Mechanism of prevention of sudden death by nadolol: differential actions on arrhythmia triggers and substrate after myocardial infarction in the dog

Slow conduction through an arc of block: A basis for arrhythmia formation postmyocardial infarction

INTRODUCTION

The electrophysiologic basis for characteristic rate-dependent, constant-late-coupled (390 + 54 milliseconds) premature ventricular beats (PVBs) present 4-5 days following coronary artery occlusion were examined in 108 anesthetized dogs.

METHODS AND RESULTS

Fractionated/double potentials were observed in injured zone bipolar and composite electrograms at prolonged sinus cycle lengths (1,296 ± 396 milliseconds). At shorter cycle lengths, conduction of the delayed potential decremented, separating from the initial electrogram by a progressively prolonged isoelectric interval. With sufficient delay of the second potential following an isoelectric interval, a PVB was initiated. Both metastable and stable constant-coupled PVBs were associated with Wenckebach-like patterns of delayed activation following an isoelectric interval. Signal-averaging from the infarct border confirmed the presence of an isoelectric interval preceding the PVBs (N = 15). Pacing from the site of double potential formation accurately reproduced the surface ECG morphology (N = 15) of spontaneous PVBs. Closely-spaced epicardial mapping demonstrated delayed activation across an isoelectric interval representing "an arc of conduction block." Rate-dependent very slow antegrade conduction through a zone of apparent conduction block (N = 8) produced decremental activation delays until the delay was sufficient to excite epicardium distal to the original "arc of conduction block," resulting in PVB formation.

CONCLUSION

The present experiments demonstrate double potential formation and rate-dependent constant-coupled late PVB formation in infarcted dog hearts. Electrode recordings demonstrate a prolonged isoelectric period preceding PVB formation consistent with very slow conduction (<70 mm/s) across a line of apparent conduction block and may represent a new mechanism of PVB formation following myocardial infarction.

Proteasome inhibition 1 h following ischemia protects GRK2 and prevents malignant ventricular tachyarrhythmias and SCD in a model of myocardial infarction

Arrhythmia-prone epicardial border zone (EBZ) tissues demonstrate decreased G protein-coupled receptor kinase-2 (GRK2) activity and increased sensitivity to isoproterenol 6–24 h after coronary artery ligation in the dog. We previously demonstrated that the ischemia-mediated decrease in GRK2 in cardiac ischemic tissue was largely blocked by proteasome blockade initiated 1 h before the onset of ischemia, and this was associated with significant cardioprotection against malignant ventricular tachyarrhythmias. For application to clinical circumstances, it is desirable to determine whether a clinical window exists following the onset of ischemia for such a protective effect. The treatment of six dogs with the selective proteasome inhibitor bortezomib 1 h after the surgical induction of left coronary artery ischemia provided 80% (EBZ) and 42% (infarct) protection (by immunoblot) against the loss of GRK2 at 24 h. There was no significant increase of heat shock protein 70(72) in the EBZ of bortezomib-treated animals compared with control. There was a striking absence of rapid (>300 beats/min) and very rapid (>360 beats/min) ventricular triplets that is highly predictive of sudden cardiac deaths (SCDs) during electrocardiogram monitoring of the first 24 h in the bortezomib-treated animals in contrast with nontreated infarcted animals. There were no SCDs in the 6 treated animals (0%) and five SCDs in the 14 control animals (36%). Assay of whole blood proteasome activity demonstrated the expected decrease over the 24-h observation period. These data support the concept that proteasome inhibition within a window of time following myocardial infarction may be of use in suppressing malignant tachyarrhythmias and SCD.

