Baroreflex stimulation in the treatment of hypertension

Electrical stimulation of the carotid sinus lowers arterial pressure and improves heart rate variability in L-NAME hypertensive conscious rats

We evaluated the effects of long-term (48 h) electrical stimulation of the carotid sinus (CS) in hypertensive rats. L-NAME-treated (10 days) Wistar rats were implanted with a catheter in the femoral artery and a miniaturized electrical stimulator attached to electrodes positioned around the left CS, encompassing the CS nerve. One day after implantation, arterial pressure (AP) was directly recorded in conscious animals for 60 min. Square pulses (1 ms, 3 V, 30 Hz) were applied intermittently (20/20 s ON/OFF) to the CS for 48 h. After the end of stimulation, AP was recorded again. Nonstimulated rats (control group) and rats without electrodes around the CS (sham-operated) were also studied. Next, the animals were decapitated, and segments of mesenteric resistance arteries were removed to study vascular function. After the stimulation period, AP was 16 ± 5 mmHg lower in the stimulated group, whereas sham-operated and control rats showed similar AP between the first and second recording periods. Heart rate variability (HRV) evaluated using time and frequency domain tools and a nonlinear approach (symbolic analysis) suggested that hypertensive rats with electrodes around the CS, stimulated or not, exhibited a shift in cardiac sympathovagal balance towards parasympathetic tone. The relaxation response to acetylcholine in endothelium-intact mesenteric arteries was enhanced in rats that underwent CS stimulation for 48 h. In conclusion, long-term CS stimulation is effective in reducing AP levels, improving HRV and increasing mesenteric vascular relaxation in L-NAME hypertensive rats. Moreover, only the presence of electrodes around the CS is effective in eliciting changes in HRV similar to those observed in stimulated rats.

A model-based method to measure baroreflex sensitivity

This paper proposes a model-based method to quantitatively measure baroreflex sensitivity in autonomic nervous regulation of cardiovascular system. The method measures the continuous blood pressure and heart rate in orthostatic scenario, models dynamics of the baroreflex firing rate, solves parameters by optimization of measured blood pressure and heart rate variations. With this model, we can get the baroreflx sensitivity (BRS) inner indicators to evaluate the status of the autonomic nervous regulation system. Experimental results have shown the validation of the quantitative measures and the effectiveness of the method.

Baroreflex Sensitivity in Children and Adolescents: Physiology, Hypertension, Obesity, Diabetes Mellitus

The increased prevalence of obesity in children and its complications have led to a greater interest in studying baroreflex sensitivity (BRS) in children. This review of BRS in children and adolescents includes subtopics on: 1. Resting values of BRS and their reproducibility, 2. Genetics of BRS, 3. The role of a primarily low BRS and obesity in the development of hypertension, and 4. Association of diabetes mellitus, BRS, and obesity. The conclusions specific to this age follow from this review: 1. The mean heart rate (HR) influences the measurement of BRS. Since the mean HR decreases during adolescence, HR should be taken into account. 2. A genetic dependency of BRS was found. 3. Low BRS values may precede pathological blood-pressure elevation in children with white-coat hypertension. We hypothesize that low BRS plays an active role in the emergence of hypertension in youth. A contribution of obesity to the development of hypertension was also found. We hypothesize that both factors, a primarily low BRS and obesity, are partially independent risk factors for hypertension in youths. 4. In diabetics, a low BRS compared to healthy children can be associated with insulin resistance. A reversibility of the BRS values could be possible after weight loss.

Long-Term Effect of the Type of Carotid Endarterectomy on Blood Pressure

BACKGROUND

The dissection of the carotid sinus nerve in eversion carotid endarterectomy (eCEA) has been hypothesized to cause an increase in postoperative blood pressure (BP). The objective of this study is to evaluate the effect of eCEA on BP and changes in BP medications over the course of year-long follow-up after eCEA compared with longitudinal incision carotid endarterectomy patch angioplasty (pCEA).

METHODS

A retrospective review of patients who underwent CEA between July 1, 2009 and June 30, 2014 in the Vascular Surgery Department at The University of Iowa Hospital and Clinics was conducted. Demographics, comorbidities, BP, and number, dosage, and type of antihypertensive medications were collected preoperatively, at 30 days, and at 12 months. The differences in BP and medications between pCEA and eCEA patients were compared. Demographic data and comorbidities were compared using t-tests and chi-squared analysis. Differences in BP and medication dosages were analyzed using multivariate analysis of variance.

