Influence of beta blockade and chemical sympathectomy on myocardial function and arrhythmias in acute ischaemia

Adrenergic-dependent Effect of Adenosine-induced Ventricular Fibrillation in the Isolated Rabbit Heart

BACKGROUND: The present study examined the contributory role of endogenous catecholamines in adenosine-induced ventricular fibrillation in isolation rabbit hearts. METHODS AND RESULTS: Cardiac catecholamine depletion was induced in eleven rabbits by the administration of 6-hydroxydopamine (2 x 30 mg/kg, every 12 hours intramuscularly). Hearts were removed 24 hours later, and subjected to 12 minutes of hypoxic perfusion followed by 40 minutes of reoxygenation while heart rate was maintained with atrial pacing. One of six, and one of five hearts from 6-hydroxydopamine treated rabbits developed ventricular fibrillation during hypoxia-reoxygenation when exposed to 3,7-dimethyl-1-propargylzanthine (DMPX) (10 µM) + adenosine (ADO) (1 µM) and DMPX (10 µM) + ADO (10 µM), respectively. In hearts from a control group, not exposed to 6-hydroxydopamine, ventricular fibrillation developed in each of five (100% incidence) hearts when perfused in the presence of DMPX (10 µM) + ADO (10 µM) (P <.05). Nadolol (1 µM), a beta-adrenoceptor DMPX (10 µM) + ADO (10 µM) treated hearts (n = 6, P <.05 vs DMPX + ADO treated hearts). To ensure catecholamine depletion, spontaneously beating isolated hearts from vehicle and 6-hydroxydopamine treated rabbits were perfused under normoxic conditions while exposed to increasing concentrations of tyramine (1, 3, 10 mM) and the change in heart rate was determined. A concentration-related, positive chronotorpic response to tyramine was obtained in hearts from the vehicle treated group that was absent in hearts from 6-hydroxy-dopamine treated rabbits or hearts perfused in the presence of nadolol. CONCLUSIONS: The results demonstrate that inhibition of the cardiac adenosine A(2) receptor, unmasks an adenosine A(1) receptor profibrillatory effect that is dependent upon endogenous cardiac catecholamines and beta-adrenoreceptor activation during myocardial hypoxia-reoxygenation.

High spinal anesthesia does not alter experimental myocardial infarction size or ischemic preconditioning

OBJECTIVE

The role of the central nervous system in the development of myocardial infarction and ventricular fibrillation in virgin and ischemically preconditioned myocardium was investigated.

DESIGN

Infarct size and ventricular arrhythmias were assessed after regional ischemia-reperfusion. Animals were randomly assigned to four groups: (1) preconditioned, central nervous system intact; (2) nonpreconditioned, nervous system intact; (3) preconditioned, nervous system blocked; and (4) nonpreconditioned, nervous system blocked. Differences in hemodynamics and infarct size were assessed with analysis of variance, and differences in ventricular fibrillation were assessed with the Kruskal-Wallis test.

SETTING

Experiments were performed in the Anesthesiology Research Laboratory at a medical center.

PARTICIPANTS

Anesthetized open-chest New Zealand white rabbits were used for these studies.

INTERVENTIONS

Rabbits underwent 30 minutes of coronary artery occlusion and 3 hours of reperfusion. The central nervous system was blocked with total spinal anesthesia. Ischemic preconditioning was elicited with 5 minutes of coronary artery occlusion and 10 minutes of reperfusion. Infarction was assessed with tetrazolium and expressed as a percentage of the risk zone (mean +/- SEM).

MEASUREMENTS AND MAIN RESULTS

Preconditioning resulted in infarct size limitation compared with the control (8% +/- 4% v 43% +/- 5%; p < 0.001) and delayed the onset of fibrillation (15.5 minutes v 11 minutes; p = 0.001). Spinal blockade neither altered nonpreconditioned infarct size nor attenuated preconditioning (32% +/- 7% v 8% +/- 3%; p = 0.04), but it was associated with ventricular fibrillation in 24/25 rabbits as compared with 6/14 rabbits without blockade. In blocked animals, preconditioning resulted in a decreased duration of fibrillation (2.5 minutes v 12.5 minutes; p = 0.0004). However, spinal blockade eliminated the preconditioning-induced delay in fibrillation (10 minutes v 12 minutes; p = NS).

