Hemodynamic and humoral characteristics of hypertension induced by prolonged stellate ganglion stimulation in conscious dogs

Autonomic nerve activity and blood pressure in ambulatory dogs

BACKGROUND

The relationship between cardiac autonomic nerve activity and blood pressure (BP) changes in ambulatory dogs is unclear.

OBJECTIVE

The purpose of this study was to test the hypotheses that simultaneous termination of stellate ganglion nerve activity (SGNA) and vagal nerve activity (VNA) predisposes to spontaneous orthostatic hypotension and that specific β₂-adrenoceptor blockade prevents the hypotensive episodes.

METHODS

We used a radiotransmitter to record SGNA, VNA, and BP in eight ambulatory dogs. Video imaging was used to document postural changes.

RESULTS

Of these eight dogs, five showed simultaneous sympathovagal discharges in which the minute-by-minute integrated SGNA correlated with integrated VNA in a linear pattern (group 1). In these dogs, abrupt termination of simultaneous SGNA-VNA at the time of postural changes (as documented by video imaging) was followed by abrupt (>20 mm Hg over four beats) drops in BP. Dogs without simultaneous on/off firing (group 2) did not have drastic drops in pressure. ICI-118,551 (ICI, a specific β₂-blocker) infused at 3 µg/kg/h for 7 days significantly increased BP from 126 mm Hg (95% confidence interval 118-133) to 133 mm Hg (95% confidence interval 125-141; P = .0001). The duration of hypotension (mean systolic BP <100 mm Hg) during baseline accounted for 7.1% of the recording. The percentage was reduced by ICI to 1.3% (P = .01).

CONCLUSION

Abrupt simultaneous termination of SGNA-VNA was observed at the time of orthostatic hypotension in ambulatory dogs. Selective β₂-adrenoceptor blockade increased BP and reduced the duration of hypotension in this model.

Nicardipine versus Nitroprusside Infusion as Antihypertensive Therapy in Hypertensive Emergencies

This prospective study compared the efficacy of nicardipine and nitroprusside for treating hypertensive emergencies by measuring haemodynamic indices and serum catecholamine levels. Patients admitted to the emergency department with a hypertensive crisis and acute pulmonary oedema received intravenous infusions of nitroprusside (starting dose 1 μg/kg per min, n = 20) or nicardipine (starting dose 3 μg/kg per min, n = 20). Both groups experienced significant declines in systolic and diastolic blood pressure after treatment, but there were no significant time-dependent differences between the groups. Heart rate decreased in the nicardipine group and increased in the nitroprusside group, but neither change was significant. Respiration rate decreased and capillary oxygen saturation rate increased after treatment in both groups. Adrenaline and noradrenaline levels decreased significantly after treatment in both groups; noradrenaline levels were significantly decreased in the nicardipine-treated group compared with the nitroprusside-treated group. Injectable nicardipine is easy to use and as effective as nitroprusside for treating hypertensive crisis with acute pulmonary oedema.

Abnormalities of autonomic nervous control in human hypertension

The pathophysiology of various stages of hypertension is different. In early hyperkinetic borderline hypertension, the sympathetic drive to the heart and blood vessels is increased while the parasympathetic cardiac inhibition is decreased. The elevated cardiac output, vascular resistance, and blood pressure at that stage can be fully normalized by autonomic blockade. As hypertension advances, a hyperkinetic circulation is less evident, since beta-adrenergic responsiveness and cardiac compliance tend to decrease. Simultaneously hypertrophy of the resistance vessels increases the baseline vascular resistance and the vessels' responsiveness to constrictive stimuli. Eventually a picture of a normal cardiac output/high vascular resistance typical for established essential hypertension emerges. As the blood vessels become hyperreactive, the same degree of vasoconstriction/blood pressure elevation can be achieved with less sympathetic tone. In that phase the sympathetic overactivity is less evident, as the brain resets itself to maintain the same blood pressure elevation with a small amount of sympathetic discharge. While sympathetic overactivity may be less evident in established hypertension, it remains an important pathophysiologic factor, not only for the maintenance of blood pressure, but also for a number of other abnormalities in hypertension. Hypertension is intimately associated with higher levels of pressure-unrelated risk for development of atherosclerosis: dyslipidemia, overweight, and hyperinsulinemia. Furthermore, a number of factors in hypertension favor a poorer outcome from coronary heart disease. These pressure-independent factors increase the risk of coronary thrombosis, arrhythmic deaths, and coronary spasms. Sympathetic overreactivity appears to be crucially implicated in the evolution of this added coronary risk in hypertension. Understanding the pathophysiology of coronary risk and its relationship to sympathetic overreactivity in hypertension is helpful in seeking further improvements in clinical practice. At present antihypertensive treatment is less efficacious in reducing coronary events in hypertension than would be expected. Judicious use of appropriate drugs promises to further improve the efficacy of antihypertensive treatment in those patients who, in addition to high blood pressure, also have other associated risk factors.

