Beta-adrenergic blockade may prolong life in post-infarction patients in part by increasing vagal cardiac inhibition

Baroreflex autonomic control in human spinal cord injury: Physiology, measurement, and potential alterations

The arterial baroreflex is a primary regulator of autonomic outflow to effectively regulate acute changes in blood pressure. After a spinal cord injury (SCI), regulation of autonomic function is disrupted, although the damage of the autonomic pathways may not necessarily be related to the severity of injury (i.e. level and completeness). Nonetheless, it can be assumed that there would be greater loss of sympathetic innervation with higher level of injury and that cardiac parasympathetic control would remain intact regardless of injury level. In those with SCI, impaired baroreflex regulation has implications not only for adequate pressure regulation, but also for long term cardiovascular health. In this review, we discuss the expected impact ofan SCI on baroreflex control and the studies that have investigated baroreflex sensitivity in this population. The data generally indicates that baroreflex sensitivity is lesser in those with chronic injuries. However, these findings are counter to the expected effect of an SCI and hence may indicate that the effect of an SCI on baroreflex control might be secondary to long term deconditioning and/or vascular stiffening of baroreceptive arteries. Furthermore, the alterations in the ability to regulate pressure do not impact the relationship between spontaneous heart rate and blood pressure variabilities. In addition, those with SCI are not adequately able to control blood pressure changes in response to orthostasis, resulting in frank hypotension in a significant proportion of those with high level injuries.

Baroreflex sensitivity is higher during acute psychological stress in healthy subjects under β-adrenergic blockade

Acute psychological stress challenges the cardiovascular system with an increase in BP (blood pressure), HR (heart rate) and reduced BRS (baroreflex sensitivity). β-adrenergic blockade enhances BRS during rest, but its effect on BRS during acute psychological stress is unknown. This study tested the hypothesis that BRS is higher during acute psychological stress in healthy subjects under β-adrenergic blockade. Twenty healthy novice male bungee jumpers were randomized and studied with (PROP, n=10) or without (CTRL, n=10) propranolol. BP and HR responses and BRS [cross-correlation time-domain (BRSTD) and cross-spectral frequency-domain (BRSFD) analysis] were evaluated from 30 min prior up to 2 h after the jump. HR, cardiac output and pulse pressure were lower in the PROP group throughout the study. Prior to the bungee jump, BRS was higher in the PROP group compared with the CTRL group [BRSTD: 28 (24-42) compared with 17 (16-28) ms·mmHg-1, P<0.05; BRSFD: 27 (20-34) compared with 14 (9-19) ms·mmHg-1, P<0.05; values are medians (interquartile range)]. BP declined after the jump in both groups, and post-jump BRS did not differ between the groups. In conclusion, during acute psychological stress, BRS is higher in healthy subjects treated with non-selective β-adrenergic blockade with significantly lower HR but comparable BP.

Novel spectral indexes of heart rate variability as predictors of sudden and non-sudden cardiac death after an acute myocardial infarction

BACKGROUND

Various indexes of 24-hour heart rate variability (HRV) have been able to predict all-cause mortality after an acute myocardial infarction (AMI), but their value in predicting specific modes of cardiac death has been limited.

AIM

The aim of this study was to assess the role of two novel spectral indexes of HRV as predictors of either sudden (SCD) or non-sudden cardiac death after an AMI. Method. We used two novel methods of spectral analysis of HRV: 1) the high-frequency (HF) spectral component, V(index), calculated as an average HF power from the most linear portion of HF power versus the R-R interval regression curve, and 2) the prevalent low-frequency oscillation of heart rate (PLF). V(index), conventional HRV measures, and PLF were analyzed from 24-hour Holter recordings of 590 patients with a recent AMI.

RESULTS

During the mean follow-up of 39+/-14 months, SCD occurred in 3% (n = 17) and non-sudden cardiac death in 5% (n = 28) of the patients. In univariate analysis, V(index) was the most potent predictor of SCD (RR: 6.0, 95% CI: 1.7-20.7, P<0.01), also remaining the most powerful predictor of SCD after adjustment for clinical variables and ejection fraction (RR: 4.2, 95% CI: 1.2-15.2, P<0.05). PLF was a potent predictor of non-sudden cardiac death (RR: 13.9, 95% CI: 5.9-32.5, P<0.001), but it did not predict SCD.

