Protective effect of beta adrenoceptor blockade in experimental coronary occlusion in conscious dogs

Effect of cardioselective beta-blockade on heart function and chest pain in acute myocardial infarction

Systolic time intervals and the a/H ratio were recorded in 20 patients with uncomplicated acute myocardial infarction over a period of five days. The initial high heart rate and systolic blood pressure and the short PEP and ICT indicating a sympathetic overactivity were spontaneously normalized during the first week of infarction. LVET was reduced indicating a fall in stroke volume and the a/H ratio was unchanged at the high levels suggestive of elevated preload or LVEDP. In 10 patients with acute myocardial infarction and recurrent chest pain recordings on noninvasive parameters were made before and 30 min after intravenous injection of practolol. In addition, 7 patients with chest pain, classified as acute myocardial infarction, were given practolol. The average dose of practolol was 17.9 mg ranging from 5 to 30 mg. An almost immediate and pronounced relief of pain was observed in all patients and no signs of impaired left ventricular function appeared. The product of systolic blood pressure and heart rate was decreased by practolol and the PEP and the ICT were prolonged to normal values while no changes were seen in LVET and a/H ratio. On 126 occasions practolol was given in dosages ranging from 5 to 30 mg (mean 8 mg) to 75 patients with acute myocardial infarction and recurrent chest pain. A satisfactory pain relief was seen on 108 occasions. It is suggested that an inappropriate sympathetic overactivity is an important factor in provoking recurrent chest pain in acute myocardial infarction. Administration of the beta-adrenergic blocking agent practolol resulted in pain relief due to reduction of heart work and in severity of myocardial ischemia. The beta-blocking agent was well tolerated in the present study. Continuous beta-blockade during the whole hospital stay to patients with acute myocardial infarction seems to be a very attractive therapy in order to preserve the ischemic myocardium and limit the size of infarction.

Importance of beta blockade in the therapy of serious ventricular arrhythmias

Beta blockers have traditionally been considered relatively poor antiarrhythmic agents for patients with ventricular arrhythmias. This view is based on the observations that beta blockers are less effective in suppressing spontaneous ventricular ectopy or inducible ventricular arrhythmias than are the class I and class III agents. However, there are convincing data that beta blockers can have a clinically important antiarrhythmic effect and prevent arrhythmic and sudden death. Beta blockers have multiple potential effects that can contribute to a therapeutic antiarrhythmic action, including an antiadrenergic/vagomimetic effect, a decrease in ventricular fibrillation threshold, and prevention of a catecholamine reversal of concomitant class I/III antiarrhythmic drug effects. Postinfarction trials, recent congestive heart failure studies, and observations in patients who are at risk for sustained ventricular arrhythmias all suggest a potent antiarrhythmic effect of beta blockade.

Proarrhythmic effects of ischemic preconditioning in anesthetized pigs

Conflicting data exist on how ischemic preconditioning affects the incidence of arrhythmias during ischemia. The present study was, therefore, performed to clarify if ischemic preconditioning alters the incidence of arrhythmias during the early phase of ischemia in pigs. Twenty-four pigs (23-36.5 kg) in pentobarbital anesthesia were preconditioned by a 10 min LAD-occlusion and 30 min reperfusion prior to a second 10 min occlusion. The first occlusion served as the preconditioning ischemic period, and the second occlusion as the test ischemic period. To verify that the preconditioning protocol induced protection, infarct size was assessed in an additional nine pigs. The arrhythmic index (arrhythmic beats/min) increased from 12+/-3 during the first occlusion to 38+/-8 during the second occlusion (P < 0.05). Four pigs (17%) fibrillated during the first ischemic period, while 10 pigs (42%) fibrillated during the second ischemic period (P < 0.05). All pigs that fibrillated during the first occlusion also fibrillated during the second occlusion. Mean heart rate was 108+/-4 beats/min before the first occlusion and increased to 113+/-4 beats/min before the second occlusion (P < 0.001). There was a positive correlation between heart rate before the second occlusion and occurrence of fibrillation during this occlusion (rs = 0.42; P < 0.05). The preconditioning protocol reduced infarct size, after a subsequent 45 min ischemic period followed by two hours of reperfusion, from 58+/-3% of area at risk to 40+/-5% (P < 0.05). In conclusion, our data show that ischemic preconditioning increases both the arrhythmic index and the incidence of ventricular fibrillation during the early phase of a subsequent ischemic period.

