The heart is a muscular organ with four chambers designed to work efficiently and continues throughout life. The muscular walls of each chamber contract in a precise sequence during each beat and expel more blood with less effort.
The contraction of the muscle fibers of the heart is controlled by an electric shock through the heart following different paths and at a certain speed. The rhythmic discharge that starts each heartbeat originates in the pacemaker of the heart (sinoatrial node), located on the wall of the right atrium. The speed of these discharges is partly dependent on nerve impulses and the amount of certain hormones in the blood.
The part of the nervous system that automatically regulates the heart rate is the autonomic nervous system, which includes the sympathetic and parasympathetic nervous systems. The sympathetic nervous system increases heart rate, the parasympathetic decreases. The sympathetic system provides the heart with a network of nerves called the sympathetic plexus. The parasympathetic system gets to the heart through a single nerve: the vagus nerve or vagus.
On the other hand, the sympathetic nervous system hormones (adrenaline and noradrenaline) also increase heart rate. Thyroid hormone also exerts the same effect. Too much thyroid hormone causes the heart to beat too quickly, whereas if there is little, it does so very slowly.
The resting heart rate is 60-100 beats per minute. However, they can be considered normal speeds much lower in young adults, especially those in good physical condition. Changes in heart rate are normal. Appear not only the effect of exercise or inactivity, but also by other stimuli, such as pain and emotions. Only when the rate is inappropriately fast (tachycardia) or slow (bradycardia) or when the electrical impulses are abnormal pathways or routes, it is considered that the heart has an abnormal rhythm (arrhythmia). The abnormal rhythm can be regular or irregular.
Traffic routes of electrical stimuli
The electrical impulses from the pacemaker are directed first to the left and right atria, therefore, cause contraction of muscle tissue in a certain sequence that determines the blood to be expelled from the atria to the ventricles. Then, the electrical impulse reaches the atrioventricular node is located between the atria and ventricles. This node holds the shock and slows its transmission to allow the atria to contract completely and the ventricles fill with blood as much as possible during ventricular diastole.
After passing through the atrioventricular node, the electrical impulse reaches the His bundle, a group of fibers that are divided into a left branch to the left ventricle and a right branch to the right ventricle. Thus, the momentum is distributed orderly on the surface of ventricles and initiate its contraction (systole), during which blood is ejected from the heart.
Various anomalies of this system of electrical impulse conduction that cause arrhythmias can range from harmless to severe life-threatening. Each variety has its own cause arrhythmia, while a cause can give rise to various types of arrhythmias. The mild arrhythmias may occur by excessive consumption of alcohol or snuff, by stress or exercise. Hyperactivity or poor performance of the thyroid and some drugs, especially those used for the treatment of pulmonary disease and hypertension, may also alter heart rate and rhythm. The most common cause of arrhythmias is heart disease, including coronary artery disease, malfunctioning valves and heart failure. Sometimes arrhythmias ensue without underlying heart disease or any other cause detectable.
Symptoms
Awareness of own heartbeat (palpitations) varies greatly from person to person. Some people can distinguish abnormal beats and others are able to receive up to a normal heartbeat. Sometimes when lying on the left side, most people perceive the heartbeat. The consciousness of one’s own heartbeat can be disturbing, but usually not the result of an underlying disease. Most often it is due to very strong contractions that appear periodically for various reasons.
The person with a certain type of arrhythmia is prone to the same arrhythmia repeatedly. Some types of arrhythmias cause few or no symptoms but can cause problems. Others do not cause major problems but never, however, do cause symptoms. Often, the nature and severity of underlying heart disease are more important than the arrhythmia itself.
When arrhythmias affect the heart’s ability to pump blood, can cause dizziness, vertigo and fainting (syncope). Arrhythmias that cause these symptoms require immediate attention.
Diagnosis
The description of the symptoms almost always allows a preliminary diagnosis and the severity of the arrhythmia. The most important considerations are whether the beats are fast or slow, regular or irregular, shorter or longer, if they appear vertigo, dizziness or weakness and even loss of consciousness and if the palpitations are associated with chest pain, shortness of breath and other uncomfortable sensations. The doctor also needs to know whether the palpitations occur when the patient is at rest or during unusual or vigorous activity and, furthermore, if they begin and end suddenly or gradually.