CONCLUSIONS

It is concluded that (1) activation of efferent sympathetic nerves is not necessary for ischemic preconditioning; (2) preconditioning delays the onset of ventricular arrhythmias; and (3) spinal blockade exacerbates ischemia-induced ventricular arrhythmias.

Anti-arrhythmic efficacy of beta-adrenergic blockade during acute ischemia in myocardium with scar

Ventricular arrhythmia production in the ischemic heart is considered to be influenced by prior infarction. Although beta-adrenergic blockade is known to have beneficial effects during acute ischemia, its anti-arrhythmic efficacy during post-infarction ischemia is not known. To explore this question, we have used a model with a relatively high incidence of ischemic arrhythmias. Mongrel dogs 2 to 3 years of age were studied intact under anesthesia. An irreversible injury of the infero-posterior myocardium was produced with an electrode catheter 1 week earlier. The arrhythmic response to acute ischemia was assessed using serial, transient 15-minute occlusions of the left-anterior descending coronary artery with a balloon catheter. During ischemia alone, the incidence of ventricular fibrillation in animals who underwent all phases of the study was 6 of 9; with atenolol (0.2 mg/kg intravenously) and ischemia, 1 of 9 (p < 0.05). To assess the role of the bradycardic response, the latter was repeated 1 week subsequently during atrial pacing at the heart rate that existed before ischemia. Fibrillation occurred in 8 of 9, a significant reversal of the therapeutic effect. To exclude the potential artifact of a fixed intervention protocol, a study was undertaken with the short-acting esmolol, in which three ischemic periods were alternated at 1-hour intervals: (A) ischemia without treatment, (B) ischemia with continuous infusion of 150 micrograms/kg/min esmolol, and (C) same as B except that heart rate was maintained by atrial pacing.(ABSTRACT TRUNCATED AT 250 WORDS)

Potential arrhythmogenic role of cyclic adenosine monophosphate (AMP) and cytosolic calcium overload: Implications for prophylactic effects of beta-blockers in myocardial infarction and proarrhythmic effects of phosphodiesterase inhibitors

Activation of the adrenergic nervous system appears to play a crucial role in the genesis of fatal arrhythmias associated with the very early stages of acute myocardial infarction. The second messenger of beta-adrenergic catecholamine stimulation, cyclic adenosine monophosphate (AMP), has established arrhythmogenic qualities, acting by an increase in cytosolic calcium, which potentially has three adverse electrophysiologic effects. First, stimulation of the transient inward current by excess oscillations of cytosolic calcium can invoke delayed afterdepolarizations, so that triggered automaticity can develop in otherwise quiescent ventricular muscle. Second, cyclic AMP can evoke calcium-dependent slow responses in depolarized fibers, so that conditions for reentry are favored. Third, excess cytosolic calcium can cause intercellular uncoupling with conduction slowing. Focal changes in cyclic AMP and cytosolic calcium promote the development of ventricular fibrillation. Beta-adrenergic blockade can limit the formation of cyclic AMP in ischemic tissue. Furthermore, by reducing sinus tachycardia it can lessen cytosolic calcium overload. Hence, beta-adrenergic blockade helps to prevent ventricular fibrillation in the early stages of acute myocardial infarction and protects from sudden death in the postinfarction phase. In congestive heart failure, abnormalities of cytosolic calcium patterns exist with cytosolic calcium overload. It is proposed that the adverse effects of phosphodiesterase inhibitors on the mortality rate in patients with congestive heart failure can be explained by increased rates of formation of cyclic AMP and the development of calcium-dependent arrhythmias. Because calcium is the ultimate messenger of cyclic AMP-induced arrhythmias and because cytosolic calcium is increased in heart failure, it will be difficult to develop positive inotropic agents that are free of the risk of sudden death.

