Effects of clonidine on vulnerability to fibrillation in the normal and ischemic canine ventricle

Effects of central sympathetic activation on repolarization-dispersion during short-term myocardial ischemia in anesthetized rats

AIMS

Sympathetic activation during myocardial ischemia enhances arrhythmogenesis, but the underlying pathophysiologic mechanisms remain unclear. We investigated the central sympathetic effects on ventricular repolarization during the early-period post-coronary artery occlusion.

MAIN METHODS

We studied 12 Wistar rats (254±2 g) for 30 min following left coronary artery ligation, with (n=6) or without (n=6) pretreatment with the central sympatholytic agent clonidine. Mapping of left and right ventricular epicardial electrograms was performed with a 32-electrode array. As an index of sympathetic activation, heart rate variability in the frequency domain was calculated. Heart rate and repolarization duration were measured with a custom-made recording and analysis software, followed by calculation of intra- and inter-ventricular dispersion of repolarization.

KEY FINDINGS

Heart rate and heart rate variability indicated lower sympathetic activation in clonidine-treated rats during ischemia. Repolarization duration in the left ventricle prolonged after clonidine at baseline, independently of heart rate, but no differences were present 30 min post-ligation. Dispersion of repolarization in the right ventricle remained stable during ischemia, whereas it increased in the left ventricle, equally in both groups. A similar trend was observed for inter-ventricular dispersion, without differences between groups.

SIGNIFICANCE

In addition to intra-ventricular repolarization-dispersion, anterior-wall myocardial ischemia may also increase inter-ventricular repolarization-dispersion. Progressive central sympathetic activation occurs during myocardial ischemia, but it does not affect intra- or inter-ventricular dispersion of ventricular repolarization during the early phase. Further research is warranted on the potential effects during subsequent time-periods.

Experimental studies of psychophysiological factors in sudden cardiac death

Earlier research in the field of sudden cardiac death is reviewed. Such studies have largely oriented towards the provocation of myocardial injury and asystole in normal animals. However, such investigations constitute an inadequate model to describe the clinical appearance of sudden death, where underlying coronary disease is often present and the precipitating event is usually ventricular fibrillation rather than asystole. This report describes a series of studies designed to investigate the processes underlying cardiac vulnerability and the influence upon it of various psychological stresses. It is concluded that the primary mediator of ventricular vulnerability is the sympathetic nervous system. The efferent vagus appears to exert some protective influence against arrhythmias due to adrenergic stimulation. An appropriate clinical strategy for the treatment of malignant arrhythmias would therefore involve attempts to decrease cardiac sympathetic drive whilst at the same time enhancing vagal tone. Treatments are described which aim to bring this situation about by the use of clonidine, morphine sulphate, l-tryptophan and tyrosine. The use of neurochemical agents in this context appears promising.

Abolition of clonidine's effects on ventricular refractoriness by naloxone in the conscious dog

The interaction between opiate and adrenergic receptors on cardiac electrophysiologic function in the conscious dog was addressed in our study. We examined the effects of opiate receptor blockade with naloxone on clonidine-induced changes in refractoriness of the cardiac ventricle. Nine dogs were chronically instrumented for recording mean arterial blood pressure, administration of drugs and for measurement of effective refractory period of the ventricle. Clonidine (10 micrograms/kg, i.v.) significantly (p less than 0.05) decreased heart rate to 72 +/- 5 beats/minute from 108 +/- 8 beats/minute; mean arterial pressure decreased significantly (p less than 0.05) to 83 +/- 3 mmHg from 91 +/- 4 mmHg. Ventricular refractoriness was increased significantly (p less than 0.05) at current levels of 7 and 10 mA and pacing rates 180 and 200 beats/minute. Naloxone (3-10 mg/kg, i.v.) abolished clonidine's effects on heart rate, mean arterial pressure and ventricular refractoriness. We conclude that ventricular refractoriness may be regulated in part by interactions between central adrenergic and opioidergic systems.

Correlation of cardiac arrhythmias with brainstem compression in patients with intracerebral hemorrhage

Neurogenically caused cardiac arrhythmias and their correlation to lesions within the central nervous system were examined prospectively in 54 patients with spontaneous intracerebral hemorrhage. All patients were examined neurologically daily for 3 weeks, with special attention given to signs of brainstem compression resulting from transtentorial herniation. Electrocardiograms were continuously recorded over an average of 5 days. A significant correlation was established between the clinical manifestations of brainstem compression and sinus arrhythmias, multifocal premature ventricular contractions, couplets, and ventricular tachycardias. An explanation for this correlation may be found in the localization of the autonomous cardiovascular centers in the hypothalamus and brainstem. Transtentorial herniation frequently leads to a bilateral lesion of these structures. However, the cardiac arrhythmias are only a partial phenomenon within a complex cardiovascular reaction.

Antifibrillatory action of the narcotic agonist fentanyl

Morphine, an opiate alkaloid with mixed mu- and delta-agonist properties, raises the ventricular fibrillation threshold in anesthetized dogs by altering autonomic tone. To elucidate further underlying structure-activity relationships, the effect of fentanyl, a nonalkaloid, mu-selective agonist in wide clinical use, was studied. Fentanyl (30 micrograms/kg) was given intravenously to 27 chloralose-anesthetized dogs, and ventricular fibrillation threshold was measured by means of the single-stimulus technique. In the baseline state fentanyl raised the ventricular fibrillation threshold by 14%. When the dogs were subjected to hemorrhagic stress, this effect was amplified to 29% (p less than 0.0001). Bilateral cervical vagotomy abolished fentanyl's antifibrillatory effect, but neither atropine sulfate (0.4 mg/kg/hr) nor atropine methylnitrate (0.5 mg/kg/hr) did so. Fentanyl's influence on the fibrillation threshold during hemorrhage was significantly reduced by bilateral stellate ganglionectomy (p less than 0.005). It is concluded that fentanyl raises the ventricular fibrillation threshold by its known sympathoinhibitory action rather than by its vagal efferent activating effect. The facts that an intact vagus is required and that hemorrhage amplifies the effect suggest that the antifibrillatory effect of fentanyl is mediated through the afferent component of the baroreflex arc.