Sotalol

Sotalol is a novel antiarrhythmic agent combining beta-adrenergic-antagonist actions with the ability to increase cardiac repolarization and refractoriness. The drug's electrophysiologic and clinical profile is different from that of conventional beta-receptor antagonists. As compared with other antiarrhythmic agents, sotalol prevents recurrences of arrhythmia in a higher proportion of patients, particularly among those presenting with ventricular tachycardia and aborted sudden cardiac death. The net hemodynamic effect of sotalol is the result of a balance between the depressant effects due to beta-receptor blockade and an action that tends to increase contractility. Although initially marketed in the United States for treatment of life-threatening ventricular arrhythmias, sotalol also has demonstrated efficacy in many patients with supraventricular arrhythmias. As with all drugs that prolong the QT interval, the syndrome of torsade de pointes is a serious potential adverse effect.

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.

Pericardial prostaglandin biosynthesis prevents the increased incidence of reperfusion-induced ventricular fibrillation produced by efferent sympathetic stimulation in dogs

This study tested the hypothesis that sympathetic neural stimulation increases the prevalence of reperfusion-induced ventricular fibrillation and explored the mechanisms by which this occurs and how it may be prevented. In anesthetized, autonomically denervated dogs, we examined the effects of bilateral ansae subclaviae stimulation (SS) and of induction of pericardial biosynthesis of prostaglandins, an intervention that reduces SS effects by acting at presynaptic sites. A 5-minute occlusion of the left anterior descending coronary artery distal to the first or second diagonal branch was performed during SS. Heart rate was maintained constant by atrial pacing. In the absence of SS, one of 23 dogs developed ventricular fibrillation during occlusion, and three of the remaining 22 dogs developed ventricular fibrillation upon reperfusion. SS did not increase the prevalence of occlusion-induced ventricular fibrillation (four of 23 dogs) but increased the prevalence of reperfusion-induced ventricular fibrillation (12 of the remaining 19 dogs, p = 0.01). SS did not affect occlusion-induced decrease in local electrogram amplitude recorded from the ischemic myocardium or myocardial blood flow to the ischemic myocardium during occlusion or reperfusion. SS, however, prevented occlusion-induced increase in diastolic excitability threshold. Instillation into the pericardial cavity of arachidonic acid solution (3 micrograms/ml) resulted in release of prostacyclin, measured by radioimmunoassay as a stable metabolite 6-ketoprostaglandin F1 alpha (63.1 +/- 11.3 ng/ml, n = 11, mean +/- SEM), and of prostaglandin E2 (7.0 +/- 0.9 ng/ml, n = 11). This pericardial solution blunted SS-induced increase in mean arterial blood pressure and reduced the prevalence of ventricular fibrillation during reperfusion (six dogs to one dog, p less than 0.05). Blood flow to the ischemic myocardium remained unaffected. Indomethacin, when added to the solution (3 micrograms/ml), reversed the effects of prostaglandin release and arrhythmia development. These data indicate that efferent sympathetic stimulation during a coronary occlusion and reperfusion sequence increases the prevalence of reperfusion-induced ventricular fibrillation that is reduced by pericardial biosynthesis of prostaglandins. Pericardial prostaglandin synthesis may serve as a unique antiarrhythmic function by regulating efferent cardiac sympathetic nerve effects.

Inducible monomorphic sustained ventricular tachycardia in the conscious pig

Sustained monomorphic ventricular tachycardia (VT) is of clinical importance but has not been readily modeled in conscious animals. Eleven pigs had myocardial infarction induced by pulling snares previously placed around the left anterior descending (LAD) coronary artery. Six days after occlusion, bipolar pacing catheters were inserted in the right ventricular apex for induction of VT. Testing was repeated in conscious pigs on 6 out of 8 to 19 days after infarction. Monomorphic VT was induced in each animal during each session, using three to four extrastimuli. VT was terminated by burst pacing in 74% of trials; average VT rate was 362 +/- 26 beats/min. VT was prevented in four of eight animals by procainamide and in five of eight animals by magnesium, but was not prevented by lidocaine or metoprolol. The model may be useful in the study of potentially malignant ventricular tachyarrhythmias, important prodromes for sudden death.