Cardiac sympathetic activity in myocardial ischemia: release and effects of noradrenaline

Sympathetic overactivity in myocardial ischemia is closely associated with the progression of myocyte injury and the incidence of malignant arrhythmias. Adrenergic stimulation of the ischemic myocardium is predominantly due to increased local noradrenaline concentrations in the heart, whereas plasma catecholamine levels are of minor relevance. During the first few minutes of ischemia, efferent sympathetic nerves are activated. Excessive accumulation of noradrenaline, however, is prevented since adenosine, formed in the ischemic myocardium, suppresses exocytotic noradrenaline release, and released noradrenaline is rapidly removed as long as catecholamine reuptake is functional. With progression of ischemia to more than 10 min, the myocardium is no longer protected against excess catecholamine accumulation in the interstitial space, since local metabolic release mechanisms become increasingly important. This release, which is independent of central sympathetic activity and from extracellular calcium, occurs in two steps: First, noradrenaline escapes from its intracellular storage vesicles and accumulates in the cytoplasm of the neuron. In a second, rate-limiting step, noradrenaline is transported across the plasma membrane into the interstitial space, using the neuronal uptake carrier in reverse of its normal transport direction. As a consequence of local metabolic catecholamine release, extracellular noradrenaline reaches 1000 times the normal plasma concentration within 20 min of ischemia. Studies using acute and chronic sympathetic denervation and antiadrenergic agents demonstrate that local metabolic, rather than centrally induced noradrenaline release is critically involved in the progression of ischemic cell damage within the occurrence of ventricular fibrillation in early ischemia. Myocardial ischemia results in a temporary supersensitivity of the myocytes to catecholamines. This is due to a twofold increase of alpha 1- and a 30% increase of beta-adrenergic receptor number at the cell surface. The sensitization of adenylate cyclase during the first 20 min of total ischemia is followed by a rapid inactivation of the enzyme. The beta-adrenergic hyperresponsiveness to catecholamines is therefore limited to the first few minutes of ischemia. The deleterious combination of extremely high noradrenaline concentrations with a temporarily enhanced responsiveness to catecholamines of the tissue is thought to accelerate the propagation of the wavefront of irreversible cell damage within the ischemic myocardium. Moreover, the inhomogenous distribution of catecholamine excess within the heart is considered to promote malignant arrhythmias by unmasking and enhancing electrophysiological disturbances in early ischemia.

Effects of beta-blockade on the incidence of ventricular tachyarrhythmias during acute myocardial ischemia: experimental findings and clinical implications

Myocardial ischemia and infarction are the most common substrates for life-threatening ventricular tachyarrhythmias. Experimental and clinical evidence suggests that enhanced activity of the sympathetic nervous system plays an important role in the genesis of ischemia-related arrhythmias. In animal experiments, beta-blockers display significant antifibrillatory effects during the acute phase of myocardial ischemia. Preconditions for their antifibrillatory effects are high serum- and tissue-concentrations, and absence of a significant partial agonist activity. During the delayed phase of ischemic arrhythmias which starts 6-8 h after coronary occlusion, beta-blockers gain significance as antiarrhythmic and potentially antifibrillatory drugs, if sympathetic activity is enhanced. The presently available evidence suggests that the potentially antifibrillatory effects of beta-blockers are at least one of the major mechanisms by which these drugs may decrease mortality when given prophylactically in patients after myocardial infarction. However, it remains to be explained why beta-blockers, in a great number of prospective randomized trials, have reduced the incidence of sudden death only by an average of about 30%. This may be the result of their "specific" mechanisms acting in the setting of acute myocardial ischemia with enhanced adrenergic tone, whereas in the remaining patients other mechanisms such as a chronic arrhythmogenic substrate may be operative. A clearer separation of these various mechanisms seems mandatory in order to allow a more specific "targeted" administration of beta-blockers. This is the more important since none of the available prospective studies that used antiarrhythmic agents has shown an improvement of prognosis, but, instead showed a worsening of the mortality rate.