Reduced parasympathetic cardiac control in patients with hypertension at rest and under mental stress

The neurogenic component in the pathogenesis of essential hypertension has predominantly been analyzed with regard to the sympathetic part of the autonomous nervous system; the parasympathetic branch has largely been neglected. We investigated whether 54 normotensive (mean causal blood pressure [cBP]: 125 +/- 6/82 +/- 4 mm Hg), 41 borderline hypertensive (cBP: 134 +/- 8/90 +/- 5 mm Hg), and 34 hypertensive men (cBP: 152 +/- 13/101 +/- 5 mm Hg) without secondary target organ damage differed in parasympathetic cardiac control. Parasympathetic cardiac control was assessed via the amount of fast fluctuations (0.15 to 0.40 Hz; vagus band) and by the amount of respiratory-linked fluctuations (mean respiratory frequency +/- 0.03 Hz) in the power spectra of continuously registered interbeat intervals under the following conditions: mean of three rest phases with 10, 5, and 5 minutes' duration (REST); mean of two modes of a reaction time task with 10 and 5 minutes' duration (RTT); mean of 5 minutes' mental arithmetic plus noise (MA). Analysis of variance (ANOVA) shows that spectral energy in the so-called vagus band reveals the most prominent differences between blood pressure groups under all conditions: REST = normotensive, 2.70 +/- 0.31; borderline hypertensive, 2.55 +/- 0.33; and hypertensive, 2.43 +/- 0.43 (F[2.126] = 6.19; p < 0.01). RTT = normotensive, 2.41 +/- 0.35; borderline hypertensive, 2.19 +/- 0.33; and hypertensive, 2.17 +/- 0.46 (F[2.126] = 6.04; p < 0.01); MA = normotensive, 2.69 +/- 0.34; borderline hypertensive, 2.52 +/- 0.33; and hypertensive, 2.38 +/- 0.46 (F[2.126] = 7.04; p < 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)

Circulating neuropeptide Y in dog plasma consists of multiple peptide fragments

Neuropeptide Y-like immunoreactivity (NPY-LI) in dog plasma was characterized and quantified using three extraction methods (Sep-Pak:acetonitrile, HCl:ethanol, and ethanol). Sep-Pak extraction yields the best recovery and preserves the integrity of the peptide. Oxidized NPY is not generated during blood collection. Using two antisera of different specificities, at least three peptide forms in normal dog arterial and venous plasma were detected. A peptide with retention times similar to oxidized NPY or peptide YY is the major component of plasma NPY-LI under basal conditions, but NPY(1-36) predominates during sympathetic stimulation. The mature peptide in dog plasma is similar to human NPY. The antiserum ABII provides a more accurate measure of circulating NPY(1-36) and its oxidized form. The antiserum ABI is useful for detecting NPY-like fragments.

Transition from high cardiac output to elevated vascular resistance in hypertension

The early phase of hypertension (borderline hypertension) is characterized by a hyperkinetic circulation caused by excessive sympathetic drive and decreased parasympathetic inhibition to the heart. In later phases the cardiac output becomes normal, but the hypertension is still neurogenic, as demonstrated by the fact that continued pharmacologic parasympathetic, beta- and alpha-adrenergic inhibition normalizes the blood pressure. In both of these phases of the process, plasma norepinephrine values are elevated. These patients also show characteristic behavioral patterns; they are outward oriented, submissive, but experience unexpressed anger and frequently harbor hostile feelings. In late phases of hypertension the cardiac output is normal and the total peripheral resistance is elevated. This hemodynamic transition can be explained by a secondary response to elevated blood pressure. The heart becomes less responsive as a result of altered receptor responsiveness and decreased cardiac compliance, whereas the responsiveness of arterioles increases because of vascular hypertrophy, which leads to changes in the wall-to-lumen ratio. However, one observation eludes explanation: the absence of plasma norepinephrine elevation in later phases of hypertension. We propose a new conceptual framework to explain the disappearance of elevated plasma norepinephrine in the course of hypertension. The concept is based on a wide range of observations with the use of various receptor-blocking agents during neurogenic pressor responses. Invariably, the blood pressure response is preserved, but the hemodynamic pattern can be altered from a high cardiac output to high total peripheral resistance or vice versa.(ABSTRACT TRUNCATED AT 250 WORDS)

Hypertension and coronary heart disease: cause and consequence or associated diseases?