Central beta-adrenergic mechanisms may modulate ischemic ventricular fibrillation in pigs

A central noradrenergic process may permit expression of the stress-related increase in cardiac vulnerability to ventricular fibrillation (VF). Thus, the effect of central beta-adrenergic receptor blockade with L-propranolol (0.01 and 0.05 mg/kg) on ischemia-induced VF vulnerability was evaluated in the psychologically stressed pig model and compared with Ringer's solution and D-propranolol (0.05 mg/kg). The ischemia of a maximum 15-minute left anterior descending coronary artery occlusion was used since we previously determined that pigs surviving 15 minutes usually do not fibrillate. Time to the onset of VF was analyzed by time-to-event analysis and ranged from 0.75 to 13.8 minutes in vulnerable pigs. Intracerebroventricular administration of L-propranolol (0.05 mg/kg) prolonged the time to VF compared with Ringer's solution and D-propranolol (p less than 0.05). The high dose of L-propranolol also reduced the incidence of VF (7/15 fibrillated) compared with Ringer's solution (12/12 fibrillated) and D-propranolol (6/7 fibrillated). The lower dose of L-propranolol was without effect on VF vulnerability (7/9) fibrillated). The plasma concentration resulting from central administration of 0.05 mg/kg L-propranolol was found to be 9.05 +/- 3.25 ng/ml, which is significantly below therapeutic antiarrhythmic blood levels. We conclude that the reduced vulnerability to ischemia-induced VF after intracerebroventricular administration of propranolol is due to alteration of a central beta-adrenergic receptor-mediated phenomenon as opposed to an effect on the heart directly or to nonspecific membrane stabilization.

Electrophysiologic and antiarrhythmic actions of nadolol: acute ischemia in the presence of previous myocardial infarction

The actions of the beta-adrenergic receptor antagonist, d,l-nadolol, were examined in anesthetized dogs subjected to circumflex coronary artery ligation in the presence of previous anterior myocardial infarction. With circumflex ligation, control dogs (N = 18) developed premature ventricular beats and ventricular tachycardia, followed by ventricular fibrillation (N = 16, 89%). Immediate arrhythmias (2 to 5 minutes) were accompanied by activation delays and continuous diastolic electrical activity in acutely ischemic epicardial tissue. Delayed arrhythmias (6 to 12 minutes) were accompanied by delayed activation and continuous diastolic electrical activity in acutely ischemic mid-myocardium. Nadolol (8 mg/kg, intravenously) (N = 10) reduced ventricular arrhythmias during both phases of arrhythmia development and increased survival (70%, p = 0.001 vs control). Nadolol failed to after activation in acutely ischemic epicardium, but prevented beat-to-beat changes in epicardial and mid-myocardial activation. Atrial pacing of nadolol-treated animals at heart rates comparable with those of the control group reversed the beneficial effects of nadolol on the development of ventricular arrhythmias and ventricular fibrillation (70%; p = 0.07 vs nadolol; p = 0.21 vs control, respectively). The beneficial effects of nadolol could not be attributed to reduced epicardial delays, but were associated with the suppression of beat-to-beat conduction abnormalities that preceded ventricular fibrillation.

Epinephrine-induced hypokalemia: the role of beta adrenoceptors

Epinephrine was infused intravenously in 9 normal volunteers to plasma concentrations similar to those found after acute myocardial infarction. This study was undertaken on 3 occasions after 5 days of treatment with placebo or the beta-adrenoceptor antagonist, atenolol, which is relatively beta 1 selective, or timolol, which blocks both beta 1 and beta 2 receptors. Epinephrine increased the systolic blood pressure (BP), decreased the diastolic BP and increased the heart rate modestly. These changes were prevented by atenolol. However, after timolol the diastolic BP rose by +19 mm Hg and heart rate fell by -8 beats/min. Epinephrine caused the corrected QT interval to lengthen (0.36 +/- 0.02 to 0.41 +/- 0.06 second). No significant changes were found in the corrected QT interval when subjects were pretreated with atenolol or timolol. The serum potassium decreased from 4.06 to 3.22 mmol/liter after epinephrine. Serum potassium decreased to a lesser extent to 3.67 mmol/liter after atenolol and actually increased to 4.25 mmol/liter after timolol. In a further study with a similar design another nonselective beta blocker propranolol also increased potassium after epinephrine. While atenolol also prevented hypokalemia in this study, it did not block the beta 2-receptor mediated decrease in diastolic BP. Epinephrine-induced hypokalemia results from stimulation of a beta-adrenoceptor linked to membrane sodium/potassium adenosine triphosphatase causing potassium influx. This appears to be predominantly mediated by beta 2 receptors although beta 1 receptors may also play a part.

The effects of the combined administration of beta-adrenoceptor antagonists and non-steroidal anti-inflammatory drugs on ligation-induced arrhythmias in rats

Pretreatment of anaesthetized rats with intravenously administered beta-adrenoceptor antagonists or non-steroidal anti-inflammatory drugs (NSAIDs) reduced the incidence and severity of ventricular arrhythmia which resulted from acute coronary artery ligation. The beta-adrenoceptor antagonists preferentially suppressed the immediate (Phase 1A) arrhythmia while the NSAIDs suppressed the delayed (Phase 1B) arrhythmias. Combined administration of beta-adrenoceptor antagonists and the NSAIDs produced a more pronounced antiarrhythmic effect than either of the drugs alone.