In general, I need some additional tests to determine the exact nature of the disease. The electrocardiogram is the main diagnostic test to detect arrhythmias and provides a graphical representation of them.
However, the electrocardiogram (ECG) only shows the heart rate during a short period and arrhythmias are generally intermittent. Therefore, a portable monitor (Holter), which takes over 24 hours, can provide more information. It can record arrhythmias occur sporadically while the patient continues normal daily activities and points in a diary of symptoms detected within 24 hours. If life-threatening arrhythmias, hospitalization is required to carry out this monitoring.
When you suspect the existence of a persistent and potentially fatal arrhythmia, invasive electrophysiological studies may be helpful. To do this, is introduced intravenously into the heart catheter containing a wire. Using a combined electrical stimulation and a sophisticated monitoring can determine the type of arrhythmia and the most likely response to treatment. The most serious arrhythmias can be detected by this technique.
Prognosis and Treatment
The outlook depends in part on whether the arrhythmia starts in the normal heart pacemaker in the atria or the ventricles. In general, starting in the ventricles are more serious, although many of them are not dangerous.
In general, arrhythmias cause no symptoms or interfere with the pumping function of the heart, so the risks are minimal. However, arrhythmias are a source of anxiety when the person becomes aware of them, so they understand their innocuous nature can provide relief. Sometimes, when changing or adjusting the medication dose, or when you stop drinking or practice of vigorous exercise, arrhythmias are more spread out or even cease.
The administration of anti-arrhythmic drugs is very useful in case of intolerable symptoms or pose a risk. There is no single drug that will cure all arrhythmias in all people. Sometimes you have to try several treatments to find one that is satisfactory. In addition, antiarrhythmic drugs can cause side effects and may worsen or even cause arrhythmias.
Artificial pacemakers, electronic devices that act in place of the natural pacemaker was programmed to mimic the normal sequence of the heart. In general, are surgically implanted under the skin of the chest and have wires coming into the heart. Due to low power circuit design and new batteries, these units have a duration of between 8 and 10 years. These new circuits have almost eliminated the risk of interference with car dealers, radar, microwave detectors and airport security. However, other computers can interfere with pacemakers, as the devices used for magnetic resonance imaging (MRI) and diathermy (physical therapy used to warm the muscles).
The most frequent use is given to the pacemaker is to treat a heart rate too slow (bradycardia). When the heart rate decreases below a certain value, the pacemaker begins to emit electrical impulses. In exceptional cases, a pacemaker is used to send a series of pulses to stop an abnormally fast heart rate (tachycardia) and to decrease its speed. These pacemakers are used only in the case of fast rhythms that start in the atria.
Sometimes, the application of electric shock to the heart to stop an abnormal rhythm and restore normal. This method is called cardioversion or defibrillation electroversión. Cardioversion can be used to treat arrhythmias that begin in the atria or the ventricles. Generally, it uses a large device (defibrillator), handled by a specialized team of doctors and nurses, to generate an electric shock to stop an arrhythmia that can cause death. However, it can be surgically implanted a defibrillator the size of a deck of cards. These small devices that automatically detect arrhythmias that can be deadly and emit a discharge, are implanted in people who otherwise might die suddenly stopped his heart. As these defibrillators do not prevent arrhythmias, these people usually also take antiarrhythmic drugs at the same time.
Certain types of arrhythmias are corrected by surgery and other invasive procedures. For example, arrhythmias caused by coronary disease are controlled by angioplasty or bypass surgery coronary artery (bypass). When an arrhythmia is caused by an irritable focus in the heart’s electrical system, this focus may be destroyed or removed. Most often, the focus is destroyed by catheter ablation (RF power output through a catheter inserted into the heart). After a heart attack may occur episodes of ventricular tachycardia that can be fatal. This arrhythmia can be caused by a damaged area of heart muscle that can be identified and extracted by open-heart surgery.
