Treatment of Ventricular Tachyarrhythmias Refractory To Shock With Beta Blockers: The SHOCK and BLOCK Trial

Sudden cardiac death (SCD) is a catastrophic event and most commonly results from acute ventricular tachyarrhythmias (abnormal and lethal heart rhythms). It is often triggered by acute coronary events, which may occur in persons without known cardiac disease or in association with structural heart disease. Advanced therapies such as thrombolytic agents, percutaneous coronary intervention, and implantable cardioverter defibrillators are of no value to thousands of victims who do not survive. Many instances of SCD cannot be predicted and any intervention directed toward the general population would have to be applied to an estimated 1000 persons for every 1 person in whom SCD might be prevented. Thus, it would be reasonable to develop new treatment strategies to improve response to resuscitative efforts.

Prompt electrical defibrillation (electrical shocks) is the treatment of choice in persons who develop SCD due to ventricular fibrillation (VF) or pulseless ventricular tachycardia (pVT). However, in up to 25% of all cardiac arrests, patients develop shock resistant VF, defined as VF persisting beyond three defibrillation attempts, and 87-97% of these patients die. Medical therapy, including antiarrhythmic agents, sympathomimetic agents (i.e. stimulants), and buffers have been relegated to a secondary role since there is little evidence that they are of benefit and there use is considered indeterminate or class IIB. Furthermore, the "Guidelines 2000 for Cardiopulmonary Resuscitation and Emergency Cardiovascular Care" of the American Heart Association and the International Liaison Committee on Resuscitation recommend antiarrhythmic drugs as "acceptable" and "probably helpful" in the treatment of VF that persists after three or more external defibrillation shocks. It has been previously reported that the survival rate of hospital patients suffering a cardiac arrest in which epinephrine was required was only 6%. Furthermore, Dorian et al reported a survival to hospital admission of 22.8% in patients suffering an out of hospital cardiac arrest and receiving amiodarone. It is believed that the acute effects of amiodarone are due to the class II or beta blocking effects of the drug.

Resuscitation can only be considered successful if the survivor has no disabling cognitive function. The American Heart Association/International Liaison Committee on Resuscitation guidelines state that with a duration of cardiac arrest of > 8 to 10 minutes, the frequency of significant, permanent neurologic damage becomes unacceptably high. Newer treatment modalities are needed to improve patient outcomes.

Epinephrine has been used during cardiopulmonary resuscitation for more than 100 years yet its use has become controversial because it is associated with increased myocardial oxygen consumption, ventricular tachyarrhythmias, and myocardial dysfunction during the period after resuscitation. The current International Guidelines on Emergency Cardiac Care cite both epinephrine and vasopressin as acceptable vasopressor drugs for treatment of refractory VF but neither drug is acknowledged to be of proven benefit.

Beta blockers might improve patient outcomes by blunting the adverse affects of a hyperadrenergic state that occurs during a cardiac arrest and by improving the balance between myocardial oxygen supply and demand. Ditchey et al showed in an animal model that pretreatment with a beta blocker prior to cardiac arrest followed by standard epinephrine therapy results in reduced myocardial injury during CPR without compromising successful defibrillation or post resuscitation left ventricular function.

The current research protocol was formulated in an attempt to develop new treatment options for patients who develop an in-hospital VF or pVT arrest refractory to electrical defibrillation with the specific goal of improving patient outcomes. The trial will utilize pre-filled, blinded syringes of Metoprolol in patients who develop an in-hospital cardiac arrest due to ventricular fibrillation or pulseless ventricular tachycardia (see study protocol).

Sudden cardiac death (SCD) claims approximately 250,000 persons annually in the United States. Ventricular fibrillation (VF) or pulseless ventricular tachycardia (pVT) appear to be responsible for 25-35% of all out of hospital episodes of sudden cardiac death. The reported average survival to hospital discharge following in-hospital arrest is just as poor at approximately 14-17% despite the development of widespread implementation of basic and advanced cardiac life support. Current American Heart Association (AHA) guidelines recommend prompt electrical defibrillation to reestablish organized electrical activity. Increasing duration of VF (i.e. "shock resistant VF") can result in two major adverse effects. First, an increased duration can reduce the ability to terminate the arrhythmia8. Second, if VF continues for more than four minutes, there is irreversible damage to the central nervous system and other organs. Despite aggressive efforts, successful resuscitation from out-of-hospital cardiac arrest occurs in only one third of patients and only about 10% of all patients are ultimately discharged from the hospital, many of whom are neurologically impaired. Also, the outcome of patients who suffer an in-hospital cardiac arrest is poor with reported survival to hospital discharge rates of 10-15%. Thus, despite improvements and advances in the treatment of heart disease, the outcome of patients experiencing SCD remains poor.

Prompt and early defibrillation of VF or pVT has become the standard of care. Drug therapy for shock resistant VF or pVT has been relegated to a secondary role since there is little evidence that these agents are of benefit. As a result, their use is considered indeterminate or class IIB. In addition, cardiac arrest and cardiopulmonary resuscitation are extreme forms of stress that lead to the highest catecholamine levels ever recorded in both human or experimental animal models. Endogenous catecholamine concentrations are high during ventricular fibrillation even in the absence of epinephrine administration. Currently, epinephrine is the vasopressor of choice for the treatment of cardiac arrest although vasopressin has been used as an alternative. Of note, vasopressin has been shown to be superior to epinephrine in patients with asystole however, its effects were similar to those of epinephrine in the management of VF or pulseless electrical activity. Furthermore, previous studies have raised concern that epinephrine's beta adrenergic effect may increase the myocardial oxygen consumption of the fibrillating heart and predispose to post-defibrillation dysfunction and cardiac arrhythmias.

Numerous animal studies have shown that beta adrenergic blockade reduces myocardial injury and improves survival. Kudenchuk, et al undertook a study in patients with out of hospital cardiac arrest due to ventricular fibrillation. Patients were randomized to receive either amiodarone or placebo after three consecutive defibrillations and one dose of epinephrine. The authors concluded that patients who received amiodarone had a higher rate of survival to hospital admission. It is felt that the beneficial effects are related to the initial class II or beta blocking properties of amiodarone. Furthermore, Dorian, et al reported a higher rate of survival to hospital admission in patients who received amiodarone as compared to lidocaine for shock resistant out-of-hospital ventricular fibrillation. Analysis from the European Myocardial Infarct Amiodarone Trial and the Canadian Amiodarone Myocardial Infarction Trial revealed an interaction between beta-blockers and amiodarone, specifically, the combination group had a better survival and the interaction was statistically significant for arrhythmic death or resuscitated arrest.