The effect of antiarrhythmic drugs on life-threatening arrhythmias induced by the interaction between acute myocardial ischemia and sympathetic hyperactivity

Transient myocardial ischemia, with attendant sympathetic hyperactivity, seems to play a major role in sudden cardiac death among patients with ischemic heart disease. Ventricular tachycardia (VT) and fibrillation (VF) are consistently and repeatedly elicited in cats by the interaction between a 2-minute occlusion of the left descending coronary artery and a 30-second stimulation of the left stellate ganglion. When three consecutive trials yield almost identical results, time alone will not modify the response and a given drug can be injected to test its efficacy with an internal control analysis. In 90 cats the efficacy of the following drugs was assessed: lidocaine (n = 11), mexiletine (n = 12), propafenone (n = 12), propranolol (n = 19), prazosin (n = 10), amiodarone (n = 14), and verapamil (n = 12). Class I antiarrhythmic drugs completely failed to afford protection and worsening of arrhythmia was observed in several instances. Propranolol and prazosin showed efficacy in approximately 80% and 60% of the animals, respectively. Amiodarone and verapamil completely prevented the onset of VT and VF. Protection from arrhythmias seems to be related to the combined presence of a noncompetitive adrenergic blockade associated with salutary effects on coronary circulation. These findings correlate with and help to explain the results of clinical trials in postmyocardial infarction patients. This model may help to provide a rational choice of antiarrhythmic drugs to be tested in clinical trials.

Prevention of ventricular fibrillation during acute myocardial infarction by intravenous propranolol

A trial of intravenous followed by oral propranolol, started within 4 h of onset of suspected myocardial infarction and continued over 27 h, was carried out in 735 patients; 364 received propranolol, 371 were controls. Ventricular fibrillation during the first 48 h after entry to the trial occurred in 2 treated patients and in 14 controls (p = 0.006). Rates of hospital mortality, complications other than ventricular fibrillation, and progression from threatened to completed infarction did not differ between treated and control patients. Ventricular fibrillation was not apparently prevented by prior beta-blocker treatment, which was not a reason for exclusion from the trial. This intravenous/oral propranolol regimen seems to prevent ventricular fibrillation due to evolving myocardial infarction.

Acute coronary artery occlusion-reperfusion arrhythmias in pigs: antiarrhythmic and antifibrillatory evaluation of verapamil, nifedipine, prenylamine and propranolol

The antiarrhythmic activity of the calcium entry blockers, verapamil, nifedipine and prenylamine, was assessed against arrhythmias occurring during 20 min of acute occlusion, or upon rapid reperfusion of the left anterior descending coronary artery (LAD) in anesthetized pigs. Propranolol, which may indirectly reduce calcium entry by blocking the facilitory action of catecholamines on slow channel conductance, was also evaluated for antiarrhythmic activity in this acute arrhythmia model. Only verapamil (0.2 mg/kg i.v.) reduced both the number of arrhythmias occurring during LAD occlusion and the incidence of ventricular fibrillation (VF) occurring after occlusion and reperfusion. Although both nifedipine (0.04-0.2 mg/kg i.v.) and propranolol (1-2 mg/kg i.v.) produced a slight but significant (P less than 0.05) dose-dependent decrease in the incidence of VF during the occlusion period only, this protection was accompanied by a significant increase in ectopic activity. The increase in ectopic activity produced by propranolol (1.0 mg/kg i.v.) persisted even in combination with verapamil (0.2 mg/kg i.v.) which given alone decreased the ectopic frequency. Prenylamine up to 5 mg/kg was without significant antiarrhythmic or antifibrillatory activity. However, unlike verapamil and nifedipine, this drug produced only slight changes in heart rate or blood pressure which suggested the presence of only minimal calcium entry blocking action on myocardial and vascular tissue at the doses we employed. Because the relative antifibrillatory efficacies of verapamil and nifedipine paralleled the relative efficacies reported for depression of atrioventricular conduction, this may implicate the slow inward current channel in the etiology of VF occurring during acute myocardial ischemia.(ABSTRACT TRUNCATED AT 250 WORDS)

Nature of inducible ventricular tachyarrhythmias in a canine chronic myocardial infarction model

A canine model suitable for serial conscious studies was developed to evaluate the nature of sustained ventricular tachyarrhythmias in chronic experimental myocardial infarction. Thirteen dogs underwent left anterior descending coronary artery ligation followed by complete reperfusion; 11 sham-operated dogs served as controls. In this model, ventricular tachyarrhythmias are inducible in most dogs with experimental infarction and in several dogs without this condition. The morphologic features, rate and drug response of the induced arrhythmias are unlike those of human ventricular tachycardia. Tachyarrhythmia induction is facilitated by anesthesia and thoracotomy. This canine infarct model does not adequately imitate human recurrent ventricular tachycardia, but may simulate human sudden cardiac death.