CONCLUSIONS

Novel spectral HRV analysis methods, V(index) and PLF, provide significant information of the risk of the specific mode of death after an AMI.

Heart rate variability--a therapeutic target?

Reduced heart rate variability (HRV) is a powerful and independent predictor of an adverse prognosis in patients with heart disease and in the general population. The HRV is largely determined by vagally mediated beat to beat variability, conventionally known as respiratory sinus arrhythmia. Thus, HRV is primarily an indicator of cardiac vagal control. It is still unclear whether the relationship between measures of cardiac vagal control and mortality is causative or mere association. Possible mechanisms by which cardiac vagal activity might beneficially influence prognosis include a decrease in myocardial oxygen demand, a reduction in sympathetic activity and a decreased susceptibility of the ventricular myocardium to lethal arrhythmia. In animals, augmentation of cardiac vagal control by nerve stimulation or by drugs is associated with a reduction in sudden death in susceptible models. In humans a number of drugs which have been shown to reduce mortality and sudden death in large randomised trials can also be demonstrated to increase HRV. As a result of this evidence, it has been suggested that the effect of drugs or other therapeutic manoeuvres on HRV might be used to predict clinical efficacy. The use of HRV as a therapeutic target is discussed in this review.

Investigating feed-forward neural regulation of circulation from analysis of spontaneous arterial pressure and heart rate fluctuations

BACKGROUND

Analysis of spontaneous fluctuations in systolic arterial pressure (SAP) and pulse interval (PI) reveals the occurrence of sequences of consecutive beats characterized by SAP and PI changing in the same (+PI/+SAP and -PI/-SAP) or opposite (-PI/+SAP and +PI/-SAP) direction. Although the former reflects baroreflex regulatory mechanisms, the physiological meaning of -PI/+SAP and +PI/-SAP is unclear. We tested the hypothesis that -PI/+SAP and +PI/-SAP "nonbaroreflex" sequences represent a phenomenon modulated by the autonomic nervous system reflecting a feed-forward mechanism of cardiovascular regulation.

METHODS AND RESULTS

We studied anesthetized rabbits before and after (1) complete autonomic blockade (guanethidine+propranolol+atropine, n=13; CAB), (2) sympathetic blockade (guanethidine+propranolol, n=15; SB), (3) parasympathetic blockade (atropine, n=16), (4) sinoaortic denervation (n=10; SAD), and (5) controlled respiration (n=10; CR). Nonbaroreflex sequences were defined as >/=3 beats in which SAP and PI of the following beat changed in the opposite direction. CAB reduced the number of nonbaroreflex sequences (19. 1+/-12.3 versus 88.7+/-36.6, P<0.05), as did SB (25.3+/-11.7 versus 84.6+/-23.9, P<0.001) and atropine (11.2+/-6.8 versus 94.1+/-32.4, P<0.05). SB concomitantly increased baroreflex sensitivity (1.18+/-0. 11 versus 0.47+/-0.09 ms/mm Hg, P<0.01). SAD and CR did not significantly affect their occurrence.

CONCLUSIONS

These results suggest that nonbaroreflex sequences represent the expression of an integrated, neurally mediated, feed-forward type of short-term cardiovascular regulation able to interact dynamically with the feedback mechanisms of baroreflex origin in the control of heart period.

Do beta-blockers have a role in the treatment of chronic heart failure?

There has been growing interest in and recognition of the role of beta-blockers in chronic heart failure (CHF). The mode of action is complex and several mechanisms have been proposed. The principal rationale for the use of beta-blockade is to counteract neurohormonal activation and its deleterious consequences in CHF. While the positive effect of this treatment on haemodynamics, exercise tolerance and quality of life, and a clear trend in favour of improved prognosis have been shown, there is still no concrete proof that beta-blockers reduce mortality in CHF. Several large-scale, prospective, randomised, placebo-controlled trials, designed to provide a definitive answer, are underway.

