Role of the nervous system in experimentally induced arrhythmias

Circadian variation of heart rate variability and the rate of autonomic change in the morning hours in healthy subjects and angina patients

BACKGROUND

Incidence of sudden cardiac death peaks during the early morning hours when there is a rapid withdrawal of vagal and an increase of sympathetic tone. The rate of autonomic change could be of prognostic importance.

PATIENTS AND METHODS

A total of 65 patients with angina pectoris, free from other diseases and drug free, were Holter monitored for 24 h. A total of 30 patients were also monitored on isosorbide-5-mononitrate (IS-5-MN) and on metoprolol respectively. A total of 33 age-matched healthy subjects served as controls. Spectral components of heart rate variability (HRV) were analysed hourly, with special reference to the rapid changes of autonomic tone during the night and early morning hours. Circadian variation was assessed in two ways: (1) Mean HRV day (8 a.m.-8 p.m.) and night (0-5 a.m.) were compared. (2) For the morning/night hours (0-10 a.m.), individual hourly values for max. and min. HRV, the difference max.-min. (gradient), the rate of change per hour between max. and min. (velocity) and the largest difference between two consecutive hours (max. velocity) were recorded and the mean value for the group calculated.

RESULTS

During the night/morning hours, healthy controls demonstrated faster HF max. velocity (P=0.002) and higher HF gradient (P=0.011) than angina patients. Metoprolol and IS-5-MN increased the HF gradient (P=0.008 and P=0.003, respectively), and metoprolol tended to increase the max. velocity (P=0.02). Metoprolol substantially decreased the LF/HF gradient (P=0.001), velocity (P=0.008) and max. velocity (P=0.0001).

CONCLUSION

Rapid vagal withdrawal seemed to be a sign of a healthy autonomic nervous system in the control group but was significantly slower in angina patients. IS-5-MN and metoprolol tended to normalise vagal withdrawal and metoprolol slowed down the rapid increase in sympathetic predominance in the morning in patients.

Patients with uncomplicated coronary artery disease have reduced heart rate variability mainly affecting vagal tone

AIM

To investigate whether uncomplicated chronic coronary artery disease causes changes in heart rate variability and if so, whether the heart rate variability pattern is different from that described in patients with acute myocardial infarction.

METHODS

Heart rate variability was studied in 65 patients with angina who had no previous myocardial infarcts, no other diseases, and were on no drug that could influence the sinus node. Results were compared with 33 age matched healthy subjects. The diagnosis of coronary artery disease in angina patients was established by coronary angiography in 58, by thallium scintigraphy in six, and by exercise test only in one. Patients and controls were Holter monitored 24 hours outside hospital, and heart rate variability was calculated in the frequency domain as global power (GP: 0.01-1.00 Hz), low frequency peak (LF: 0. 04-0.15 Hz), high frequency peak (HF: 0.15-0.40 Hz), LF/HF in ms(2), and in the time domain as SDNN (SD of normal RR intervals), SDANN (SD of all five minute mean normal RR intervals), SD (mean of all five minute SDs of mean RR intervals), rMSSD (root mean square of differences of successive normal RR intervals) (all in ms), and pNN50 (proportion of adjacent normal RR intervals differing more than 50 ms from the preceding RR interval) as per cent.

RESULTS

The mean age in patients and controls was 60.4 (range 32-81) and 59.1 (32-77) years, respectively (NS), the male/female ratio, 57/65 and 24/33 (NS), and the mean time of Holter monitoring, 23.0 (18-24) and 22.8 (18-24) hours (NS). Mortality in angina patients was 0% (0/65) at one year, 0% (0/56) at two years, and 3% (1/33) at three years. Compared with healthy subjects angina patients showed a reduction in GP (p = 0.007), HF (p = 0.02), LF (p = 0.02), SD (p = 0.02), rMSSD (p = 0.01), and pNN50 (p = 0.01). No significant difference was found in RR, LF/HF, SDNN, or SDANN.