Assessment of Efficacy of Pharmacotherapy for Ventricular Tachycardia

The presence and sinister prognosis of sustained ventricular tachycardia was recognised early in the twentieth century in patients with serious cardiac disease. Treatment was difficult and evolved slowly. The development of antiarrhythmic drug therapy was frequently based on chance clinical observations and on the assessment of drug effects in animal models of arrhythmia that bore little resemblance to the actual clinical scenarios in which the drugs were to be employed. Even early reports of antiarrhythmic drug use were tempered by awareness of serious adverse side effects. Many drugs were brought into wide-spread clinical use without the background of large randomised trials of efficacy. Assessment of drug efficacy for ventricular tachycardia was frequently based on the effects of an administered drug on inducibility of tachycardia with invasive electrophysiologic techniques. Suppression of inducibility was suggested to be a marker of drug efficacy. Similarly, suppression of spontaneously occurring ventricular ectopic beats was also used as a predictor of drug effect. However, both predictive techniques were hindered by inherent baseline variability. It was subsequently demonstrated that mode of induction of ventricular tachycardia could vary widely at repeat inductions. Antiarrhythmic drugs rarely suppressed inducibility of VT but could alter mode of induction. Techniques were developed to estimate true drug effects by quantitating and allowing for random variability in mode of tachycardia induction. In particular, reproducibility of tachycardia induction was enhanced when baseline and drug studies were performed at short intervals. Even with these techniques, prediction of long-term drug efficacy in individual patients remained difficult and acute drug testing served principally to demonstrate the fact that drug therapy was more likely to facilitate induction of tachycardia than to suppress it (pro-arrhythmic effect). Large clinical trials also demonstrated the potent pro-arrhythmic effects of drug therapy especially when sodium-channel blocking drugs were used. By the end of the twentieth century, antiarrhythmic drugs were used primarily as adjuncts to device therapy for patients at risk of life-threatening ventricular arrhythmias.

Proteasome degradation of GRK2 during ischemia and ventricular tachyarrhythmias in a canine model of myocardial infarction

Arrhythmia-prone subepicardial border zone (EBZ) tissue demonstrates decreased G protein receptor kinase 2 (GRK2) activity and increased sensitivity to isoproterenol 6–24 h after coronary artery ligation (CAL) in the dog. With the use of a semiquantitative immunofluorescence technique, the relative fluorescence intensity (RF) of GRK2 in EBZ decreased to 24% of that in a remote site (RS) ( P < 0.01, n = 30 cells from 3 dogs), whereas GRK5 RF did not change. Confocal studies of cardiac tissue from transgenic mice overexpressing GRK2 validated the use of a semilogarithmic relationship between RF and GRK2 activity. As shown with the use of quantitative real-time RT-PCR, both GRK2 and GRK5 mRNA were not decreased at 24 h in EBZ ( n = 6 dogs) relative to RS control, indicating that the decrease of GRK2 in the EBZ is likely due to posttranscriptional degradation following CAL. Pretreatment of six dogs with the selective proteasome inhibitor bortezomib provided 100% (EBZ) and 50% (infarct) protection against loss of GRK2 at 24 h. There was an absence of rapid (>300 beats/min) and very rapid (>360 beats/min) ventricular triplets that are highly predictive of sudden cardiac death during ECG monitoring in the bortezomib-pretreated animals in contrast to nonpretreated infarcted animals. We have demonstrated that the dramatic decrease in GRK2 in cardiac ischemic tissue can be largely blocked by prior proteasome blockade and that this is associated with significant cardioprotection against malignant ventricular tachyarrhythmias.

Tumor Necrosis Factor α, Rapid Ventricular Tachyarrhythmias, and Infarct Size in Canine Models of Myocardial Infarction