Comparison of standard autonomic tests and power spectral analysis in normal adults

Standard autonomic measures [heart rate response to deep breathing (HR[DB]), systolic blood pressure response to orthostatic load, the 30:15 ratio, and the Valsalva ratio (VR)] and spectral measures of the heart rate (HR) and the arterial blood pressure (ABP) (MF: mid-frequency band at 0.05-0.15 Hz; HF: high-frequency band at 0.15-0.33 Hz) were performed in 50 healthy subjects. The supine HR-HF and the tilt ABP-MF were taken as indicators of parasympathetic and sympathetic outflow, respectively. The transfer function magnitude of HR related to the ABP in the mid-frequency band estimated the baroreflex sensitivity. The HR[DB] and the 30:15 ratio were correlated with the parasympathetic spectral measure, and the VR was, surprisingly, only correlated with the sympathetic spectral measure. Significant baroreflex contribution was only evident for the 30:15 ratio. The spectral HR data were highly correlated with their corresponding spectral data of ABP. These results provide insights into autonomic regulation, but further studies on both basic physiological mechanisms of these methods and their clinical value have to be performed before a broad application can be recommended.

Spontaneous cardiac baroreflex in humans. Comparison with drug-induced responses

We compared two methods of assessment of baroreflex sensitivity in eight supine healthy volunteers during repeated baseline measurements and various conditions of cardiac autonomic blockade. The spontaneous baroreflex method involved computer scanning of recordings of continuous finger arterial pressure and electrocardiogram to locate sequences of three or more beats in which pressure spontaneously increased or decreased, with parallel changes in pulse intervals. The mean regression slope of all these sequences during each study condition was considered to represent the mean spontaneous baroreflex slope. In the drug-induced method, sigmoidal curves were constructed from data obtained by bolus injections of phenylephrine and nitroprusside; the tangents taken at the resting pressure of each of these curves were compared with the mean spontaneous baroreflex slopes. The two methods yielded slopes that were highly correlated (r = .96, P < .001), with significant but similar intraindividual baseline variability. Atropine virtually eliminated the baroreflex slope; subsequent addition of propranolol did not alter it further. Propranolol or clonidine alone increased average baroreflex slope to the extent that they increased resting pulse interval (r = .69 to .83). The spontaneous baroreflex method provides a reliable, noninvasive assessment of human vagal cardiac baroreflex sensitivity within its physiological operating range.

Use of beta-adrenoceptor blockers in patients with congestive heart failure

The beneficial effect of chronic beta-blockade in patients with congestive heart failure has been repeatedly shown since its introduction into treatment for this condition in 1975. Still this kind of therapy remains controversial, it is sometimes regarded as a therapeutic paradox, and its use is mainly limited to specialist centers. Various favorable effects of beta-blockers in patients with heart failure due to idiopathic dilated cardiomyopathy and ischemic heart disease have been demonstrated, the principal among them being reduction in energy requirements and ischemia, antiarrhythmogenic effect, improvement of diastolic function, protection of myocytes against catecholamine overload, centrally mediated increase in vagal tone, upregulation of beta-adrenergic receptors, and possible blockade of autoantibodies against beta 1-receptors. Although most of the studies used metoprolol, these effects may be relevant to certain other beta-blockers. Despite very solid pathophysiological and pharmacological rationales for the use of beta-blockade, a major obstacle for a general acceptance of this therapeutic concept is the striking contrast between hemodynamic changes during the acute effect and long-term treatment. When titrated carefully from very low doses and used with a true commitment to long-term treatment, beta-blockers have been shown to prevent further deterioration of heart failure and to improve hemodynamics, exercise tolerance, quality of life, and prognosis.

Relation between tonic sympathetic and vagal control of human sinus node function

BACKGROUND

Clinical conditions, such as heart failure or myocardial infarction are associated with enhanced sympathetic and reduced parasympathetic activity as compared to normal controls. The reciprocal alteration in cardiac autonomic tone likely contributes to the electrical instability of the myocardium. Little information is available on the relation between sympathetic and vagal cardiac control in healthy human subjects.