Relationship between sympathetic neural activity, coronary dynamics, and vulnerability to ventricular fibrillation during myocardial ischemia and reperfusion

The relationship between neural sympathetic discharge and vulnerability to ventricular fibrillation during myocardial ischemia and reperfusion was studied in 26 chloralose-anesthetized dogs. Preganglionic cardiac sympathetic impulse activity and ventricular fibrillation thresholds were separately determined before and during a 10-minute period of left anterior descending coronary artery occlusion and during release-reperfusion. Within 2 minutes of occlusion the ventricular fibrillation threshold was significantly decreased (from 25 +/- 1.3 to 16 +/- 2.3 mA, p less than 0.05) corresponding with the period of maximal activation of cardiac sympathetic preganglionic fibers (from 4.4 +/- 0.2 to 6.3 +/- 0.5 impulses/sec). Coronary sinus blood flow and oxygen tension decreased significantly. All these changes persisted for 5 to 6 minutes, thereafter returning to control levels despite continued obstruction of the coronary artery. A transient but significant reduction in ventricular fibrillation threshold also occurred with release of the occlusion but was unaccompanied by increases in sympathetic neural discharge. Bilateral stellectomy completely prevented the ventricular fibrillation threshold changes observed during coronary artery occlusion. However, there was no change in coronary sinus oxygen tension or blood flow. During reperfusion, stellectomy increased rather than decreased vulnerability to ventricular fibrillation. Stellectomy augmented the reactive hyperemic response to release-reperfusion. These findings indicate that enhanced cardiac sympathetic neural activity contributes to ventricular vulnerability associated with coronary artery obstruction. An opposite action results during release-reperfusion. Cardiac sympathetic neural discharge, by reducing the magnitude of reactive hyperemic response through influence on coronary vascular tone, exerts an antifibrillatory effect.

A double-blind trial of metoprolol in acute myocardial infarction. Effects on ventricular tachyarrhythmias

During a double-blind trial in which patients with suspected myocardial infarction received metoprolol or placebo, we analyzed the occurrence of ventricular tachyarrhythmias. Metoprolol (15 mg intravenously) was given as soon as possible after admission, and thereafter 200 mg was given daily for three months. Antiarrhythmic drugs were given only for ventricular fibrillation and sustained ventricular tachycardia (greater than 60 beats per second). Definite acute myocardial infarction developed in 809 of the 1395 participants, and probable infarction in 162. Metoprolol did not influence the occurrence of premature ventricular contractions or short bursts of ventricular tachycardia. However, there were 17 cases of ventricular fibrillation in the placebo group (697 patients) and only 6 in the metoprolol group (698 patients, P less than 0.05). During the hospital stay significantly fewer patients receiving metoprolol (16) than placebo (38) (P less than 0.01) required lidocaine. In a separate analysis of 145 patients, metoprolol did not influence the occurrence of premature ventricular contractions or short bursts of ventricular tachycardia during the first 24 hours of treatment. Despite a lack of effect on less serious ventricular tachyarrhythmias, metoprolol had a prophylactic effect against ventricular fibrillation in acute myocardial infarction.

The effect of experimentally induced myocardial ischemia on the norepinephrine metabolism of the dog heart

Myocardial ischemia is known to provoke an excess in circulating norepinephrine and thus be related to an increased irritability of the heart. In the present experiments, we studied the norepinephrine and potassium content, oxygen tension, and pH values of the effluent of coronary sinus after thoracotomy and catheter placement into the coronary sinus. Once a steady state was reached, the measurements were repeated in the fifth, tenth, 20th, and 60th minutes of experimental myocardial ischemia provoked by coronary ligation of the left anterior descending coronary artery. The parameters obtained were compared to the corresponding values measured in the peripheral vessels. The results indicate an increased release and probably an increased turnover of norepinephrine in the ischemic myocardium. The role of metabolism acidosis in the changes in norepinephrine metabolism was suggested. It is assumed that intracellular acidosis is involved in the enhanced accumulation and release of norepinephrine in the damaged myocardium and that an increase of norepinephrine concentration in the myocardium may be considered a risk factor in supporting heart function.

