Cardioadrenergic factor in essential hypertension

Cardiac autonomic mechanisms associated with borderline hypertension under varying behavioral demands: evidence for attenuated parasympathetic tone but not for enhanced beta-adrenergic activity

Elevated blood pressure in psychophysiological studies of borderline hypertension is frequently attributed to the effects of increased sympathetic tone, and with few exceptions, the potential parasympathetic contributions have not been considered. Furthermore, of the investigations that have addressed vagal influences upon blood pressure, most have employed invasive pharmacological assessment of parasympathetic tone. In this study, cardiac parasympathetic and beta-adrenergic influences in borderline hypertension were evaluated noninvasively employing respiratory sinus arrhythmia as a vagal index and preejection period as a sympathetic index of cardiac functioning. Subjects were 30 borderline hypertensive and 23 normotensive males (age range, 24-45 years). The ECG, blood pressure, impedance cardiography, and respiration were measured during two baselines (initial and post-task), a memory-comparison reaction time task, the cold pressor, and CO2-rebreathing. Results indicated tonic differences between groups in all cardiovascular variables across tasks, with the exception of pre-ejection period, which showed no group effects at all. Hypertensives additionally manifested somewhat heightened systolic blood pressure reactivity and attenuated cardiac parasympathetic responsivity to specific tasks. Our findings provide no support for an exaggerated cardiac beta-adrenergic tonic level or reactivity in borderline hypertensives. On the other hand, the consistently lower magnitude of respiratory sinus arrhythmia in our hypertensives suggests that reduced parasympathetic control may be involved in the pathophysiology of hypertension.

Relation of blood pressure measured in several positions to the subsequent development of systemic hypertension. The Normative Aging Study

To assess the relation of initial blood pressure (BP) taken in sitting, supine and standing positions to subsequent development of systemic hypertension, 1,564 men were followed for an average of 6.6 years. The men were participants in the Normative Aging Study, a longitudinal study of aging initiated in 1963 at the Veterans Administration Outpatient Clinic in Boston. After controlling for sitting levels of systolic BP and diastolic BP in a Cox regression model, supine systolic BP was found to be a significant predictor of subsequent hypertension. This effect of position was not solely a result of decreases in measurement error associated with using additional BP measurements. Although the physiologic mechanism for the observed predictive influence of postural BP on the development of hypertension are unknown, these findings may have important influences on identifying subjects at risk and also providing insight into disease pathogenesis.

Rest and exercise hemodynamic effects of sequential alpha-1-adrenoceptor (trimazosin) and beta-adrenoceptor (propranolol) antagonism in essential hypertension

The efficacy of acute beta blockade in essential hypertension is limited by reflex vasoconstriction. The aim of this study was to determine whether the latter response was modified by prior selective alpha-1-adrenoceptor blockade. A single-blind, within-patient, placebo-controlled evaluation of the immediate hemodynamic effects of sequential alpha-1 (trimazosin)- and beta (propranolol)-adrenoceptor blockade was undertaken in 10 men (34 to 58 years) with previously untreated essential hypertension. The study commenced with a 4-minute control period of constant-load (600 to 900 kpm/min) upright bicycle exercise, and measurements were made before (control) and 30 minutes after intravenous trimazosin (2 mg/kg) and exercise was then repeated; measurements at rest were again made 4 minutes after intravenous propranolol (0.2 mg/kg) before a final exercise period. Trimazosin at rest reduced systolic and diastolic arterial pressure and systemic vascular resistance without change in heart rate, cardiac output, or left ventricular (LV) filling pressure. During upright bicycle exercise the reductions in blood pressure were sustained without change in their rest-to-exercise increments. Other circulatory variables did not differ from control values. At rest the addition of propranolol further reduced systolic arterial pressure. Heart rate and cardiac output fell and systemic vascular resistance increased to its pretreatment control value. During exercise the changes at rest were sustained and the rest-to-exercise increments in blood pressure, heart rate, and cardiac output were reduced. LV filling pressure was significantly increased. In conclusion, alpha-1-adrenoceptor blockade modified the adverse effects of acute beta blockade at rest but not during exercise.

Current problems in hypertension

Although the beneficial effects of treating hypertension remain firmly established, concern has recently arisen that certain antihypertensive drugs may adversely modify cardiovascular risk factors. Equipotent blood pressure-lowering agents may also have differing effects on cardiac structure and function. Thus, established approaches to antihypertensive therapy are being questioned and revised or new approaches are being suggested.

Systolic time intervals: a review of the method in the non-invasive investigation of cardiac function in health, disease and clinical pharmacology

Measurement of systolic time intervals is a valuable, non-invasive procedure to assess left ventricular performance, particularly when influenced by drugs. In this review, we discuss various factors affecting systolic time intervals, the therapeutic implications of the technique and its place among other non-invasive tests of cardiac function.