Etanercept (2 mg/kg), a TNFalpha sequestrant, was administered 24 hours and 1 hour before LAD coronary artery ligation to examine the role of TNFalpha on lethal ventricular tachyarrhythmias and myocardial necrosis. Dogs treated with etanercept had decreased very rapid (>360 bpm) ventricular triplets (6 +/- 1/h, n = 8) 2 to 24 hours following coronary artery ligation compared with saline (21 +/- 6/h, n = 10, P < 0.05). This was concordant with 8-fold salvage of beta-adrenergic receptor kinase 1 (betaARK) activity compared with control (33.8 +/- 7.2% versus 4.3 +/- 2.2% of unoperated control tissue, P < 0.01, n = 5). Salvage of betaARK occurred without change in the thickness of the epicardial tissue overlying the infarct. In dogs pretreated with etanercept before a 2-hour occlusion/4-hour reperfusion of the LAD coronary artery, infarct mass decreased by 61% (% area at risk) and 55% (% left ventricular mass) in the etanercept group (n = 8) compared with saline (n = 9, P < 0.05). This was concordant with an etanercept-mediated six-fold decrease in leukocyte accumulation within ischemically injured myocardium. TNFalpha antagonism decreases malignant ventricular tachyarrhythmias and may relate to partial protection of normal betaARK-mediated desensitization of beta-adrenergic receptors. TNFalpha sequestration also decreases infarct size in an occlusion/reperfusion model of myocardial ischemia.

Independent contribution of catecholamines to arrhythmogenesis during evolving infarction in the isolated rat heart

Ventricular fibrillation (VF) in conscious rats with coronary artery ligation occurs in two phases, before (phase 1) and after (phase 2) 90 min of ischaemia respectively. The mechanisms of phase 2 VF are not established. Interestingly, phase 2 VF is absent in isolated (denervated) buffer-perfused rat hearts. We investigated whether catecholamine supplementation (to mimic sympathetic drive) was sufficient to restore phase 2 VF in such hearts. Isolated rat hearts (n=10 per group) underwent coronary ligation for 240 min. At 90 min, during a period of relative electrical stability, the perfusion solution was switched from standard (Krebs) to identical solution or Krebs containing catecholamines (313 nM noradrenaline and 75 nM adrenaline) with or without 10 microM trimazosin (an alpha(1)-adrenoceptor antagonist) or 10 microM atenolol (a beta(1)-adrenoceptor antagonist). Although in all groups the incidence of phase 1 VF was high (80 - 100%), the temporal distribution of VF was monophasic, i.e. only one heart in one group developed phase 2 VF (P=NS). Other ventricular arrhythmias (e.g., tachycardia; VT) exhibited a similar temporal distribution. Nevertheless, haemodynamic changes confirmed sympathomimetic effects of catecholamines, e.g., heart rate was increased from 278+/-7 beats min(-1) in controls to 335+/-8 beats min(-1) (P<0.05) by catecholamines, an effect that could be blocked by atenolol (285+/-7 beats min(-1)) but not by trimazosin (342+/-12 beats min(-1)). Coronary flow was correspondingly increased from 7.7+/-0.7 ml min(-1) g(-1) to 16.5+/-1.3 ml min(-1) g(-1) (P<0.05); this effect could be blocked by atenolol (8.1+/-0.6 ml min(-1) g(-1)) and was enhanced by trimazosin (20.7+/-2.4 ml min(-1) g(-1)). In conclusion, despite evidence of adequate alpha- and beta-adrenoceptor activation, catecholamine supplementation to isolated buffer-perfused rat hearts was insufficient to restore phase 2 VF. It therefore appears unlikely that catecholamines alone mediate phase 2 VF.

Ischemic inactivation of G protein-coupled receptor kinase and altered desensitization of canine cardiac beta-adrenergic receptors

BACKGROUND

G protein-coupled receptor kinases (GRKs) modulate myocardial beta-adrenergic receptor (betaAR) signaling. We examined whether GRK activity was altered 6, 24, and 96 hours after left anterior descending coronary artery ligation (LAD CAL) in the dog.