METHODS AND RESULTS

Heart period changes in response to autonomic blockades were measured in 16 young, healthy human subjects. Adrenergic and cholinergic blockades were induced by i.v. propranolol (0.2 mg/kg) and atropine (0.04 mg/kg) in two opposite orders on two occasions; interindividual correlations were performed between the R-R interval responses to propranolol and to atropine obtained under the various blockade conditions, and the magnitude of the responses were compared by a drug x order two factorial ANOVA design. It was found, that previous adrenergic blockade did not reduce the extent of cardioacceleration produced by subsequent cholinergic blockade and that the R-R interval responses to atropine and to subsequently given propranolol did not share significant variance across subjects (r = 0.22, P = 0.234). Also, no interindividual correlation was found between the R-R interval responses to propranolol and to atropine, with the influence of the other, respective, autonomic division already blocked (r = 0.42, P = 0.114).

CONCLUSIONS

Under resting conditions, activity levels of cardiac vagal and sympathetic outflows are not related across young, healthy human subjects and peripheral interaction is not manifest between the autonomic divisions.

Human autonomic rhythms: vagal cardiac mechanisms in tetraplegic subjects

1. We studied eight young men (age range: 20-37 years) with chronic, clinically complete high cervical spinal cord injuries and ten age-matched healthy men to determine how interruption of connections between the central nervous system and spinal sympathetic motoneurones affects autonomic cardiovascular control. 2. Baseline diastolic pressures and R-R intervals (heart periods) were similar in the two groups. Slopes of R-R interval responses to brief neck pressure changes were significantly lower in tetraplegic than in healthy subjects, but slopes of R-R interval responses to steady-state arterial pressure reductions and increases were comparable. Plasma noradrenaline levels did not change significantly during steady-state arterial pressure reductions in tetraplegic patients, but rose sharply in healthy subjects. The range of arterial pressure and R-R interval responses to vasoactive drugs (nitroprusside and phenylephrine) was significantly greater in tetraplegic than healthy subjects. 3. Resting R-R interval spectral power at respiratory and low frequencies was similar in the two groups. During infusions of vasoactive drugs, low-frequency R-R interval spectral power was directly proportional to arterial pressure in tetraplegic patients, but was unrelated to arterial pressure in healthy subjects. Vagolytic doses of atropine nearly abolished both low- and respiratory-frequency R-R interval spectral power in both groups. 4. Our conclusions are as follows. First, since tetraplegic patients have significant levels of low-frequency arterial pressure and R-R interval spectral power, human Mayer arterial pressure waves may result from mechanisms that do not involve stimulation of spinal sympathetic motoneurones by brainstem neurones. Second, since in tetraplegic patients, low-frequency R-R interval spectral power is proportional to arterial pressure, it is likely to be mediated by a baroreflex mechanism. Third, since low-frequency R-R interval rhythms were nearly abolished by atropine in both tetraplegic and healthy subjects, these rhythms reflect in an important way rhythmic firing of vagal cardiac motoneurones.

Parasympathetic nervous system control of heart rate responses to stress in offspring of hypertensives

The elevated heart rate response to stress in normotensive offspring of hypertensives (PH+) has been suggested to be a function of sympathetic nervous system activity. This study examined whether parasympathetic nervous system activity may also underlie familial differences in the heart rate response. Twenty-four subjects, half of whom were PH+, were exposed to four stressor tasks administered in counterbalanced order. Stressors were chosen based on previous research that suggested vagal contributions to the heart rate response. Stressors were a cold pack to the forehead, isometric hand grip, a noxious film, and a shock-avoidance video game task. Physiological measures included heart rate (HR), respiration rate (RR), and respiratory sinus arrhythmia (RSA). RSA values were corrected for corresponding RR by analysis of covariance. Familial differences in HR were observed in response to the hand grip and video game tasks. However, in both cases analyses suggest that familial differences in reactivity were a function of primarily sympathetic as opposed to parasympathetic influences. Familial differences in RSA were not observed for rest or tasks. This study found no evidence for parasympathetic mediation of familial differences in the heart rate response to the stressors employed.