RESULTS

In total, 363 CEA procedures were performed during the study period, of which 275 procedures were included in the final analysis. Thirty percent of the patients underwent eCEA. Fifty-four percent of the patients who underwent eCEA and 60% of the patients who underwent pCEA were symptomatic. Thirty-day mortality was 1.4% and 12-month mortality was 6.4% for the entire population. No postoperative neurologic deficits, including stroke, were observed. Analysis of BP did not yield a significant difference among preoperative, 30-day, and 12-month follow-up measurements (P = 0.893). There was no significant change to the number and total daily dose of BP medications preoperatively, at 30 days, or at 12 months.

CONCLUSIONS

There is no statistical difference in mortality, morbidity, and patency rates at 30 days and 12 months between pCEA and eCEA. eCEA is also not associated with long-term BP changes compared with pCEA.

Synaptic-like vesicles and candidate transduction channels in mechanosensory terminals

This article summarises progress to date over an exciting and very enjoyable first 15 years of collaboration with Bob Banks. Our collaboration began when I contacted him with (to me) an unexpected observation that a dye used to mark recycling synaptic vesicle membrane at efferent terminals also labelled muscle spindle afferent terminals. This observation led to the re-discovery of a system of small clear vesicles present in all vertebrate primary mechanosensory nerve terminals. These synaptic-like vesicles (SLVs) have been, and continue to be, the major focus of our work. This article describes our characterisation of the properties and functional significance of these SLVs, combining our complementary skills: Bob’s technical expertise and encyclopaedic knowledge of mechanosensation with my experience of synaptic vesicles and the development of the styryl pyridinium dyes, of which the most widely used is FM1-43. On the way we have found that SLVs seem to be part of a constitutive glutamate secretory system necessary to maintain the stretch-sensitivity of spindle endings. The glutamate activates a highly unusual glutamate receptor linked to phospholipase D activation, which we have termed the PLD-mGluR. It has a totally distinct pharmacology first described in the hippocampus nearly 20 years ago but, like the SLVs that were first described over 50 years ago, has since been little researched. Yet, our evidence and literature searches suggest this glutamate/SLV/PLD-mGluR system is a ubiquitous feature of mechanosensory endings and, at least for spindles, is essential for maintaining mechanosensory function. This article summarises how this system integrates with the classical model of mechanosensitive channels in spindles and other mechanosensory nerve terminals, including hair follicle afferents and baroreceptors controlling blood pressure. Finally, in this time when there is an imperative to show translational relevance, I describe how this fascinating system might actually be a useful therapeutic drug target for clinical conditions such as hypertension and muscle spasticity. This has been a fascinating 15-year journey in collaboration with Bob who, as well as having an astute scientific mind, is also a great enthusiast, motivator and friend. I hope this exciting and enjoyable journey will continue well into the future.

Anatomical Considerations on Surgical Anatomy of the Carotid Bifurcation

Surgical anatomy of carotid bifurcation is of unique importance for numerous medical specialties. Despite extensive research, many aspects such as precise height of carotid bifurcation, micrometric values of carotid arteries and their branches as their diameter, length, and degree of tortuosity, and variations of proximal external carotid artery branches are undetermined. Furthermore carotid bifurcation is involved in many pathologic processes, atheromatous disease being the commonest. Carotid atheromatous disease is a major predisposing factor for disabling and possibly fatal strokes with geometry of carotid bifurcation playing an important role in its natural history. Consequently detailed knowledge of various anatomic parameters is of paramount importance not only for understanding of the disease but also for design of surgical treatment, especially selection between carotid endarterectomy and carotid stenting. Carotid bifurcation paragangliomas constitute unique tumors with diagnostic accuracy, treatment design, and success of operative intervention dependent on precise knowledge of anatomy. Considering those, it becomes clear that selection and application of proper surgical therapy should consider anatomical details. Further research might ameliorate available treatment options or even lead to innovative ones.

A novel approach to the design of an artificial bionic baroreflex

This paper presents a computationally efficient method to design an artificial bionic baroreflex. This work is built upon a physiology-based mathematical model of autonomic-cardiac regulation describing the regulation of heart rate and blood pressure as well as a system identification technique to identify a subject-specific mathematical model for each subject. The control strategy to regulate blood pressure is developed based upon the in-vivo baroreflex mechanism. A unique strength of the proposed method is its capability to determine the modulating baroreflex functions on the sympathetic and parasympathetic nerve activities. This method can be used in the treatment of individuals with baroreflex failure through overriding the corresponding nerves to properly regulate blood pressure. In fact, nerve overriding causes heart rate and arterial stiffness to adjust such that blood pressure reaches a proper range to provide enough oxygenated-blood to the critical organs.