Arrhythmia susceptibility and myocardial composition in diabetes. Influence of physical conditioning

Abnormal myocardial composition in diabetes mellitus has been described, but the effects on ventricular vulnerability have not been defined. We have assessed the susceptibility to arrhythmias in a canine model after 1 yr of mild diabetes induced by alloxan. Since physical conditioning can affect metabolic abnormalities in diabetes, this intervention has also been evaluated. Group 1 served as controls and groups 3 and 4 were diabetic. Animals in the latter group as well as nondiabetic controls of group 2 were exercised on a treadmill for the last 8 mo of the experiment. After 1 yr, anesthesia was induced with chloralose for vulnerability studies. The ventricular fibrillation threshold of 24.4 +/- 1.9 mA in group 3 was significantly less than in normals (45.1 +/- 2.2). Spontaneous arrhythmias were also more prevalent in diabetics during acute ischemia (group 3-A). Increased ventricular vulnerability after epinephrine infusion was present in the sedentary diabetes despite normal ventricular function responsiveness. In a superfused preparation of myocardium, resting membrane potential and action potential amplitude were normal in diabetics, and beta-adrenergic stimulation shortened repolarization more than in controls. Myocardial collagen concentrations, which included an interfibrillar distribution on morphologic examination, were increased in group 3. In the trained diabetics of group 4 the basal vulnerability thresholds and responses to epinephrine were normal. While myocardial collagen levels were normal, cholesterol and triglyceride increments persisted. Thus, in mild experimental diabetes, enhanced susceptibility to arrhythmias exists; this susceptibility may be based on a combination of nonhomogenous collagen accumulation affecting local conduction and increased electrophysiologic sensitivity to catecholamines.

Effects of beta-adrenergic inhibition on scar formation after myocardial infarction

In view of clinical interest in the efficacy of beta-adrenergic blockade during acute myocardial infarction (AMI), we have determined the long-term effect of therapy on scar formation after experimental myocardial ischemia. Intact anesthetized dogs underwent acute occlusion of the left anterior descending coronary artery, by means of a balloon catheter, which permitted monitoring of the aortic-peripheral coronary artery pressure gradient during the 4-hour period of balloon inflation. Practolol administration was begun 15 minutes after the onset of ischemia in group A. Control animals (group B) received procainamide to approximate the antiarrhythmic action of beta blockade. Only group A exhibited significant reduction in the ST segments during acute ischemia. Chronic therapy was maintained for 1 month and the mature scar formed in the myocardium was assessed after 4 months. The extent of subendocardial scar was similar in both groups but subepicardial scar formation was significantly less in group A. There was also a significant decrease in the percentage of total myocardium involved with scar in this treatment group. Although thinning of the left ventricular wall was similar for both groups in the central scar region, this process was significantly reduced at the lateral margin in group A. Thus, specific beta-receptor blockade during acute myocardial ischemia and sustained during the repair process can result in a reduced quantity and altered distribution of mature scar.

The effect of beta-adrenoceptor blocking agents, with differing ancillary properties, on the arrhythmias resulting from acute coronary artery ligation in anaesthetized rats

The effects of several beta-adrenoceptor blocking agents, [+), (-) and (+/-)-oxprenolol, p-oxprenolol, practolol, propranolol and timolol) were investigated on the ventricular arrhythmias occurring within the first 30 min of acutely ligating the main left coronary artery in anaesthetized rats. The degree of cardiac and vascular beta-adrenoceptor blockade was also assessed. All the compounds exhibited antiarrhythmic activity under these conditions. The degree of cardiac beta-adrenoceptor blockade required for this protection was less for the cardioselective agents, p-oxprenolol and practolol, than for the non-selective beta-adrenoceptor blocking agents. A comparison of the two isomers of oxprenolol demonstrated that the (-)-isomer markedly suppressed ischaemic arrhythmias (ventricular ectopic beats, incidence and duration of ventricular tachycardia and duration of ventricular fibrillation) more effectively than the (+)-isomer. Compounds possessing intrinsic sympathomimetic activity (ISA) caused less marked haemodynamic changes (in equivalent beta-blocking doses) than those that did not possess this ancillary property. The membrane stabilizing activity of oxprenolol and p-oxprenolol did not appear to contribute to the antiarrhythmic activity of these agents; however, the membrane stabilizing activity of propranolol may contribute to its effectiveness. In all the drugs studied, the main pharmacological property required to suppress early postischaemic arrhythmias is blockade of cardiac beta-adrenoceptors.