Myocardial infarction without coronary occlusion. A morphologic study in sheep

The macroscopic, histologic and enzyme-histochemical characteristics of the myocardial lesion obtained after heating of a thermoprobe in a branch of the left coronary artery in sheep is reported. In 13 sheep such myocardial lesions were produced distal to the location of the thermoprobe. Alterations were observed in accordance with generally accepted morphologic criteria for myocardial infarction. The coronary artery branch in which the thermoprobe was located showed erythrocyte and platelet aggregates immediately after the heating episode, which disappeared within a few min, as demonstrated by coronary arteriography. Injection of radiolabelled microspheres into the coronary circulation after induction of the myocardial lesion, cryosectioning of the heart and autoradiography revealed a lack of blood flow in the damaged myocardial region. We consider this new method a suitable tool for further studies on the complex pathology involved in the development of myocardial infarction.

Initiation of ventricular fibrillation outside hospital in patients with acute ischaemic heart disease

Factors which may predispose to ventricular fibrillation were assessed in 48 consecutive patients who developed ventricular fibrillation outside hospital after the arrival of a mobile coronary care team. Before the initiation of ventricular fibrillation, late cycle ventricular extrasystoles were recorded in 38 patients (79%) and R on T extrasystoles in 27 patients (56%). R on T extrasystoles were relatively infrequent--an average of three occurred during a mean monitoring time of 27 minutes. Multifocal extrasystoles occurred in only three patients (6%), consecutive extrasystoles in 14 (29%), and self-terminating ventricular tachycardia in three (6%). Ventricular extrasystoles occurring at more than five a minute were uncommon during this phase. The time from "warning arrhythmias" to the development of ventricular fibrillation in many patients was short, thus limiting the administration of antiarrhythmic agents. There was a significant increase in the heart rates recorded immediately before ventricular fibrillation when compared with those documented initially. Thus, an increase in heart rate appeared to be a predisposing factor in the initiation of ventricular fibrillation. An R on T extrasystole was the most important factor in the initiation of ventricular fibrillation: it occurred in 33 patients (69%). In nine (19%) ventricular tachycardia and in six (12%) a late cycle extrasystole or idioventricular rhythm initiated ventricular fibrillation.

Effect of reserpine upon the haemodynamic course of recovery following experimental myocardial infarction

Acute haemodynamic consequences of coronary artery ligation were evaluated in twenty-four anaesthetized open-chest dogs, nine of which were pretreated with reserpine. The following parameters were measured before, and at 15-min intervals following ligation for at least five hours : ECG, mean arterial blood pressure, aortic blood flow, left ventricular pressure, heart rate, peripheral resistance (P.R.), end-diastolic pressure, dP/dtmax, and "internal work" (I.W. = heart rate x dP/dtmax). It was found that aortic flow was similar in control and reserpine-pretreated dogs (753 +/- 43 vs. 744 +/- 57 ml/min respectively), even though heart rate, blood pressure and other parameters were significantly higher in the control animals. Furthermore, the controls could be divided into two groups : recoverers (R) and non-recoverers (N), on the basis of late stage haemodynamic differences. The ratio of PR/IW taken within one hour of ligation was significantly higher in the R group (496 +/- 69) and reserpine group (479 +/- 30) than in the N group (242 +/- 28), and could predict course of recovery in each dog studied. It is concluded that the presence of myocardial catecholamines may be deleterious to the ischemic heart when the PR or IW are disproportionately altered.

Activity and duration of action of pindolol and alprenolol compared in healthy volunteers

An improvement in the prognosis of myocardial infarction has been reported after long term treatment with alprenolol 200 mg twice daily. Therefore, an experiment was carried out to find the dose of pindolol given once daily which would show cardiac beta-adrenoceptor blockade at least equipotent to that obtained during treatment with alprenolol 200 mg twice daily. Cardiac beta-adrenoceptor blocking activity and its time course during treatment with pindolol (15 mg and 20 mg given once daily) and alprenolol (200 mg given 12-hourly) for three days were compared in 6 healthy volunteers. The reduction in exercise-induced tachycardia as a measure of cariac beta-adrenoceptor blockade was significantly greater after pindolol 15 mg and 20 mg than after alprenolol 200 mg. On the morning of the fourth day, i.e. 24 h after the last dose of pindolol and only 12 h after the last dose of alprenolol, the effects of pindolol at both dose levels were slightly greater than those of alprenolol. This difference was not statistically significant. It can be concluded that pindolol 15 mg once daily produces a cardiac beta-adrenoceptor blockade at least equipotent to that of alprenolol 200 mg given 12-hourly.