Premature atrial
An atrial premature contraction is a heartbeat caused by electrical activation of the atria before a normal heartbeat.
The atrial extrasystoles occur as extra heartbeats in healthy people and only rarely cause symptoms. The precipitating factors are alcohol consumption and administration of cold preparations that contain drugs that stimulate the sympathetic nervous system (such as ephedrine or pseudoephedrine) or drugs used to treat asthma.
Diagnosis and treatment
Are detected with a physical examination and confirmed by an electrocardiogram (ECG). If treatment is necessary because the ectopic beats occur frequently and generate intolerable palpitations, given a beta blocker to reduce heart rate.
Paroxysmal atrial tachycardia
Paroxysmal atrial tachycardia is a uniform heart rate, fast (160 to 200 beats per minute) that happens all of a sudden and starts in the atria.
There are several mechanisms that produce paroxysmal atrial tachycardias. The rapid rate may be due to premature atrial beats send a pulse through an anomalous pathway to the ventricles.
This fast heart rate usually has a beginning and an end sudden and can last from a few minutes to several hours. Is experienced in most cases as an annoying throbbing and is often accompanied by other symptoms such as weakness. Generally, the heart is normal, these episodes are more unpleasant than dangerous.
Treatment
The episodes of arrhythmia can often be interrupted by one or several maneuvers that stimulate the vagus nerve and that, consequently, reduce the heart rate. These maneuvers, which are usually directed by a physician, include having the patient simulate evacuation efforts, rubbing his neck just below the angle of the jaw (which stimulates the sensitive area of the carotid artery called the carotid sinus) and dip face into a bowl of very cold water. These maneuvers are more effective if they are made just beginning the arrhythmia.
If these procedures do not work, the arrhythmia is usually stopped if the person goes to sleep. But usually, people seek the help of a doctor to end the inning. In general, it is easily stopped administering intravenous doses of verapamil or adenosine. When drugs do not work should be resorted to cardioversion (electric shock applied to the heart).
Prevention is more difficult than the treatment, but there are several drugs that are effective, given alone or in combination. On rare occasions, it may be necessary to destroy an anomalous pathway in the heart through a catheter ablation (RF power through a catheter inserted into the heart).
Atrial fibrillation and flutter
Atrial fibrillation and atrial flutter are very fast electrical impulses that produce a very rapid atrial contraction, which causes the ventricles to contract in a faster and less efficient than normal.
These rhythms may be sporadic or persistent. During atrial fibrillation and flutter, the atrial contractions are so fast that the walls of the atria quiver just, so not having a real contraction, blood is not pumped into the ventricles. In atrial fibrillation, the atrial rhythm is irregular, so the ventricular rhythm is also, in the flutter, atrial and ventricular rhythms are generally uniform. In both cases, the ventricles beat more slowly than the atria because the atrioventricular bundle and can not conduct electrical impulses at speeds as high and only one of every two or four pulses get through. However, still the beating of the ventricles is so fast that they can not completely filled. Therefore, the heart pumps insufficient amounts of blood, lowers blood pressure and there is a potential risk of heart failure appear.
Atrial fibrillation or flutter may occur with no other visible signs of heart disease, but most often be an underlying problem such as rheumatic heart disease, a coronary artery disease, hypertension, alcohol abuse or excessive production of thyroid hormone (hyperthyroidism).
Symptoms and Diagnosis
Symptoms of atrial fibrillation or flutter depend on the frequency with which the ventricles contract. If this is not very fast (less than 120 beats per minute) did not produce symptoms, whereas higher frequencies cause unpleasant palpitations or chest discomfort. In atrial fibrillation, the patient may feel the irregular heart rhythm.
The reduced pumping ability of the heart can cause weakness, fainting and choking. Some people, especially the elderly, develop heart failure, chest pain and shock.