METHODS AND RESULTS

GRK activity was measured in arrhythmogenic subepicardial border zone (EBZ) tissue overlying the infarct and from nonischemic remote-site (RS) subepicardial tissue from the same animal. GRK activity in the ischemic EBZ was 15% of RS (P:=0.03, n=6) 24 hours after CAL and appeared to start as early as 6 hours through 96 hours. GRK activity and immunoblot data demonstrated a marked decrease of GRK2 but not GRK5 at 24 hours. EBZ tissue exhibited high-affinity binding for (-)-isoproterenol (K:(i) of 0. 076+/-0.026 nmol/L [SEM]) at 24 hours, which was not significantly different from control tissue from nonoperated animals (1.2+/-0.8 nmol/L, P:>0.05, n=6). A significantly lower K:(i) of 13.8+/-2.8 nmol/L (P:<0.001, n=6) was observed for RS taken from the ischemic animals. This was reflected by a 4-fold increase in the EC(50) of isoproterenol-stimulated adenylyl cyclase activity from 18 nmol/L in EBZ tissue to 73 nmol/L in RS (P:<0.05, n=4).

CONCLUSIONS

There is a selective decrease in GRK2 activity and a loss of the ability of the arrhythmia-prone EBZ tissue to desensitize to beta-adrenergic stimulation 24 hours after CAL. This correlates temporally with a second (late) peak in sudden cardiac death previously observed between 6 and 24 hours in dog and rat models of myocardial infarction.

Suppression of sustained ventricular tachyarrhythmias: a comparison of d,l-sotalol with no antiarrhythmic drug treatment

OBJECTIVES

This study evaluates the clinical efficacy of d,l-sotalol in patients with sustained ventricular tachyarrhythmias.

BACKGROUND

D,l-sotalol is an important antiarrhythmic agent to prevent recurrences of sustained ventricular tachyarrhythmias (VT/VF). However, evidence is lacking that an antiarrhythmic agent like d,l-sotalol can reduce the incidence of sustained ventricular tachyarrhythmias in comparison to no antiarrhythmic drug treatment.

METHODS

A prospective study was performed in 146 consecutive patients with inducible sustained ventricular tachycardia or ventricular fibrillation. In 53 patients, oral d,l-sotalol prevented induction of VT/VF during electrophysiological testing and patients were discharged on oral d,l-sotalol (sotalol group). In 93 patients, VT/VF remained inducible and a defibrillator (ICD) was implanted. After implantation of the device patients were randomly assigned to oral treatment with d,l-sotalol (ICD/sotalol group, n=46) or no antiarrhythmic medication (n=47, ICD-only group).

RESULTS

During follow-up, 25 patients (53.2%) in the ICD-only group had a VT/VF recurrence in comparison to 15 patients (28.3%) in the sotalol group and 15 patients (32.6%) in the ICD/sotalol group (p=0.0013). Therapy with d,l-sotalol, amiodarone or metoprolol was instituted in 12 patients (25.5%) of the ICD-only group due to frequent VT/VF recurrences or symptomatic supraventricular tachyarrhythmias. In nine patients, 17% of the sotalol group, an ICD was implanted after VT/VF recurrence, three patients (5.7%) received amiodarone. Total mortality was not different between the three groups.

CONCLUSIONS

D,l-sotalol significantly reduces the incidence of recurrences of sustained ventricular tachyarrhythmias in comparison to no antiarrhythmic drug treatment.