Device therapy to modulate the autonomic nervous system to treat heart failure

Heart failure is the final common pathway in many forms of heart disease, and is associated with excessive morbidity and mortality. Pathophysiologic alterations in the interaction between the heart and the autonomic nervous system in advanced heart failure have been noted for decades. Over the last decade, great advances have been made in the medical and surgical treatment of heart failure - and some of these modalities target the neuro-cardiac axis. Despite these advances, many patients progress to end-stage heart failure and death. Recently, device-based therapy targeting the neuro-cardiac axis with various forms of neuromodulatory stimuli has been shown to improve heart function in experimental heart failure models. These include spinal cord stimulation, vagal nerve stimulation, and baroreflex modulation. Human trials are now underway to evaluate the safety and efficacy of these device-based neuromodulatory modalities in the heart failure population.

Resistant hypertension, secondary hypertension, and hypertensive crises: diagnostic evaluation and treatment

Hypertension is a very common modifiable risk factor for cardiovascular morbidity and mortality. Patients with hypertension represent a diverse group. In addition to those with primary hypertension, there are patients whose hypertension is attributable to secondary causes, those with resistant hypertension, and patients who present with a hypertensive crisis. Secondary causes of hypertension account for less than 10% of cases of elevated blood pressure (BP), and screening for these causes is warranted if clinically indicated. Patients with resistant hypertension, whose BP remains uncontrolled in spite of use of 3 or more antihypertensive agents, are at increased cardiovascular risk compared with the general hypertensive population. After potentially correctible causes of uncontrolled BP (pseudoresistance, secondary causes, and intake of interfering substances) are eliminated, patients with true resistant hypertension are managed by encouraging therapeutic lifestyle changes and optimizing the antihypertensive regimen, whereby the clinician ensures that the medications are prescribed at optimal doses using drugs with complementary mechanisms of action, while adding an appropriate diuretic if there are no contraindications. Mineralocorticoid receptor antagonists are formidable add-on agents to the antihypertensive regimen, usually as a fourth drug, and are effective in reducing BP even in patients without biochemical evidence of aldosterone excess. In the setting of a hypertensive crisis, the BP has to be reduced within hours in the case of a hypertensive emergency (elevated BP with evidence of target organ damage) using parenteral agents, and within a few days if there is hypertensive urgency, using oral antihypertensive agents.

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.

The Walter B. Cannon Memorial Award Lecture, 2009. Physiology in perspective: The wisdom of the body. In search of autonomic balance: the good, the bad, and the ugly

Walter B. Cannon's research on the sympathetic nervous system and neurochemical transmission was pioneering. Wisdom has endowed our body with a powerful autonomic neural regulation of the circulation that provides optimal perfusion of every organ in accordance to its metabolic needs. Exquisite sensors tuned to an optimal internal environment trigger central and peripheral sympathetic and parasympathetic motor neurons and allow desirable and beneficial adjustments to physiologic needs as well as to acute cardiovascular stresses. This short review, presented as The Walter B. Cannon Memorial Award Lecture for 2009, addresses the mechanisms that disrupt sensory signaling and result in a chronic maladjustment of the autonomic neural output that in many cardiovascular diseases results in excessive increases in the risks of dying. The hopes for any reduction of those risks resides in an understanding of the molecular determinants of neuronal signaling.

The ion channel ASIC2 is required for baroreceptor and autonomic control of the circulation

Arterial baroreceptors provide a neural sensory input that reflexly regulates the autonomic drive of circulation. Our goal was to test the hypothesis that a member of the acid-sensing ion channel (ASIC) subfamily of the DEG/ENaC superfamily is an important determinant of the arterial baroreceptor reflex. We found that aortic baroreceptor neurons in the nodose ganglia and their terminals express ASIC2. Conscious ASIC2 null mice developed hypertension, had exaggerated sympathetic and depressed parasympathetic control of the circulation, and a decreased gain of the baroreflex, all indicative of an impaired baroreceptor reflex. Multiple measures of baroreceptor activity each suggest that mechanosensitivity is diminished in ASIC2 null mice. The results define ASIC2 as an important determinant of autonomic circulatory control and of baroreceptor sensitivity. The genetic disruption of ASIC2 recapitulates the pathological dysautonomia seen in heart failure and hypertension and defines a molecular defect that may be relevant to its development.

Reduction of blood pressure in patients with treatment-resistant hypertension

BACKGROUND

Resistant hypertension is a common clinical problem, and patients with resistant hypertension have increased cardiovascular risk. It is a subset of the hypertensive population that is little studied and poorly characterized.