In atrial fibrillation, the atria do not empty completely into the ventricles with each beat, so the blood left inside can become stagnant and clot. Even clots can break off pieces that go into the left ventricle, enter the general circulation and can reach a smaller artery and block it (embolism). However, most often, the pieces of a clot breaks loose shortly after the atrial fibrillation returns to normal rhythm, either spontaneously or through the application of a treatment. The blockage of an artery in the brain can cause a stroke that, in rare cases, the first sign of atrial fibrillation.
The diagnosis of atrial flutter or fibrillation is made from symptoms and is confirmed by an electrocardiogram (ECG). In atrial fibrillation, the pulse is irregular, whereas in atrial flutter has a tendency to be steady but fast.
Treatment
Treatments for atrial fibrillation and atrial flutter aim to control the speed of contraction of the ventricles, to treat the disorder responsible for the abnormal rhythm and restore normal heart rhythm. In atrial fibrillation is provided in addition, a treatment to prevent blood clots and stroke.
First, decrease the ventricular rate to increase the pumping efficiency of the heart. To this effect, digoxin, a drug that slows the impulse conduction to the ventricles. When digoxin is not effective, combined with another drug (a beta-blocker such as propranolol or atenolol, a blocker or calcium channel blockers, such as diltiazem or verapamil), which increases their effectiveness.
Treatment of the underlying disease rarely improves atrial arrhythmias, unless the condition is hyperthyroidism.
Sometimes, atrial flutter or fibrillation may revert to a normal rhythm spontaneously, but more often it is necessary to intervene to get this normal. Although this reversal can be achieved with certain antiarrhythmic drugs, electric shock (cardioversion) is the most effective treatment. The success of the means used depends on the time since the onset of abnormal heart rhythm (the odds of success are lower after six months or more), the degree of dilatation of the ventricles and the seriousness of the disease has reached underlying heart. Although conversion is achieved, the risk of arrhythmia recurrence is high even when given preventive drugs such as quinidine, procainamide, propafenone or flecainide.
If all other treatments are unsuccessful, the atrioventricular node is destroyed using catheter ablation (RF power through a catheter inserted into the heart). This procedure interrupts the conduction from the atria in fibrillation to the ventricles, so that is required to place a permanent artificial pacemaker for the ventricles to contract.
The risk of blood clots is higher in people with atrial fibrillation and left atrial enlargement or mitral valve disease. The risk of a clot breaks loose and causes a stroke is particularly high in patients with intermittent but long-lasting episodes of atrial fibrillation or atrial which has been converted into normal rhythm. Since any person with atrial fibrillation runs the risk of stroke, generally, apply an anticoagulant to prevent clot formation, unless a specific reason not to (for example, hypertension) . However, this treatment carries a risk of bleeding that can lead to hemorrhagic stroke and other bleeding complications. Consequently, the potential benefits and risks must be considered in each individual.
Wolff-Parkinson-White
The Wolff-Parkinson-White is a cardiac arrhythmia in which electrical impulses are conducted over an accessory pathway from the atria to the ventricles, which causes episodes of tachycardia.
The Wolff-Parkinson-White is the most common disorders affecting accessory pathways. Although they are present at birth, only these accessory pathways conduct impulses through the heart at times. They can manifest early, during the first year of life or late, for example, at age 60.
Symptoms and Diagnosis
The Wolff-Parkinson-White cause sudden episodes of tachycardia with palpitations. During the first year of life, babies can begin to show symptoms of heart failure if the episode is prolonged. Sometimes, they seem to run out of breath or lethargic, stop eating well or have quick and visible pulsations in the chest.
The first episodes may occur between 10 and 25 years. Episodes typically begin all of a sudden, often during an exercise. They may last only a few seconds or persist for several hours, rarely more than twelve hours. In a young person and, moreover, with a good health, episodes produce minimal symptoms, but palpitations are annoying and stressful and can cause fainting or heart failure. Tachycardia sometimes is transformed in atrial fibrillation. The latter is particularly dangerous in about one percent of people suffering from Wolff-Parkinson-White, because the accessory pathway can conduct impulses to the ventricles faster more efficiently than normal. The result is a very rapid ventricular rate, which can be fatal. Not only is a very ineffective heart beat so fast, but the rapid heart rate may progress to ventricular fibrillation, which leads to death immediately.