Sympathetic contribution to the cardiac response to stress in hypertension

Studies of cardiac performance in hypertension have often been restricted to cardiac output determinations, although the latter alone are inadequate for that purpose. To define the range of cardiac performance in hypertension, the response of left ventricular filling pressure to increased workload (static exercise) was determined in 39 subjects--eight normotensive (NT) volunteers, seven patients with borderline hypertension (BLH), and 24 essential hypertensives (EH), of age-matched groups. A rise of mean pulmonary wedge pressure (PWP) by 5 mm Hg or more during maximum handgrip (HG) was considered "abnormal" for a workload (SBP x HR x 10(-3)) increase of greater than or equal to 25%. All NT subjects and all patients with BLH as well as 16 of the 24 EH (EH-I) showed normal cardiac performance by this definition. In contrast, PWP increased greater than or equal to 5 mm Hg during HG in eight patients with EH (EH-II). The calculated increase in cardiac workload was not significantly different among the four groups (+5, 5.8, 5.4 and 5.5 respectively). Beta blockade (propranolol, 10 mg i.v.) slowed heart rate in all subjects and reduced SBP x HR product in all groups both at rest and during HG. Responses of PWP to HG were widely divergent in the different patients. However, as a group those patients with "impaired cardiac performance" before propranolol (EH-II) had a greater reduction in performance following propranolol than EH-I or NT. This study suggests that adrenergic support of cardiac performance might be important in some hypertensive patients with no evidence of heart failure.

Physiologic regulation of arterial pressure. An overview

Research of the past 30 years has produced information showing the close interrelationships of neural mechanisms, the renin-angiotensin-aldosterone system, and salt balance as determinants of arterial pressure, both normal and elevated. Contemporary emphases are on central sites of neural regulation of arterial pressure, and an interesting sidelight of the conventional approaches is the possibility that the endorphin-enkephalin system may have a role in hypertension. Salt balance is critically important for people with salt-dependent hypertension; mechanisms of this dependency have not been defined but possible candidates are activation of the sympathetic nervous system, release of a natriuretic factor that causes vasoconstriction as well as salt loss, and renal-neural interactions.

Pindolol and systolic time intervals in patients with hypertension

Two studies of systolic time intervals (STIs) in patients with mild to moderate hypertension (HBP) revealed that no mean change in systolic intervals occurred with pindolol therapy, although some patients had significant alterations in their STIs. Pindolol responders with normal pretreatment preejection period to left ventricular ejection time (PEP/LVET) ratios had a significant increase in this ratio following pindolol therapy, whereas those with abnormal pretreatment PEP/LVET ratios had improvement in this ratio on administration of the drug. Patients on propranolol showed no change in PEP/LVET ratio. Propranolol administration slowed heart rate and lengthened Q-S2, S1-S2, and LVET, however, without altering the Q-S2 and LVET index, indicating that the changes were caused by the effect of propranolol on the heart rate alone. Chlorthalidone in high doses significantly reduced the Q-S2 index and the S1-S2 index, indicating that these changes were not caused by alteration of the heart rate. The second study suggests that STIs may provide a predictive clue for clinical response to pindolol. Patients with normal cardiac function (group I) are more likely to respond to pindolol than are those with abnormal cardiac function (group II). Directionally opposite changes in STIs in the two subgroups suggest different mechanisms for changing cardiac function. Pindolol's dual role as a beta-blocking agent with intrinsic sympathomimetic activity is proposed as a possible explanation, beta-blocking effects predominating in group I and sympathomimetic activity balancing the beta effect in group II.

Echocardiographic dimensions in borderline and sustained hypertension

Echocardiographic dimensions and hemodynamic indexes were determined in 42 patients with borderline hypertension and in 22 with sustained hypertension and the results were compared with findings in 33 normal subjects. In patients with borderline hypertension the thickness of the interventricular septum (IVS) was significantly increased (P less than 0.001), whereas the posterior wall PW) thickness remained within normal range. The IVS/PW ratio was significantly elevated (P less than 0.001) and was correlated negatively with the cardiac index (P less than 0.01) and positively with the preejection period (P less than 0.01). In patients with sustained hypertension a symmetric cardiac hypertrophy was observed, and there was a significant positive correlation between the IVS/PW ratio and diastolic pressure (P less than 0.05). The study suggested that (1) in sustained hypertension the symmetric cardiac hypertrophy was secondary to a progressive increase in pressure load, whereas (2) in borderline hypertension the asymmetric cardiac hypertrophy could not be directly related to the level of blood pressure and was probably associated with abnormalities of the sympathetic nervous system.

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.

Pathogenesis of essential hypertension

It is proposed that essential hypertension is an exaggeration of the tendency for blood-pressure to rise with age, that it results from a repeated sequence in which a small rise in pressure, possibly resulting from an autonomic nervous overactivity, produces changes in the kidney which maintain the rise of pressure and become the basis for a further rise.

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.

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

Recent evidence has suggested that cardiac factors may play a role in the evolution of arterial hypertension. To test the possibility that an increase in cardiac-performance can lead to a sustained increase in systemic blood pressure, we electrically stimulated the left stellate ganglion of six conscious dogs continuously for a 7-day period and monitored cardiac output and arterial blood pressure. In all six dogs, stimulation elicited an abrupt rise in systemic blood pre-sure that was entirely due to rise in cardiac output that lasted at least 6 hours. After 1 day of continuous stimulation, cardiac output retured to control values, but blood pressure remained elevated. After 7 days of stimulation, blood pressure was increased by an average of 25 mm Hg and peripheral resistance by 35 plus and minus 4%. Measurements of blood volume, plasma renin activity, circulating catecholamines (three of the six dogs), and sodium balance showed that none of these factors could explain the development pf this sustained hypertension. Pharmacologic blockade with phenoxybenzamine prevented in large part the rise in blood pressure in short-term stellate ganglion stimulations, whereas propranolol had very little effect on the pressor response, although it nearly abolished the increase in cardiac output. The data indicate that continuous stimulation of the stellate ganglion in conscious dogs leads to substained rises in both blood pressure and peripheral resistance; these changes are apparently mediated by increased activity of the sympathetic nervous system.