CONCLUSIONS

Uncomplicated coronary artery disease without previous acute myocardial infarction was associated with reduced high and low frequency heart rate variability, including vagal tone. SDANN and SDNN, expressing ultra low and very low frequencies which are known to reflect prognosis after acute myocardial infarction, were less affected. This is in agreement with the good prognosis in uncomplicated angina in this study.

Regulatory peptides as modulators of vagal influence on cardiac rhythm

When the right vagus nerve of anesthetized cats was stimulated with repetitive bursts of pulses, decelerated heart rate became synchronized to the rhythm of the vagal bursts. Each burst applied to the vagus was followed by a single heart contraction. Within defined limits an increase in the frequency of vagal bursts evoked a proportional acceleration of the heart, whereas a decreased frequency diminished the heart rate. Therefore, over the range of synchronization the heart rate was precisely controlled by changing the vagal stimulation rate. We concluded that the chronotropic effect evoked by vagal bursts was composed of two functionally different types of influence, namely, inhibitory tonic and synchronizing. The vagotropic influence of intravenously injected regulatory peptides was found to be selective for either the tonic or synchronizing component. For instance, dalargin (D-Ala2-Leu5-Arg6-enkephalin) and neokyotorphin selectively diminished the inhibitory tonic vagal influence, whereas delta sleep inducing peptide and neurotensin potentiated it. The magnitude of synchronizing vagal influence was not modified by these peptides. In contrast, secretin selectively inhibited the synchronizing vagal effect, but the tonic one was not affected. Somatostatin potentiated the synchronizing effect but diminished the tonic one. These data support the hypothesis that certain regulatory peptides can modulate the effects of repetitive vagal bursts on pacemaker activity.

The anti-arrhythmic effect of D-Ala 2, Leu 5, Arg 6-enkephalin and its possible mechanism

We studied antiarrhythmic action of D-Ala 2, Leu 5, Arg 6-enkephalin (dalargin) in experiments on male rats. Dalargin is reported to prevent heart rhythm disturbance and heart electrical stability decrease in experimental coronary occlusion, postinfarction, cardiosclerosis and emotional stress. Dalargin prevents acute myocardial ischaemia-induced increase of cAMP content in blood serum and cardiac muscle, as an indirect feature of its antiadrenergic activity. D-Ala 2, Leu 5, Arg 6-enkephalin leads to a decrease of cAMP content in myocardium and blood plasma, which presumably indicates a decrease of sympathetic tone. The data strongly suggest that cGMP content increase and somatostatin level decrease in cardiac muscle play a significant role in antiarrhythmic action of dalargin.

A reduction in the correlation dimension of heartbeat intervals precedes imminent ventricular fibrillation in human subjects

Reduced reflexive control of heartbeat intervals occurs with advanced heart disease and is an independent risk factor for mortality. Based on a previous study of experimental myocardial infarction in pigs, we hypothesized that a deterministic measure of heartbeat dynamics, the correlation dimension of R-R intervals (D2), may be a better predictor of risk than a stochastic measure, such as the standard deviation (SD). We determined the point estimates of the heartbeat D2 (i.e., PD2s) in Holter electrocardiographic recordings from 11 high-risk patients who manifested ventricular fibrillation (VF) during the recording and in high-risk controls having only nonsustained ventricular tachycardia (14 patients) or premature ventricular complexes (13 patients). We found that PD2 reduction (i.e., PD2s < 1.2) precedes lethal arrhythmias by hours, but is not reduced in high-risk controls (p < 0.001; sensitivity, 91%; specificity, 85%). Heartbeat SD did not discriminate among the patients. Thus PD2 of heartbeat intervals may provide an important diagnostic test and early warning sign of VF.