Cardiac arrhythmias and the autonomic nervous system

The multiple facets of cardiac arrhythmias and their relationship with the autonomic nervous system can be investigated by studying the spontaneous heart rate behavior through ambulatory ECG recordings, an approach that complements the limitations of invasive electrophysiologic investigations. Information obtained from heart rate behavior is more reliable in the absence of structural heart disease and ventricular hypertrophy/failure, during which compensatory mechanisms involving the autonomic nervous system tend to limit reflex changes in heart rate. Thus, in such situations, less marked sinus rhythm variations preceding the arrhythmia onset do not imply a more limited influence of the autonomic nervous system, and the sensitivity of the electrophysiologic substrate may otherwise vary. These two factors may combine to form the basis of the "adrenergic paradox" that implies that the more marked the autonomic nervous system dependence of tachyarrhythmias, the less obvious its evidence. Assessment of the QT interval dynamicity may also allow one to evaluate the modulation of autonomic neural effects on the ventricular tissues. Finally, it may be difficult to distinguish clearly autonomic nervous system dependence from rate dependence: the latter frequently conditions the behavior of the trigger whereas the former mainly concerns the electrophysiologic substrate. There are many examples of the importance of the autonomic nervous system as a determinant of cardiac arrhythmias. In the atrium, either limb of the autonomic nervous system, particularly the parasympathetic limb, can generate atrial fibrillation. The absence of structural heart disease defines pure electrophysiologic substrates responsible for benign forms of ventricular tachycardia as well as potentially lethal tachyarrhythmias of the long QT syndrome and its variants. In both, the role of the autonomic nervous system is essential, although the therapeutic consequences are crucial only in the latter. In the presence of heart disease and, in particular, heart failure, the autonomic nervous system behavior is more difficult to assess than in the absence of structural heart disease. This does not mean that its role is less crucial. In this situation the beneficial effects of beta blockers may be as important as in normal hearts although physicians should be more cautious when heart failure is present.

Predictive value of electrophysiologic studies during treatment of ventricular tachycardia with the beta-blocking agent nadolol. The Working Group on Arrhythmias of the French Society of Cardiology

Sixty patients with recurrent inducible sustained ventricular tachycardia were prospectively treated with nadolol (40 or 80 mg/day). Old myocardial infarction was present in 43 patients and dilated cardiomyopathy in 12. In group I (n = 36), nadolol was given alone, whereas in group II (n = 24), previously ineffective treatment with amiodarone was continued in combination with nadolol. Left ventricular ejection fraction was higher in patients in group I (0.40 +/- 0.12) than in group II (0.30 +/- 0.10, p less than 0.01) patients. Electrophysiologic study was repeated after short-term treatment with nadolol, which was continued regardless of the results of this test, according to the scheme of the parallel approach. Recurrence of spontaneous tachycardia or sudden death occurred in 21 patients after 10 +/- 9.2 months; sustained tachycardia was inducible in 19 on nadolol therapy. The remaining 39 patients (of whom 21 had inducible tachycardia while taking the drug) have had no recurrence of tachycardia after 27.8 +/- 9.3 months of follow-up study. Sensitivity, specificity and predictive value of a positive and negative test were 90.5%, 46%, 47.5% and 90%, respectively. The results differ between group I and group II patients, the latter having a high percent of false positive responses. This difference is even more obvious with respect to left ventricular ejection fraction: the predictive value of a positive test was 86% when ejection fraction was greater than 0.40 and 39% when it was less than 0.40.(ABSTRACT TRUNCATED AT 250 WORDS)

Seasonal variation in sudden cardiac death after experimental myocardial infarction

The authors studied the incidence of sudden death by monitoring the ECG after ligation of the left anterior descending coronary artery in 184 dogs. A significant number of sudden deaths (46 dogs) occurred in the cold weather months, November-February (42%), compared to the summer months, July and August (6%). All deaths resulted from ventricular tachyarrhythmias (greater than or equal to 300/min) and occurred between 13 and 22 hours after coronary artery ligation. The survivors (138 dogs) were subjected to electrophysiological study, during which a significantly higher number showed induced sustained monomorphic ventricular tachycardia (VT) (heart rate greater than or equal to 300/min) during the winter months than during the summer months. Heart weight and infarct mass were not significantly different throughout the year. Higher sympathetic tone or catecholamine levels may account for the seasonal variation in sudden death during evolving myocardial infarction.