Sudden cardiac death and the potential role of beta-adrenoceptor-blocking drugs

Sudden cardiac death is a major health problem in the industrially developed countries. The risk of sudden cardiac death may be reduced by early detection of coronary heart disease, elimination of the risk factors, treatment of the ischaemia in patients known to have coronary heart disease and suppression of ventricular arrhythmias. Of all the therapeutic measures currently available to reduce the risk of sudden cardiac death, beta-adrenoceptor-blocking drugs (beta blockers) appear to be the most effective. In this paper their actions are reviewed and evidence for their efficacy is presented.

Respiratory sinus arrhythmia, cardiac vagal tone, and respiration: within- and between-individual relations

Respiratory sinus arrhythmia (RSA) is frequently employed as an intra- and interindividual index of cardiac parasympathetic tone, although the relationship of RSA to interindividual differences in cardiac vagal tone remains questionable. Our study examined between- and within-subject relations among RSA, cardiac vagal tone, and respiratory parameters. Twenty-nine young adults performed two sessions of tasks under no medication and single and double autonomic blockade (intravenously administered propranolol and atropine). Parasympathetic tone was determined from heart period responses to complete vagal blockade. Results indicated the following. Resting RSA does not accurately predict individual differences in cardiac vagal tone. However, RSA and heart period together do predict such individual differences reasonably well. The relationship between individual variations in RSA and vagal tone is not improved by controlling respiratory parameters. Substantial cardiac vagal activity occurs during inspiration, and intraindividual variations in respiratory measures confound the association between RSA and cardiac vagal tone.

A view from Europe

The pharmacologic approach to coronary protection, defined here as the prevention or delay of sudden death and myocardial infarction (without negatively affecting noncardiac mortality), is critically discussed. The value of pharmacologically treating mild hypertension and mild hypercholesterolemia is questioned, and the need for well-designed, randomized clinical trials with definitive endpoints to determine a drug's cardioprotective capability is emphasized. Based on such studies, it is concluded that some (but perhaps not all) beta-receptor antagonists as well as aspirin have been shown to protect against sudden cardiac death. Trials of thiazide diuretics, calcium antagonists, and angiotensin-converting enzyme inhibitors have not shown a reduction in sudden cardiac death, despite having individual benefits with respect to other aspects of cardiovascular disease. The demonstration that some beta blockers are cardioprotective is discussed in terms of the pathophysiology of sudden cardiac death, and differences in the pharmacokinetic profiles of individual agents.

MRC trial of treating hypertension in older adults

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The effect of chronic timolol in an animal model for myocardial infarction

The effect of no drug or timolol (5 mg/kg, PO, for 1, 2, or 8 weeks on postganglionic cardiac sympathetic neural discharge, blood pressure, heart rate and beta-receptor density after acute coronary occlusion of the left anterior descending artery was compared. Beta-receptor density, determined by binding of 3H-dihydroalprenolol, was examined in the myocardium (LA = left atrium, RA = right atrium, LV1 = proximal and LV2 = distal left anterior descending artery distribution, LV3 = posterior left ventricle, S = septum, and RV = right ventricle). In control cats (no coronary occlusion or timolol) beta-receptor density of LV2 and LV3 was greater (P less than .05) than LA, RA, LV1, and RV. LV3 was greater (P less than .05) than S and RA, and LA was less than S. Longer treatment with timolol increased beta-receptor density. When compared with no timolol, beta-receptor density was greater in RA after 8 weeks and in LV1 after 2 weeks and not different in LV2 and S. Beta-receptor density and LV3 and RV were greater after 8 weeks than after 1 week or no timolol. Spearman rank correlation coefficients between dose and beta-receptor density revealed an increase (P less than .05) for all heart areas. Heart rate did not vary before timolol and was decreased after all doses of timolol. Timolol increased the mean times to coronary occlusion-induced death although the increase was not statistically significant. Timolol did not prevent postganglionic cardiac sympathetic neural discharge associated with arrhythmia. Timolol may increase beta-receptor density and decrease synaptic norepinephrine, causing a decreased release per cardiac sympathetic nerve impulse. Alternatively, molecules of timolol may accumulate in nerve endings and be released in greater concentrations at the receptors. This could explain the protection against coronary occlusion-induced arrhythmia and death.