The diagnosis of Wolff-Parkinson-White with or without atrial fibrillation is made by an electrocardiogram (ECG).
Treatment
The episodes of arrhythmia are often interrupted by one or several maneuvers that stimulate the vagus nerve and, consequently, reduce the heart rate. These maneuvers, under medical supervision, are to make similar efforts to escape, rubbing his neck just below the angle of the jaw (which stimulates the sensitive area of the carotid artery called the carotid sinus) and immerse the face in a container ice-cold water. These exercises work best when done just beginning the arrhythmia. If you do not have the desired effect, are given drugs such as verapamil or adenosine intravenously to stop the arrhythmia. For long-term prevention of episodes of tachycardia are given other antiarrhythmic drugs.
Digoxin can be supplied to infants and children under 10 years to suppress episodes of rapid heart rate. Adults should not take digoxin because it speeds up the accessory pathway conduction and increases the risk of ventricular fibrillation. For this reason, the drug is discontinued before reaching puberty.
The destruction through the accessory pathway catheter ablation (RF power through a catheter inserted into the heart) is effective in more than 95 percent of cases. The risk of death during the procedure is less than 1 in 1,000 people. Catheter ablation is particularly useful in young patients, because, otherwise, would face an indefinite treatment with antiarrhythmic drugs.
Premature ventricular
A ventricular extrasystole (premature ventricular contraction) is a heartbeat caused by electrical activation of the ventricles before the normal heartbeat.
This type of arrhythmia is common and not indicative of danger when there is no associated heart disease. However, when often manifest in a person suffering from heart failure, aortic stenosis or has had a heart attack may represent the onset of dangerous arrhythmias such as ventricular fibrillation and cause sudden death.
Symptoms and Diagnosis
The isolated premature ventricular contractions have little effect on the heart’s pumping action, and usually produce no symptoms, unless they are too frequent. The main symptom is the perception of a strong beat or out of place.
The premature ventricular contractions are diagnosed with an electrocardiogram (ECG).
Treatment
In principle, the only treatment is to reduce the causes of stress and avoiding alcohol and prepared for the cold counter containing active ingredients that stimulate the heart. In general, only prescribed drug treatment if symptoms are intolerable or when the heart rate tracing suggests danger. Given its relative safety, beta-blockers are the first choice. However, many patients do not want to take them due to the feeling of laziness that can cause.
After a heart attack, and if frequent premature ventricular contractions can reduce the risk of sudden death taking beta-blockers and undergoing coronary bypass surgery (bypass) to relieve the underlying obstruction. Antiarrhythmic drugs suppress PVCs, but they can also increase the risk of a fatal arrhythmia. They should therefore be used with caution in selected patients after cardiac sophisticated studies and related risk assessment.
Ventricular tachycardia
Ventricular tachycardia is a ventricular rate of at least 120 beats per minute.
Sustained ventricular tachycardia (ventricular tachycardia lasting at least 30 seconds) occurs in several cardiac diseases that cause serious injury to the ventricles. Most often it manifests several weeks or months after a heart attack.
Symptoms and Diagnosis
Ventricular tachycardia is almost always accompanied by palpitations. Sustained ventricular tachycardia can be dangerous and often requires emergency treatment, because the ventricles can not fill properly or exercise its pumping function. Blood pressure tends to decrease and heart failure occurs. Likewise, there is the risk of aggravation of ventricular tachycardia and ventricular fibrillation becomes (a form of cardiac arrest). Although ventricular tachycardia may produce few symptoms, even at frequencies up to 200 beats per minute is extremely dangerous.
The diagnosis of ventricular tachycardia is performed by an electrocardiogram (ECG).