Adrenergic effects on reentrant ventricular rhythms in subacute myocardial infarction

BACKGROUND

Reentry has been shown to be a mechanism of ventricular arrhythmias elicited by programmed premature stimulation in the subacute ischemic period of dogs subjected to myocardial infarction. The spatial distribution of refractoriness in these hearts has been shown to play an important part in the formation of functional arcs of conduction block during programmed ventricular stimulation. Because the adrenergic nervous system influences cardiac arrhythmias and myocardial infarction can directly affect sympathetic innervation in the heart, we investigated the role of the sympathetic nervous system on reentry in the canine heart 4 days after infarction.

METHODS AND RESULTS

The influences of adrenergic stimuli on the initiation of reentrant ventricular excitation were studied using a 128-channel computerized recording system in the canine heart 4 days after ligation of the left anterior descending coronary artery. Bilateral stimulation of the ansae subclavia preferentially improved conduction of premature beats in the normal zones. This corresponded to an improvement in excitability, as measured by a decrease in stimulus strength at the same premature coupling interval as control. Consequently, the effective refractory period was preferentially shortened at normal sites but not at ischemic sites. Both of these changes contributed to a shift of the arc of functional conduction block toward more normal tissue. As a result, sites proximal to the arc of functional conduction block had more time to recover excitability and thereby were available to be reexcited by the distal activation wave front. Conversely, intravenous infusion of norepinephrine preferentially shortened the effective refractory period of sites in the ischemic zone, thereby indicating that denervation hypersensitivity had occurred at these sites. The spatial dispersion of refractoriness and the arc of functional conduction block were significantly reduced in size. As a consequence, previously inducible reentrant rhythms were no longer inducible.

CONCLUSIONS

Sympathetic stimulation can be considered an arrhythmogenic intervention, whereas norepinephrine infusion may be considered antiarrhythmic in this experimental model.

Application of chaos theory to biology and medicine

The application of "chaos theory" to the physical and chemical sciences has resolved some long-standing problems, such as how to calculate a turbulent event in fluid dynamics or how to quantify the pathway of a molecule during Brownian motion. Biology and medicine also have unresolved problems, such as how to predict the occurrence of lethal arrhythmias or epileptic seizures. The quantification of a chaotic system, such as the nervous system, can occur by calculating the correlation dimension (D2) of a sample of the data that the system generates. For biological systems, the point correlation dimension (PD2) has an advantage in that it does not presume stationarity of the data, as the D2 algorithm must, and thus can track the transient non-stationarities that occur when the systems changes state. Such non-stationarities arise during normal functioning (e.g., during an event-related potential) or in pathology (e.g., in epilepsy or cardiac arrhythmogenesis). When stochastic analyses, such as the standard deviation or power spectrum, are performed on the same data they often have a reduced sensitivity and specificity compared to the dimensional measures. For example, a reduced standard deviation of heartbeat intervals can predict increased mortality in a group of cardiac subjects, each of which has a reduced standard deviation, but it cannot specify which individuals will or will not manifest lethal arrhythmogenesis; in contrast, the PD2 of the very same data can specify which patients will manifest sudden death. The explanation for the greater sensitivity and specificity of the dimensional measures is that they are deterministic, and thus are more accurate in quantifying the time-series. This accuracy appears to be significant in detecting pathology in biological systems, and thus the use of deterministic measures may lead to breakthroughs in the diagnosis and treatment of some medical disorders.

Effects of transdermal scopolamine on heart rate variability in normal subjects

A decrease in cardiac parasympathetic tone is a recognized finding in patients with ischemic heart disease, sudden cardiac death and heart failure, correlating closely with disease severity and overall survival. To study the clinical potential of vagomimetic intervention, the effect of transdermal scopolamine on fluctuations in heart rate was studied in 32 healthy adult subjects using both time-domain (mean RR interval, standard deviation of the mean RR interval, mean of the differences between consecutive RR intervals) and frequency-domain measures (spectrum analysis of 128 consecutive RR intervals) of heart rate variability. After an exposure of 24 hours, transdermal scopolamine resulted in a significant increase in all indexes tested. The increase was most pronounced in the 0.25-Hz respiratory peak of the RR interval power spectrum, compatible with a strong vagomimetic mode of action of transdermal scopolamine. Results indicate that transdermal scopolamine may have potential merit as a selective vagotonic agent in certain patients with myocardial infarction, heart failure or ventricular arrhythmias.