The effect of beta-adrenergic blockade on vulnerability to ventricular fibrillation and inducibility of ventricular arrhythmia in short- and long-term feline infarction models

Previous investigation, predominantly in the short-term canine model, has documented a potent antifibrillatory effect of beta-adrenergic blockade. To determine whether the protection afforded by beta blockade is species- and model-specific, we studied 23 chloralose-anesthetized cats. Eight animals were studied over a short term and underwent serial determinations of the ventricular fibrillation (VF) threshold prior to and 1 minute after occlusion of the left anterior descending coronary artery (LAD) and immediately following reperfusion of a 10-minute occlusion. Beta-blocking doses of intravenous propranolol (P) (0.5 mg/kg) attenuated the fall in VF threshold during acute ischemia. Increasing the dose of P to 1 mg/kg did not provide further protection, nor did P protect against reperfusion VF. The other 15 animals underwent a preliminary surgical procedure during which the LAD was completely and irreversibly occluded (nine animals) or in which a sham procedure was performed (six animals). Two weeks later, we measured ventricular refractoriness at several left ventricular sites, ventricular inducibility using programmed electrical stimulation, and VF thresholds both before and after administration of intravenous P (1 mg/kg). Ventricular refractory periods in the infarcted zones were significantly increased compared with normal sites and with values obtained in sham-operated animals. In addition, VF thresholds in the infarcted animals were lower than those obtained in the sham-operated group. Before treatment, a reproducible sustained ventricular tachyarrhythmia was induced by means of programmed stimulation in seven of the nine chronically infarcted animals but in none of the sham-operated animals (p less the 0.02).(ABSTRACT TRUNCATED AT 250 WORDS)

Comparison of immediate versus day to day variability of ventricular tachycardia induction by programmed stimulation

The use of programmed stimulation to assess long-term oral antiarrhythmic drug efficacy for ventricular tachycardia is complicated by the fact that mode of ventricular tachycardia induction varies from day to day in the absence of drug therapy. The purpose of this prospective study was to assess whether mode of ventricular tachycardia induction is more reproducible within one study than from day to day. Thirty-nine consecutive patients with documented sustained ventricular tachyarrhythmias secondary to coronary artery disease underwent three inductions of ventricular tachycardia at 15 min intervals in the absence of drug therapy. A stimulation protocol in which the only major variable was the number of extrastimuli required for tachycardia induction was used. Subsequent day to day variability in mode of tachycardia induction was also assessed in the same patients at two further drug-free inductions at intervals of 5 +/- 2 days. The number of extrastimuli required for tachycardia induction was significantly more reproducible at the immediate repeat studies than from day to day (69% of patients versus 31%, p less than 0.01). From these data, probability tables were derived that show the likelihood that changes in inducibility at subsequent tachycardia inductions are due to chance. The QRS configuration of induced ventricular tachycardia was also more reproducible at the immediate studies (64% versus 26%, p less than 0.01). Basic electrophysiologic and stimulation variables differed over a significantly wider range from day to day than at the immediate studies.(ABSTRACT TRUNCATED AT 250 WORDS)

Propranolol blocks ventricular refractory period changes with orthostatic stress in humans

The purpose of this study was to test the hypothesis that orthostatic stress shortens the right ventricular effective refractory period by reflex activation of beta-adrenergic receptors. Twelve patients undergoing electrophysiologic testing for standard clinical indications were studied. After a full electrophysiologic study, patients underwent graded lower body negative pressure before and after administration of either propranolol (0.2 mg/kg intravenously) in Group I or atropine (0.035 mg/kg intravenously) in Group II. Before the addition of drugs, lower body negative pressure produced decreases in systolic blood pressure and significant increases in sinus rate. The effective refractory period shortened from 214 +/- 8 (mean +/- SEM) to 206 +/- 7 ms at -40 cm H2O and to 197 +/- 4 ms at -60 cm H2O lower body negative pressure. After propranolol, Group I patients had no change in right ventricular effective refractory period despite similar changes in sinus rate and systolic blood pressure. In group II patients, atropine did not alter effective refractory period responses to lower body negative pressure. Thus, reflex adjustments to orthostatic stress result in shortening of right ventricular effective refractory period mediated by way of beta-adrenergic mechanisms. These findings constitute the first evidence that sympathetic influences mobilized by the body can directly modulate ventricular electrophysiologic changes.