Numerous epidemiologic and experimental studies have shown that increased arterial pressure accelerates atherosclerotic disease including coronary heart disease. Left ventricular hypertrophy also develops along with the development of hypertension. In uncomplicated stages of hypertension, left ventricular hypertrophy may enable the heart to maintain its performance against an increased afterload. Development of coronary heart disease in hypertensive heart disease could be one of the factors that impairs cardiac function in later stages of the disease so that the function of the heart can no longer match the increased afterload. It can also be argued that atherosclerotic disease commonly contributes to an increase in systolic blood pressure. Short-term increases in arterial pressure due to angina pectoris or other stressful events may trigger permanent hypertension; this is an interesting alternative that is yet to be investigated. Thus, it is possible that a cause/consequence relationship is involved between hypertension and coronary heart disease as opposed to a simple association. Furthermore, this cause/consequence relationship could sometimes be bidirectional.

Physiologic considerations in left ventricular hypertrophy

Left ventricular hypertrophy is both a target organ response to hypertensive vascular disease as well as a factor that might be responsible for other cardiovascular events. Recent work confirms that the increased cardiac mass associated with hypertension results as a structural adaptation to the increased afterload imposed on the heart. Initially there is a transient period of hyperfunction that is followed by the sustained structural adaptative period of stable hyperfunction. Even before left ventricular failure supervenes, the ventricular mass demonstrates impaired contraction. This article reviews the hemodynamic evidence in favor of this sequence of events but, in addition, points to the pathophysiological and clinical factors that may be responsible for the increased cardiac mass in addition to the pressure overload. These include: the pressor mechanisms per se; the age, sex, and race of the patient; and coexisting diseases. Some of these factors may account in part for the regression of cardiac mass with antihypertensive therapy. However, until we understand more clearly those factors that transduce the physical stimulus for hypertrophy into biochemical events, we shall neither understand completely the development of this structural adaptation of the heart nor its regression with treatment.

Neurophysiology and neuropharmacology of cardiovascular regulation and stress

Evidence has accumulated over the past several years indicating that environmental factors can have a substantial influence on cardiovascular dynamics. It has been hypothesized by many investigators that through these influence environmental stressors may be important to the etiology and maintenance of cardiovascular diseases. Since the nervous system is intimately involved in the regulation of cardiovascular function it may be assumed that environmental influences on cardiovascular dynamics are to a large extent mediated by the nervous system. This assumption is supported by the literature reviewed which indicates that there are many nervous system nuclei and neurotransmitter systems involved in the regulation of cardiovascular dynamics which are also involved in an organisms adjustment to environmental stressors. The conclusion is reached that further multidisciplinary research will reveal underlying neurophysiological and neuropharmacological mechanisms responsible for stress induced cardiovascular disease and lead to new methods of treatment.

Do arterial chemoreceptors play a role in the pathogenesis of hypertension?

Although the arterial chemoreflex exerts a powerful influence upon the cardiovascular system, this reflex has until now been a disregarded factor in hypertension research. By comparing the physiological effects of chemoreceptor excitation to disarrangements present during the early labile phase of hypertension in spontaneously hypertensive rats, we found remarkable similarities. A search through the literature as well as our experimental data fully confirm this association. Many factors, among them decreased blood flow through the glomic tissue, lead to chemoreceptor stimulation. The origins of the chemoreceptor arteries are located in areas very susceptible to atherosclerotic changes which can lead to ischemia of chemoreceptor tissue. We are led to hypothesize that arterial chemoreflex is a significant factor in the etiology of essential hypertension.

Systemic arterial hypertension associated with cardiac surgery

Significant hypertension can develop in 15 to 40 percent of patients undergoing various types of cardiac surgery. These hypertensive episodes can occur at almost any time before, during or after open or closed chest operations. The various hypertensions encountered in this context do not form a homogeneous entity; they are nt due to the same causes and do not necessarily develop by the same mechanisms. Their frequency and seriousness have been demonstrated by reports from many centers: hence, the urgent need for accurate definition of their various types to allow correct identification and therapy. A classification based on well defined clinical events is therefore proposed and possible mechanisms for the more common types of hypertension are reviewed. Prophylactic measures nclude reassurance, attention to details of anesthesia and maintenance of preoperative antihypertensive therapy when indicated; for patients with coronary artery disease, preventive nitrate therapy as well as prompt attention to chest pain is essential. Both general and specific antihypertensive measures to control the more common types of hypertension complicating cardiac surgery are outlined.