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.

The effect of C1 spinal cord transection or bilateral adrenal vein ligation on thioridazine-induced arrhythmia and death in the cat

The phenothiazine thioridazine 1 mg/kg/min was infused intravenously into three groups of cats: (1) thioridazine alone (N = 5), (2) after bilateral adrenal ligation (N = 4), and (3) after spinal cord section at the atlanto-occipital junction (C1; N = 6). The times to arrhythmia and death with thioridazine alone were 47.8 +/- 7.8 and 72.8 +/- 5.6 minutes respectively. After bilateral adrenal ligation, arrhythmia and death occurred at 41.1 +/- 5.2 and 53.1 +/- 5.8 minutes, respectively, which showed no increase (P greater than .05) from thioridazine alone. After spinal cord section, thioridazine-induced arrhythmia and death occurred at 74.0 +/- 13.7 and 85.7 +/- 13.8 minutes, respectively, which were not increased (P greater than .05) when compared with thioridazine alone. The results of this study suggest that neither adrenomedullary catecholamines nor the central sympathetic component above C1 plays a significant role in acute thioridazine-induced arrhythmia. The action of thioridazine to induce arrhythmia in spite of transection of the spinal cord or bilateral adrenal vein ligation suggests that its cardiotoxicity is a result of a direct myocardial effect. Thioridazine depressed blood pressure without producing the sustained reflex tachycardia normally seen with hypotension. This suggests that the agent may modify the baroreceptor reflex arc.

Role of vagus in the antagonism of ouabain induced arrhythmias in dogs by beta-adrenoceptor antagonists and related drugs

The effects of propranolol and related drugs were investigated on ouabain-induced ventricular tachycardia (VT) in dogs with intact and ablated vagi. Propranolol and UM-272 completely antagonized the ouabain VT in dogs with intact vagi, whereas timolol was ineffective. Bilateral vagotomy completely abolished the effect of UM-272 and reduced the effect of propranolol. Diphenylhydantoin, however, reversed ouabain VT in dogs with both intact and ablated vagi. It is inferred that the vagus plays a significant role in the arrhythmolytic effect of propranolol and UM-272.

Parasympathetic effects on electrophysiologic properties of cardiac ventricular tissue

The physiologic importance of parasympathetic influence on the sinoatrial and atrioventricular nodes is well established, but the importance of parasympathetic modulation of ventricular function remains controversial. Recognized effects of muscarinic cholinergic stimulation on ventricular automaticity and ventricular repolarization, the ability of muscarinic cholinergic agonists to antagonize catecholamine effects in the ventricle and proposed mechanisms for these effects are described. Anatomic studies have demonstrated a great abundance of cholinergic nerve endings in association with the ventricular conducting system. Stimulation of the vagus nerve or addition of muscarinic cholinergic agonists suppresses ventricular automaticity in most species and antagonizes isoproterenol-induced action potential shortening and isoproterenol-restored slow response action potentials. In vivo, interactions between the parasympathetic and sympathetic nervous systems occur at multiple levels. Muscarinic cholinergic agonists inhibit release of norepinephrine from sympathetic nerve terminals, inhibit catecholamine-stimulated adenylate cyclase activity and alter cyclic guanosine monophosphate (GMP) and possibly cyclic adenosine monophosphate (AMP) levels. Evidence is also presented that, in vivo, parasympathetic effects on ventricular electrical function might influence the pathophysiologic milieu responsible for initiation or termination of certain ventricular arrhythmias. Vagal influences appear to be protective against certain digitalis-induced arrhythmias and protective in certain experimental acute myocardial infarctions. In human beings, there appears to be tonic vagal tone in the ventricle and vagal stimulation terminates certain types of ventricular tachycardia. The evidence presented supports a physiologic role of parasympathetic stimulation in altering ventricular electrical function.