Effect of propranolol on enzymatic and histochemical estimates of infarct size in experimental myocardial infarction

Acute myocardial infarction was produced in anesthetized dogs by ligation of the left anterior descendens coronary artery. Propranolol (2 mg/kg i.v.) administered 4 hours post ligation was examined for its ability to reduce infarct size estimated by histochemical and enzymatic methods. There was a signficant correlation between these two methods in their estimation of infarct size. Treatment with propranolol significantly decreased infarct size estimated with both methods. It is concluded that some portion of the myocardium can be protected against infarction by pharmacologic intervention as late as 4 hours after the onset of coronary artery occlusion.

Central neurochemical factors related to serotonin metabolism and cardiac ventricular vulnerability for repetitive electrical activity

Sympathetic neural activity modifies cardiac excitability and lowers the threshold of the vulnerable period for ventricular fibrillation. Sympathetic neural traffic to the heart can be diminished by administering serotonin precursors that localize in the central nervous system. In this study anesthetized dogs were injected with either of the serotonin precursors L-tryptophan or 5-hydroxy-L-tryptophan in conjunction with the monoamine oxidase inhibitor pheneizine and the selective peripheral L-amino acid decarboxylase inhibitor carbidopa. Ventricular vulnerability was evaluated by measuring the repetitive extrasystole threshold. A sustained increase of 50 percent in this threshold resulted only with use of biochemical measures that presumably increase serotonin levels in the central nervous system. Thus neuropharmacologic measures affecting central sympathetic activity alter cardiac vulnerability and may protect against ventricular fibrillation.

Reduction in mortality after myocardial infarction with long-term beta-adrenoceptor blockade. Multicentre international study: supplementary report

In a controlled multicentre trial carried out to assess the value of long-term practolol treatment after myocardial infarction the provisional results showed a significant reduction in mortality, though some of the data were lacking. These have now been included and the results updated. The final figures for all deaths were 78 in the placebo group of 1533 patients and 48 in the practolol group of 1520 patients. The reduction in mortality (38%) was significant at the 1% level. The figures for non-fatal reinfarction (97 in the placebo group, and 75 in the practolol group) were not significantly different. Patients with pre-entry anterior infarction, and especially those with a diastolic blood pressure equal to or below the mean (78 mm Hg) at entry to the trial, were at high risk but benefited particularly well from beta-adrenoceptor blockade. After pre-entry inferior infarction the percentage reduction in deaths occurring within two hours after symptoms of a new event was similar to that after anterior infarction, but the incidence of death more than two hours after the event was greater in the practolol-treated group. Thus the difference between groups in total deaths after pretrial inferior infarction was marginal. Until the results of further trials are reported long-term beta-adrenoceptor blockade (possibly up to two years) is recommended after uncomplicated anterior myocardial infarction.

The effect of beta-adrenoreceptive antagonists on the morbidity and mortality in cardiovascular disease

The effect is critically reviewed of chronic administration of beta-adrenoreceptor antagonists on morbidity and mortality in angina pectoris and myocardial infarction. There is inconclusive evidence that the abrupt cessation of therapy with these agents may increase morbidity and mortality in angina pectoris. The type of anginal patient in which this may occur is not yet defined and neither is the mechanism. There is no evidence that beta-antagonists prolong the life of anginal patients. In the acute phase of experimental myocardial infarction, these drugs reduce mortality but do not do so in man. There is increasing evidence that they prolong life if administered to patients surviving the acute phase of myocardial infarction. The need for further studies is discussed.

Effects of allopurinol, propranolol and methylprednisolone on infarct size in experimental myocardial infarction

With use of a canine model of occlusion of the left anterior descending coronary artery and an intracellular lactic dehydrogenase stain to measure infarct size directly, the effects of allopurinol, methylprednisolone sodium succinate and propranolol were studied. Allopurinol did not influence the extent of myocardial necrosis, whereas both methylprednisolone and propranolol significantly reduced myocardial infarct size. Possible mechanisms of action and clinical applicability of these agents are discussed.

Anti-arrhythmic action of nadolol, a beta-adrenergic receptor blocking agent

The anti-arrhythmic action of 2,3-cis-1,2,3,4-tetrahydro-5-[(2-hydroxy-3-tert-butylamino)propoxy]2,3-naphthalenediol (nadolol) was evaluated and compared with that of propranolol in several experimental models of cardiac arrhythmias. Both nadolol and propranolol antagonized isoproterenol-induced tachycardia and ouabain-induced arrhythmias in cats, antagonized coronary artery ligation-induced ventricular fibrillation and suppressed ventricular ectopic activity during vagal stimulation in dogs. In contrast to propranolol, nadolol was considerably weaker in suppressing existing digoxin-induced arrhythmias, lacked local anesthetic activity and did not depress the heart in dogs. Because of these findings, it is concluded that the anti-arrhythmic activity of nadolol is apparently related to blockade of beta-adrenergic receptors.