Unilateral stellate block in the treatment of hypertension after coronary bypass surgery. Implications of a new therapeutic approach

Unilateral stellate ganglion block (right or left) was achieved by local injection of 15 ml of lidocaine in 27 patients with hypertension after coronary bypass surgery. The stellate block led to rapid and sustained control of blood pressure in 18 patients (9 of 15 with right stellate block and 9 of 12 with left stellate block). The reduction in arterial pressure was associated with significant (P less than 0.01) reductions in total peripheral resistance and heart rate but no significant changes in cardiac output or central venous or left atrial pressures. This hemodynamic pattern as well as effectiveness of a unilateral approach suggests that the stellate block reduced arterial pressure by interrupting the afferent limb of a pressor reflex from the heart or great vessels, or both. The procedure was free from side effects and helped avoid prolonged parenteral administration of potent antihypertensive drugs.

Hemodynamics of postmyocardial revascularization hypertension

The evolution of hemodynamic variables during the development of postcoronary bypass hypertension was investigated with use of serial cardiac output determination (indocyanine green dye) in 17 patients. Seven of the 17 patients remained normotensive (Group I) during the follow-up period of 4 to 6 hours after operation, whereas 10 (Group II) had a steady increase in blood pressure (173/101 mm Hg +/- 5.9/2.4 [mean +/- standard error] from 132/78 +/- 4.0/2.5 mm Hg immediately postoperatively, P less than 0.001) during the same time interval. Patients in Group I had no significant change in cardiac output, total peripheral resistance or heart rate. In contrast, patients who became hypertensive had a significant increase in total peripheral resistance (47 +/- 2.9 units/m2 from an initial level of 38 +/- 2.5 units/m2, P less than 0.001) with no significant change in cardiac index (2.73 +/- 0.17 versus 2.66 +/- 0.25 liters/min per m2, P greater than 0.10). Their heart rate, which was rapid initially (102 +/- 3.7 beats/min), remained unchanged during the hypertensive episode (103 +/- 3.0 beats/min). The mean rate of left ventricular ejection was not reduced by the increase in pressure and even tended to increase further in all but one patient. Central venous pressure (measured in all patients) and left atrial pressure (measured in eight patients) remained constant throughout the study in both Groups I and II. The results suggest that the mechanism underlying this type of hypertension is a generalized hemodynamic disturbance possibly related to overall sympathetic overdrive rather than the result of improved cardiac performance induced by myocardial revascularization.

Sustained hypertension after section of the glossopharyngeal nerve

A case of sustained hypertension produced by unilateral section of the glossopharyngeal nerve is reported. Special tests revealed baroreceptor dysfunction. The hypertension was responsive to propranolol therapy. Review of the literature revealed no other documented cases of sustained hypertension although transient hypertension is common.

Hemodynamic lesions in hypertension

Essential hypertension is increasingly recognized as a nonhomogenous disorder by various methods of study. The hemodynamic approach, coupled with clinical determination of the range and lability of blood pressure, has resulted in the description of several subgroups: labile hypertension with normal or elevated cardiac output, fixed or established hypertension with varying cardiac output and advanced hypertension with normal or low cardiac output. There is a tendency to postulate that these categories are stages of one disorder, but this remains to be proved. Still other patients have been described who may be further set off by exceptionally labile or hyperkinetic features. In some hypertensive patients, the peripheral resistance is normal; however, regardless of its numerical value, it is now considered to be increased if it fails to decrease normally in the presence of elevated cardiac output. Because an elevated cardiac output is the hemodynamic function that differentiates these groups, and renovascular hypertension as well, it is the focus of much current work. New interest in the central blood volume, the peripheral veins, and the portal veins and splanchnic circulation is focused on their connection with cardiac out-put. Newly appreciated, too, is the existence of parasympathetic inhibition in hypertension, which not only contributes to elevations of heart rate, cardiac output and possibly renin secretion, but also depresses baroreflex responses. Thus far, hemodynamic and endocrine mechanisms of hypertension have not been studied together, except possibly through the blood volume, which remains a highly controversial topic. In this paper, some recent work in the above areas is reviewed and emphasis is given to studies in man.

Hyperkinetic heart in severe hypertension: a separate clinical hemodynamic entity

A long-term study of established hypertension helped identify a well defined group of 10 patients who differed both clinically and hemodynamically from 59 patients with the more frequent form of this disease. Their cardiac output was significantly increased (P less than 0.001) despite a severe elevation of arterial pressure (average 212/125 mm Hg plus or minus 13.5/7.3[standard error[). All had labile hypertension of long standing (16.2 years average) that was difficult to control and always symptomatic; in all, the diagnosis of pheochromocytoma had to be specifically excluded. Increased myocardial contractility was suggested by (1) significant elevation of the rate of rise of isovolumic pressure (P less than 0.001), and (2) high ratio of cardiac output to cardiopulmonary volume (P less than 0.005). Beta adrenergic blockade with propranolol helped to alleviate symptoms and to control tachycardia but failed by itself to reduce arterial pressure.