Failure of methylprednisolone to prevent nonuniform cardiac accelerator nerve discharge associated with coronary occlusion-induced arrhythmia: evidence against prostaglandin modulation of autonomic cardioaccelerator neural discharge in the anesthetized cat

Postganglionic cardiac sympathetic neural discharge in the minute prior to arrhythmia produced by acute occlusion of the left anterior descending coronary artery was nonuniform, i.e., both increases or decreases occurred. It is hypothesized that the nonuniform neural discharge is transmitted to the heart, causing nonuniform changes in excitability and conduction which produce ventricular arrhythmias. Prostaglandins have been shown to exhibit both arrhythmogenic as well as antiarrhythmic actions. Methylprednisolone (30 or 50 mg/kg, i.v., a known inhibitor of prostaglandin synthesis) was given 30 minutes prior to coronary occlusion of the left anterior descending artery. Both doses of methylprednisolone failed to: exert any effect on the mean postganglionic cardiac sympathetic nerve discharge prior to coronary occlusion; eliminate the nonuniform neural discharge associated with occlusion-induced arrhythmia; increase the time to arrhythmia or death; and decrease the number of cats dying after acute coronary occlusion. Thus, it is hypothesized that methylprednisolone failed to prevent the arrhythmogenic actions of prostaglandins. The failure of methylprednisolone to decrease the time to arrhythmia suggests that, in this study, an antiarrhythmic mechanism for prostaglandins does not exist. The failure of methylprednisolone to prevent the occlusion-induced nonuniform cardiac sympathetic discharge suggests that prostaglandin modulation of the autonomic balance on the heart is not due to a direct action on the postganglionic cardiac sympathetic nerve.

Autonomic dysfunction in epilepsy: characterization of autonomic cardiac neural discharge associated with pentylenetetrazol-induced epileptogenic activity

The purpose of this study was to determine if epileptogenic activity is associated with changes in autonomic cardiac neural discharge and the development of arrhythmias. Nine cats, anesthetized with alpha-chloralose, received pentylenetetrazol (PTZ), 10, 20, 50, 100, 200, and 2,000 mg/kg, intravenously at 10 min intervals. The following were monitored: neural discharge from 1 to 3 postganglionic cardiac sympathetic branches (8 cats, 17 nerves) and the vagus (7 cats, 8 nerves); the electrocorticogram; blood pressure; heart rate; and lead II electrocardiogram. Autonomic dysfunction was manifested by: the observation that autonomic cardiac nerves did not always respond in a predictable manner to changes in blood pressure; the development of a marked increase in variability in mean autonomic cardiac sympathetic and parasympathetic neural discharge; and the fact that the very large increase in the variability of the discharge rate of parasympathetic nerves was seen after PTZ, 50 mg/kg, but did not develop until 100 mg/kg in sympathetic neural discharge. This autonomic imbalance was associated with both interictal and ictal epileptogenic activity. Almost invariably, interictal discharge occurred after PTZ, 10 mg/kg; with higher doses, the duration of ictal activity increased although interictal discharges were present. The altered cardiac autonomic neural discharge was associated with minimal epileptogenic activity in the form of interictal discharges and cardiac arrhythmias which may contribute to sudden unexplained death of the epileptic.

Influence of cardiac rhythm disturbances and antiarrhythmic drugs on the efflux of PGE, PGF2 alpha, cyclic AMP, and cyclic GMP in canine coronary sinus blood