Effect of autonomic neural influences on the cardiovascular changes induced by coronary occlusion

The influence of vagal stimulation and/or beta-adrenergic receptor blockade on the heart rate, blood pressure, contractile force, and cardiac rhythm was evaluated in chloralose-anesthetized cats subjected to occlusion of the anterior descending coronary artery. Occlusion performed in 25 control animals produced significant decreases in heart rate (minus 22.9 plus or minus 4.4 beats per minute), blood pressure (minus 19.2 plus or minus 2.4 mm. Hg), and contractile force (minus 21.6 plus or minus 6.3 per cent). Death due to ventricular fibrillation occurred in five out of 25 animals. Coronary occlusion performed in the presence of vagal nerve stimulation resulted in similar decreases in blood pressure, whereas the decreases in contractile force were significantly greater than in control animals. In addition, the time to onset of the arrhythmias occurring in the vagus-stimulated group was increased. Death due to ventricular fibrillation was similar to control animals (i.e., two of seven, or 28 per cent). Blockade of beta-adrenergic receptors with propranol (0.75 mg. per kilogram) resulted in the usual decreases in rate, pressure, and force with occlusion but the duration of arrhythmias was shortened. The incidence of ventricular fibrillation was not different from that of the control animals. The combination of propranolol and vagal stimulation also failed to confer protection against ventricular fibrillation. Propranolol was observed to prevent the large decrease in contractile force seen with vagal stimulation. These results suggest that: (1) increasing vagal tone above the level existing after acute myocardial infarction does not decrease mortality, (2) propranolol pretreatment does not affect the incidence of ventricular fibrillation induced by coronary occlusion, (3) the duration of the arrhythmia after coronary occlusion is effectively shortened with propranolol, and (4) cardiac beta-adrenergic receptors do not appear to be involved in the decreases in heart rate, blood pressure, and contractile force seen with coronary occlusion.

Modification of ventricular fibrillation latency following coronary artery occlusion in the conscious pig

Abrupt occlusion of the left anterior descending coronary artery was performed on 45 unanesthetized farm pigs in order to evaluate the relative effects on the latency to ventricular fibrillation (VFL) of 1) adaptation of the animals to the laboratory, and 2) beta-receptor blockade by propranolol. Compared to control values, VFL was greatly lengthened (i.e., VF delayed or prevented) by adaptation (P smaller than 0.01), was shortened by large (2 MG/KG) doses of racemic propranolol in unadapted animals (P smaller than 0.02), and was again increased while under the influence of the drug, by adaptation (P smaller than 0.051). Neither a lower (0.02 mg/kg) dose of racemic propranolol, 2 mg/kg of dextropropranolol, nor ventricular pacing to a higher heart rate had an effect on VFL. The results suggest that reduced psychological stress was very effective in retarding or preventing the onset of VF, that low doses of propranolol were ineffective, and that higher doses were deleterious in unadapted animals.

The Myocardial Depressant Effect of Beta-Receptor Blocking Agents

dl -Propranolol, d -propranolol, and practolol were administered in increasing doses to conscious dogs before and after acute coronary artery occlusion. dl -Propranolol and practolol caused dose-dependent decreases in cardiac output, stroke volume, stroke work, and maximum rate of left ventricular pressure rise before and after coronary artery occlusion. These parameters were reduced 25-40% after the largest cumulative doses--1.43 mg/kg for dl -propranolol and 52 mg/kg for practolol--were administered. The degree of beta-receptor blockade increased in proportion to the logarithm of plasma drug concentration. After occlusion, both drugs produced identical decreases in cardiac output at equivalent degrees of beta-receptor blockade. Neither drug affected resting heart rate before infarction, and neither drug reduced the tachycardia induced by coronary artery occlusion; this tachycrotic response probably resulted from cardiac vagal withdrawal. Mean systemic arterial blood pressure and total peripheral vascular resistance increased following beta-receptor blockade before infarction, most likely because of increased peripheral vascular alpha-adrenergic activity. These two effects did not occur after coronary artery occlusion, probably because vascular alpha-adrenergic activity had already been stimulated. d -Propranolol produced none of these effects. We concluded that the cardiodepressant effect of beta-receptor blocking agents in the doses administered was caused by their inhibition of endogenous sympathetic tone to the heart rather than by their quinidinelike action.