolic velocities of the common and internal carotid arteries 1 and 4 months after initial implantation, indicating a lack of injury, remodeling, or stenosis. CONCLUSION The current data suggest that the CSLs used with the Rheos System are not associated with the development of carotid stenosis or injury. These short-term data support the concept of CSL placement and merit long-term investigation in a larger multicenter prospective trial.

Current challenges for clinical trials of cardiovascular medical devices

Several features of cardiovascular devices raise considerations for clinical trial conduct. Prospective, randomized, controlled trials remain the highest quality evidence for safety and effectiveness assessments, but, for instance, blinding may be challenging. In order to avoid bias and not confound data interpretation, the use of objective endpoints and blinding patients, study staff, core labs, and clinical endpoint committees to treatment assignment are helpful approaches. Anticipation of potential bias should be considered and planned for prospectively in a cardiovascular device trial. Prospective, single-arm studies (often referred to as registry studies) can provide additional data in some cases. They are subject to selection bias even when carefully designed; thus, they are generally not acceptable as the sole basis for pre-market approval of high risk cardiovascular devices. However, they complement the evidence base and fill the gaps unanswered by randomized trials. Registry studies present device safety and effectiveness in day-to-day clinical practice settings and detect rare adverse events in the post-market period. No single research design will be appropriate for every cardiovascular device or target patient population. The type of trial, appropriate control group, and optimal length of follow-up will depend on the specific device, its potential clinical benefits, the target patient population and the existence (or lack) of effective therapies, and its anticipated risks. Continued efforts on the part of investigators, the device industry, and government regulators are needed to reach the optimal approach for evaluating the safety and performance of innovative devices for the treatment of cardiovascular disease.