Sympathetic denervation limited to a region of acutely ischemic canine myocardium increases excitability threshold and duration of bipolar electrograms

This study determines the direct effects of sympathetic denervation on excitability threshold and bipolar electrograms in acutely ischemic myocardium. Regional denervation was performed by application of phenol to the epicardium surrounding the ischemic zone in order to eliminate the possible hemodynamic effects that global cardiac denervation may exert on the ischemic zone. Data were obtained during serial occlusions (less than or equal to 6 minutes in duration) of left anterior coronary artery in open-chest dogs with sympathetic denervation performed before the last occlusion. Late diastolic threshold was measured every 5 seconds by a constant voltage pacemaker which automatically registered threshold in stimulus duration. During ischemia, regional denervation (n = 9) increased peak excitability threshold from 240 +/- 51 (standard error of the mean) to 552 +/- 182 mus (p less than 0.05) and prolonged electrographic duration in epicardium from 19 +/- 3 to 25 +/- 4 ms (p less than 0.025) and in endocardium from 20 +/- 3 to 25 +/- 4 ms (p less than 0.01). Phenol application did not alter aortic pressure, ischemic wall motion (sonomicrometer technique), or ischemic zone blood flow (microsphere technique). Thus, acute sympathetic denervation when limited to ischemic myocardium increases the peak excitability threshold and concomitantly prolongs duration of bipolar electrograms.

Antiarrhythmic effects of aspirin during nonthrombotic coronary occlusion

To study the action of aspirin upon the myocardium per se, independent of thrombosis, coronary occlusion with a balloon catheter was induced in 53 anesthetized dogs divided into two groups. One group (N = 20) was treated daily with aspirin (600 mg/dog) for seven days and another (N = 33) was untreated. Left ventricular hemodynamics and precordial ECG mapping were used to assess the influence of myocardial ischemia over a four hour period. There were no significant differences in left ventricular function or extent of injury as judged by ECG mapping between the two groups. However, there was a significant decrease in the incidence of ventricular fibrillation in the treated dogs (5% vs 39%). Serial plasma samples for free fatty acid determination showed a significant rise in the untreated group. Aspirin blocked the FFA increment in the treated animals. Tissue samples from the ischemic area of left ventricle exhibited a significant reduction of the sodium and water increments, as well as a lesser potassium loss in the treated animals compared to the controls and may have been the basis for the lower incidence of arrhythmias. Since infusion of 51Cr labelled platelets showed no myocardial accumulation of platelets in either group, microthrombi did not appear to contribute to the observed differences.

Ischemic heart failure: sustained inotropic response to small doses of I-epinephrine without toxicity

As a prelude to a study of severe ischemic heart failure, the therapeutic response of the ischemic ventricle to epinephrine and acetylstrophanthidin in nontoxic doses was determined in 24 intact anesthetized dogs undergoing a first episode of acute regional ischemia. A thrombotic obstruction was produced in the left ventricular dysfunction. The elevation of end-diastolic pressure and reduced stroke volume in control dogs were not significantly altered by administration of strophanthidin. Epinephrine (0.05 mug/kg per min) elicited a significant reduction in end-diastolic pressure and increase in stroke volume. The latter was not attended by an increased incidence of ventricular fibrillation, whereas fibrillation occurred in half of the group given strophantihidin. Thus, the catecholamine was selected to study pump failure. Severe ischemic heart failure was assessed in two groups with scar from previous infarction for up to 4 hours. By 60 minutes of ischemia the increase in end-diastolic pressure and volume and decrease in stroke volume and ejection fraction were comparable in both groups. Thereafter, alternate animals received small doses of epinephrine (0.05 to 0.15 mug/kg per min) with graded increments at 60 minute intervals to counter tachyphylaxis and findings were compared with those in control dogs. Over the subsequent 3 hours, there was progressive deterioration of left anterior descending coronary artery, affecting ventricular function in the untreated group with an increase in end-diastolic pressure from 10 plus or minus 1 to 33 plus or minus 2.4 mm Hg. End-diastolic volume increased by 63 percent; stroke volume and ejection fraction decreased by 48 and 66 percent, respectively. The infusion of epinephrine was attended by a significantly lower end-diastolic pressure of 20 plus or minus 2.5 mm Hg, whereas end-diastolic volume, stroke volume and ejection fraction were restored to control levels after 4 hours of ischemia. Mortality in the untreated group was 62 percent by 4 hours; all seven animals in the treated group survived.