OBJECTIVE

The purpose of this review is to discuss resistant hypertension, its recognition and diagnostic workup and management, and to present current data about the disease from the latest research.

METHODS

We define resistant hypertension and differentiate it from pseudoresistance. We identify diagnostic tests that may be done on patients with resistant hypertension. Last, we discuss therapeutic approaches to resistant hypertension, focusing on pharmacological treatment, and present an algorithm that may be used by the clinician in treating a patient with resistant hypertension.

CONCLUSION

Resistant hypertension is a significant clinical problem commonly encountered by clinicians. Patients with resistant hypertension have increased cardiovascular risk. In evaluating for resistant hypertension, it is important to recognize elements that contribute to pseudoresistance to treatment. Secondary causes of hypertension are common in patients with resistant hypertension and should be included in the diagnostic workup. Pharmacological treatment for resistant hypertension entails choosing medications with complementary mechanisms of action, optimizing diuretic use, and considering the use of mineralocorticoid antagonists as an add-on agent to the antihypertensive regimen.

Acute and chronic electrical activation of baroreceptor afferents in awake and anesthetized subjects

Electrical stimulation of baroreceptor afferents was used in the 1960's in several species, including human beings, for the treatment of refractory hypertension. This approach bypasses the site of baroreceptor mechanosensory transduction. Chronic electrical stimulation of arterial baroreceptors, particularly of the carotid sinus nerve (Hering's nerve), was proposed as an ultimate effort to treat refractory hypertension and angina pectoris due to the limited nature of pharmacological therapy available at that time. Nevertheless, this approach was abandoned in the early 1970's due to technical limitations of implantable devices and to the development of better-tolerated antihypertensive medications. More recently, our laboratory developed the technique of electrical stimulation of the aortic depressor nerve in conscious rats, enabling access to hemodynamic responses without the undesirable effect of anesthesia. In addition, electrical stimulation of the aortic depressor nerve allows assessment of the hemodynamic responses and the sympathovagal balance of the heart in hypertensive rats, which exhibit a well-known decrease in baroreflex sensitivity, usually attributed to baroreceptor ending dysfunction. Recently, there has been renewed interest in using electrical stimulation of the carotid sinus, but not the carotid sinus nerve, to lower blood pressure in conscious hypertensive dogs as well as in hypertensive patients. Notably, previous undesirable technical outcomes associated with electrical stimulation of the carotid sinus nerve observed in the 1960's and 1970's have been overcome. Furthermore, promising data have been recently reported from clinical trials that evaluated the efficacy of carotid sinus stimulation in hypertensive patients with drug resistant hypertension.

Sympathetic neural mechanisms in human hypertension

Compared with substantial clinical research on the renin-angiotensin-aldosterone system (RAAS), much less is known about the importance of the sympathetic nervous system as a therapeutic target to slow the initiation and progression of human hypertension. Using microelectrode recordings of sympathetic activity and radiotracer measurements of regional norepinephrine spillover in hypertensive patients, recent research has advanced several provocative findings with novel--but still largely potential--therapeutic implications for clinical hypertension. These include a stronger scientific rationale for using 1) combined alpha/beta-blockers in the early phases of primary hypertension and obesity-related hypertension; 2) RAAS blockers as central sympatholytics in hypertension associated with chronic kidney disease; and 3) a higher dialysis dose--either nocturnal or short daily hemodialysis--to reduce uremic stimulation of a blood pressure--raising reflex arising in the failing kidneys. New outcomes trials are needed if we are to translate this largely theoretical body of research into clinical practice.

Baroreflex stimulation: A novel treatment option for resistant hypertension

Hypertension is a major public health problem in both developing and developed countries. Despite the increasing awareness of hypertension and its implications among patients and the treating physicians, the prevalence of resistant hypertension remains high and is expected to increase. Many patients fail to reach their target blood pressure (BP) despite the wide availability of several antihypertensive agents and the continued recommendation of dietary and lifestyle modifications. Stimulation of the carotid sinus results in lowering of BP by initiating the baroreflex and, in so doing, reducing sympathetic tone and increasing renal excretory function, in part, by exerting inhibitory effects on renin secretion. Recent evidence from experimental studies suggests that the baroreflex may be more important in the setting of chronic hypertension than originally believed. In early-phase clinical trials that did not include control arms, implantation of a baroreflex stimulator yielded a sustained decrease in BP. An ongoing larger clinical trial with appropriate control arms is further exploring the safety and efficacy of the device. This article describes the history and potential mechanisms of action of this device including its extensive preclinical development and movement to human clinical trials.