In anaesthetized open-chest dogs, cardiac arrhythmias (CA) were induced by cumulative intravenous doses of aconitine or ouabain. Aconitine in a dose which did not induce CA had no influence on the PGE and PGF2 alpha effluxes into coronary sinus blood (CSB), whereas the PGE efflux into CSB increased after a subtoxic dose of ouabain. However, both PGE and PGF2 alpha effluxes were increased, when CA had developed. During aconitine induced CA, the PGE efflux was 6.5-fold and that of PGF2 alpha had increased by 80%. During ouabain induced CA, the effluxes of both PGs were about 3-fold. Propranolol and lidocaine decreased the PGF2 alpha efflux into CSB by about 50% and the PGE efflux was doubled after lidocaine and decreased after propranolol by about a third. The increased PGE efflux into CSB during CA was normalized after propranolol and quinidine if the CA was abolished or the cardiac rhythm improved. Lidocaine did not modify the increase in PGE efflux, despite the abolishment of CA. The increase in PGF2 alpha efflux was not influenced by antiarrhythmic drugs. The cyclic AMP and cyclic GMP in CSB remained unchanged during ouabain induced arrhythmias or after propranolol. The increased efflux of PGE into CSB during aconitine and ouabain induced CA and its abolishment by propranolol support the hypothesis that PGE participates in the modulation of increased sympathetic tone during CA.

Effect of timolol on autonomic neural discharge associated with ouabain-induced arrhythmia

The purpose of this study was to determine the effect of the beta-blocking agent timolol on postganglionic cardiac and preganglionic splanchnic sympathetic and vagal neural discharge, ouabain-induced arrhythmia, heart rate and mean arterial blood pressure. Cats were anesthetized with alpha-chloralose, given atropine, and pretreated with timolol (5 mg/kg, i.v. infused at a rate of 0.5 mg/kg/min for 10 min). Bolus injections of ouabain (25 microgram/kg, i.v.) were given every 15 min until the animals died; the first injection was given 15 min after the end of the timolol infusion. When compared with cats (n = 13) receiving only ouabain, timolol (n = 11 cats) increased the time to ouabain-induced arrhythmia and death from 23 +/- 3 to 48 +/- 7 and 46 +/- 3 to 76 +/- 9 min, respectively (p less than 0.05). Both heat rate and mean arterial blood pressure had decreased from 137 +/- 4 to 104 +/- 6 beats/min and 133 +/- 6 to 103 +/- 7 mm Hg, respectively (P less than 0.05); ouabain did not reverse the decreases. Neural activity monitored from the vagus and from the postganglionic cardiac and preganglionic splanchnic sympathetic nerves was not significantly altered by the infusion of timolol. The administration of ouabain after timolol did not alter splanchnic nor vagal discharge. Most important was the observation that postganglionic cardiac sympathetic neural discharge exhibited both increases and decreases, i.e., a nonuniform neural discharge, at the time of ouabain-induced arrhythmia and that the ouabain-induced nonuniformity did not occur in the cats pretreated with timolol. Thus, the protective effect of timolol may be due, in part, to prevention of the nonuniform postganglionic cardiac sympathetic neural discharge and to the prevention of ouabain-induced increases in vagal discharge. The establishment of beta-blockade and a direct negative inotropic action may also have contributed to the antiarrhythmic action of timolol.

Nonuniform cardiac sympathetic nerve discharge: mechanism for coronary occlusion and digitalis-induced arrhythmia

This study examined nonuniform postganglionic cardiac sympathetic neural discharge as a possible mechanism involved in the production of coronary occlusion or ouabain-induced arrhythmias. After acute occlusion of the left anterior descending coronary artery in 12 cats, anesthetized with alpha-chloralose and pretreated with atropine, arrhythmia occurred within 3 min in eight animals; three of these died in ventricular fibrillation. In recordings from 15 nerves in the eight animals with arrhythmia, spontaneous discharge increased in nine nerves, decreased in five nerves, and showed no change in one nerve. This nonuniform neural discharge was associated with the development of arrhythmia after occlusion. In four of the cats, neural discharge did not change within the first 3 min after coronary artery occlusion and arrhythmia did not occur. Development of ouabain-induced arrhythmia was accompanied by a nonuniform pattern in the neural discharge (13 cats). This discharge may alter ventricular excitation and conduction to produce arrhythmia.