Hemodynamic and electrophysiologic effects of a new alpha 2-adrenoceptor agonist, rilmenidine, for systemic hypertension

The hemodynamic and electrophysiologic effects of rilmenidine were examined after single oral administration to hypertensive patients. In 8 untreated hypertensive patients, cardiac output, pulmonary pressure and blood pressure were measured before and for 10 hours after the administration of 25 micrograms/kg of rilmenidine (1.3 to 2.4 mg, mean 1.88). In addition, electrophysiologic investigations were performed before and 2 hours after administration. Hemodynamics were repeated in 8 other hypertensive patients receiving 50 micrograms/kg rilmenidine (3.0 to 4.8 mg, mean 3.85 mg). The electrophysiologic study was repeated in 8 other hypertensive patients receiving 50 micrograms/kg of rilmenidine (3.2 to 4.4 mg, mean 3.90). In contrast to the results obtained at the dose of 50 micrograms/kg, there was no significant variation in pulmonary arterial pressure, cardiac index or stroke index after administration of 25 micrograms/kg. No significant variation was observed in heart rate, sinus function, conduction parameters or atrial, nodal and ventricular refractory periods after administration of 25 and 50 micrograms/kg. Rilmenidine, after single oral administration at the 25 micrograms/kg dose, led to a significant reduction in blood pressure and peripheral resistance without any significant change in cardiac output; the 25- and 50-micrograms/kg doses led to no alteration in heart rate and cardiac electrophysiology.

Influence of site of stimulation on measurement of ventricular vulnerability in the normal and ischemic feline heart

Site of stimulation is an important variable in the inducibility of ventricular tachycardia in both non-human animal infarction models and in humans; that is, proximity of the stimulating electrode to the site of reentry facilitates the induction of sustained arrhythmia. Whether site of stimulation is decisive in measurement of vulnerability to ventricular fibrillation (VF) during acute coronary occlusion has not been fully evaluated. We measured VF thresholds in 9 chloralose-anesthetized cats at 2 right ventricular and 3 left ventricular sites (2 endocardial, 3 intramural) before and after abrupt occlusion of the anterior descending coronary artery. VF thresholds were measured using a single stimulus of increasing intensity delivered during ventricular drive. Although VF thresholds were lower at endocardial sites, there were no significant differences in VF threshold among any of the sites tested at control. After occlusion, VF thresholds fell to a similar extent at all 5 sites tested. The percent reduction in VF threshold at any site was not influenced by the sequence of testing. VF may be precipitated from multiple sites and, unlike ventricular tachycardia, does not represent an isolated focus of arrhythmogenicity.

The repetitive extrasystole as an index of vulnerability to ventricular fibrillation during myocardial ischemia in the canine heart

The repetitive extrasystole threshold (RET) is a reliable measure of vulnerability to ventricular fibrillation (VF) during diverse interventions in the normal heart. Whether this relationship also holds during varying degrees of myocardial ischemia has not been adequately explored. In 15 chloralose-anesthetized dogs, circumflex coronary blood flow (CBF) was decreased progressively with the use of an externally applied balloon occluder. There was a statistically significant correlation between the RET and ventricular fibrillation threshold (VFT) until left circumflex coronary artery flow was reduced by 90% of control values (r = 0.92). During reductions of CBF of 90% or greater, the VFT fell more than the RET and the RET/VFT ratio was disrupted. Total coronary occlusion, whether performed abruptly or gradually (5 minutes), likewise resulted in a disproportionate decline in VFT. During sustained total coronary occlusion, the VFT recovered to control values within 15 minutes, and the relationship between the RE and VF thresholds was restored. We conclude that the vulnerable period threshold for provoking repetitive extrasystole is a reliable index of vulnerability to VF during myocardial ischemia and remains so until nearly total occlusion of a major coronary vessel.

Effect of alpha-adrenergic receptor stimulation on ventricular electrical properties in the normal canine heart

We examined the effects of alpha-adrenergic stimulation on ventricular excitability, refractoriness, and vulnerability to fibrillation. Methoxamine or phenylephrine was infused in five dogs each before and after aortic arch and carotid sinus baroreceptor denervation, in doses which increased mean arterial blood pressure by 20 to 30 mm Hg. Methoxamine or phenylephrine caused an increase in the ventricular fibrillation threshold (VFT) (from 27% to 41%) and in the repetitive extrasystole threshold (RET) (from 28% to 39%). This effect was abolished by baroreceptor denervation. Neither drug altered mid-diastolic threshold or effective refractory period duration either before or after denervation. We conclude that alpha-receptor activation exerts no direct effect on ventricular excitability or refractoriness in the normal intact heart.

Tyrosine administration decreases vulnerability to ventricular fibrillation in the normal canine heart

Intravenous infusion of tyrosine (1, 2, or 4 milligrams per kilogram) for 20 to 30 minutes caused dose-dependent increases in the ventricular fibrillation threshold in normal dogs. Administration of valine, a neutral amino acid that competes with tyrosine for uptake at the blood-brain barrier, in a dose equimolar to the most effective dose of tyrosine, slightly decreased the ventricular fibrillation threshold when given alone and significantly blocked elevation of the ventricular fibrillation threshold after tyrosine infusion. Hence, tyrosine, presumably acting in the central nervous system, can protect against certain ventricular arrhythmias.