Effects of adenosine on autonomic control of heart rate in man

Six healthy subjects (two female) aged 23-40 years participated in a double-blind randomized cross-over study to investigate autonomic mechanisms involved in the chronotropic effect of adenosine in conscious man. Adenosine was infused in increasing doses following saline, propranolol (0.2 mg kg-1 body weight) or propranolol (0.2 mg kg-1 plus atropine (0.04 mg kg-1). Heart rate and blood pressure were measured supine, on standing and during a Valsalva manoeuvre. Plasma catecholamines were measured in the supine and standing positions. Following saline, adenosine (up to 120 micrograms kg-1 min-1) caused a dose-related increase in heart rate (mean +/- SD maximum increase 18 +/- 8 bpm; P less than 0.01). The change in heart rate with adenosine after propranolol (12 +/- 9 bpm; P less than 0.05) did not differ significantly from the corresponding change following saline but was abolished by propranolol plus atropine, which, in turn, was associated with a mean maximum decrease in heart rate of 5 +/- 3 bpm (P less than 0.01). The increase in heart rate during the initial 30 s on standing was augmented with adenosine compared with saline (16 +/- 5 bpm; P less than 0.01). A significant increase in plasma noradrenaline on standing was also found with adenosine compared with saline (6.37 +/- 2.86 vs. 4.77 +/- 1.79 nmol 1(-1); P less than 0.05). The heart rate response to the Valsalva manoeuvre was not affected by adenosine. These results suggest that the positive chronotropic effect of infused adenosine in conscious man may in part be caused by an inhibition of cardiac vagal tone.(ABSTRACT TRUNCATED AT 250 WORDS)

The effect of beta blockers on cardiac neural discharge associated with coronary occlusion in the cat

The effect of timolol on postganglionic cardiac sympathetic neural discharge, blood pressure, heart rate, and rhythm changes associated with acute coronary occlusion of the left anterior descending artery was examined and compared with the effects of the beta blockers practolol and metoprolol. Timolol (5 mg/kg, IV) was infused 15 minutes prior to coronary occlusion in cats anesthetized with alpha-chloralose. Control heart rate fell from 129 +/- 10 to 106 +/- 2 one minute prior to coronary occlusion and remained at 106 +/- 2 beats/minute in the minute prior to arrhythmia. Control blood pressure fell from 126 +/- 20 to 91 +/- 19 and stabilized at 99 +/- 19 mm Hg one minute prior to coronary occlusion. Mean time to arrhythmia and death was 4.7 +/- 2.3 and 68.0 +/- 51.0 minutes (P greater than .05 vs no drug), respectively. Three cats died and two were sacrificed six hours after coronary occlusion. Blood pressure fell to 86 +/- 20 mm Hg two minutes after coronary occlusion, rose to 95 +/- 23 mm Hg at ten minutes, and remained there for ten minutes. Timolol did not alter postganglionic cardiac sympathetic neural discharge prior to coronary occlusion. Two minutes after coronary occlusion, mean postganglionic cardiac sympathetic neural discharge was 128 +/- 27 and increased to 139 +/- 36 impulses/second (% control) 4 minutes after coronary occlusion. A similar trend was found for the data recorded in 15 nerves (eight cats) in which coronary occlusion was initiated without timolol. The data suggest that a difference exists among beta blockers because prior to coronary occlusion, the cardioselective drugs metoprolol (1, 5, and 10 mg/kg, IV) and practolol (8 mg/kg, IV) depressed postganglionic cardiac sympathetic neural discharge whereas noncardioselective timolol did not. Because all three beta blockers increased the times to arrhythmia and death (although the increase was significant only after metoprolol and practolol), the acute protective mechanism does not appear to be due primarily to a depression of spontaneous sympathetic neural discharge.