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.

Prevention of postischemic ventricular fibrillation late after right or left stellate ganglionectomy in dogs

To gain insight into the differences in antiarrhythmic potential of right vs left stellate ganglionectomy, 72 dogs were randomized to either unilateral stellectomy or second intercostal space thoracotomy and left circumflex coronary arteriovenous pedicle occlusion was performed, without vagotomy, a mean of 8 weeks later under anesthesia. The type and timing of ventricular ectopic beats, including both nonsustained and sustained ventricular tachycardia and ventricular fibrillation, were investigated. Several covariates, including postischemic electrocardiographic changes, were considered. Both right and left stellate ganglionectomy reduced the incidence of early (0 to 10 min) (p = .004 and p = .001, respectively) and total (0 to 60 min) (p = .009 and p = .008, respectively) ischemia-induced ventricular fibrillation, and improved outcome (p = .0013 and p = .0012, respectively). Early sustained ventricular tachycardia was similarly reduced (p = .02) in both stellectomized groups. By contrast, neither the type nor the time distribution of the other forms of ventricular arrhythmias differed significantly among the randomized groups. The multivariate Cox's regression model showed that ST segment elevation at 3 min postocclusion, unilateral stellate ganglionectomy (either right or left), sex, and weight were significant independent predictors of the incidence of ventricular fibrillation during the occlusion period. Lower ST segment elevation and reduced incidence of sustained ventricular tachycardia in the early postischemic period might explain improved outcome in stellectomized dogs by Cox analysis. The side of intervention (either stellectomy or sham operation) did not influence survival; however, left-sided interventions were more effective than right-sided ones. These results confirm the previously reported antifibrillatory effect of left and indicate like effects of right stellate ganglionectomy in a randomized experimental study.

Reduction in digitalis-associated postinfarction mortality with nadolol in conscious dogs

Previously, we have demonstrated an increased incidence of lethal ischemic arrhythmias in postinfarction dogs with clinically observable serum digoxin concentrations, and a significant reduction in digitalis-related lethal ischemic arrhythmias after subacute left stellectomy. In the present study, the protective actions of acute beta-adrenoceptor blockade with nadolol, 1.0 mg/kg administered intravenously immediately preceding the induction of posterolateral myocardial ischemia, were assessed in conscious dogs with recent, small anterior myocardial infarctions pretreated with digoxin, 0.0125 mg/kg/day intravenously, for 5 to 7 consecutive days (total n = 11). A cohort of postinfarction dogs pretreated with digoxin alone served as a control group (total n = 26). Pre vs postdigoxin electrophysiologic testing indicated reductions in myocardial refractoriness in ventricular noninfarct and infarct zones in both treatment groups, whereas the administration of nadolol tended to reverse the reductions in ventricular refractoriness. Arrhythmia-related deaths in response to posterolateral myocardial ischemia were reduced from 12 of 20 (60%) in the digoxin control group to 2 of 10 (20%) in the digoxin + nadolol group (p = 0.039). Serum digoxin concentrations (1.29 +/- 0.14 ng/ml vs 1.39 +/- 0.24 ng/ml), underlying anterior myocardial infarct size (6.9 +/- 1.5% vs 4.6 +/- 0.9% of left ventricle), and developing posterolateral myocardial infarct size (22.8 +/- 2.5% vs 17.5 +/- 3.6% of left ventricle) did not differ significantly between the digoxin and digoxin + nadolol groups. Acute beta-adrenoceptor blockade with nadolol appears to reduce digitalis-mediated ischemic postinfarction mortality, possibly because of a salutary increase in ventricular refractoriness.