Treatment
Should be treated any episode of ventricular tachycardia that causes symptoms and those longer than 30 seconds, even if they are asymptomatic. When episodes cause a drop in blood pressure below normal values, is needed cardioversion. To suppress the ventricular tachycardia is administered lidocaine or a similar drug intravenously. If episodes of ventricular tachycardia persist, an electrophysiology study is done and tested other drugs. The study results can decide the most effective drug to prevent recurrences. Sustained ventricular tachycardia is caused by a small abnormal area in the ventricles, which can sometimes be removed surgically. In some people with ventricular tachycardia unresponsive to medical therapy, can implant a device called an automatic defibrillator cardioversion.
Ventricular fibrillation
Ventricular fibrillation is an uncoordinated series of contractions and potentially fatal ventricular ineffective very quickly, caused by multiple chaotic electrical impulses.
Ventricular fibrillation is electrically similar to atrial fibrillation, except that it has a much more serious prognosis. In ventricular fibrillation, the ventricles quiver and not simply carry out coordinated contractions. Because the heart does not pump blood, ventricular fibrillation is a form of heart failure and is fatal unless treated immediately.
The causes of ventricular fibrillation are the same as those of heart failure. The most common cause is insufficient blood flow to the heart muscle because of coronary artery disease or stroke. Other causes include shock and very low concentration of potassium in the blood (hypokalemia).
Symptoms and Diagnosis
Ventricular fibrillation causes unconsciousness within seconds. If no treatment is applied immediately, usually produced convulsions and permanent brain damage after 5 minutes, because the oxygen no longer reaches the brain. Then comes death.
The physician considers the diagnosis of ventricular fibrillation in case of sudden collapse. During the scan are not detected no pulse or heartbeat, nor blood pressure is detected. The diagnosis is confirmed with an electrocardiogram (ECG).
Treatment
Ventricular fibrillation should be treated as an emergency. Cardiopulmonary resuscitation (CPR) should be initiated before they pass a few minutes and as soon as possible should be a cardioversion (electric shock applied on the chest). The following drugs are administered to maintain normal heart rhythm.
When ventricular fibrillation occurs within hours of a heart attack and the person is not in shock or have heart failure, immediate cardioversion is successful in 95 percent of cases and the prognosis is good. The shock and heart failure are indicative of serious injuries in the ventricles, when present, even the immediate cardioversion is only a success rate of 30 percent and 70 percent of these survivors died after resuscitation .
Heart block
Heart block is a delay in electrical conduction through the atrioventricular node, located between the atria and ventricles.
Heart block is classified as first-degree block, second degree or third degree, depending on the delay of electrical conduction to the ventricles is mild, intermittent or complete.
In the first-degree heart block, each impulse from the atria reach the ventricles but is delayed a fraction of a second when flowing through the atrioventricular node. This problem is driving asymptomatic. The first-degree heart block is common among well-trained athletes, adolescents, young adults and people with a high activity of the vagus nerve. However, the condition can also be seen in rheumatic fever and cardiac involvement in sarcoidosis and its origin may be the administration of some drugs. The diagnosis is based on observation of conduction delay on an electrocardiogram (ECG).
In the second-degree heart block not all impulses reach the ventricles. This blocking occurs when the heart beats slow or irregular. Some forms of second-degree block progress at the third level.
In the third-degree heart block, the impulses from the atria and ventricles are aimed at completely blocked and the heart rate and rhythm are determined by the activity of the atrioventricular node or the ventricles themselves. Without the stimulation of normal cardiac pacemaker (sinoatrial node), the ventricles beat very slowly, less than 50 beats per minute. The third-degree heart block is a serious arrhythmia that affects the heart’s pumping ability. Fainting (syncope), dizziness and sudden heart failure are common symptoms. When the ventricles beat at a rate greater than 40 beats per minute, symptoms are less severe but include fatigue, low blood pressure when standing and choking. The atrioventricular node and ventricles are not only slow as substitutes for natural pacemaker, but even they are often irregular and inefficient.
Treatment
The first-degree block requires no treatment, even when caused by heart disease. Some cases of second-degree block may require an artificial pacemaker. The third-degree block almost always require an artificial pacemaker. In an emergency you can use a temporary pacemaker until a permanent one can be implemented. Usually, people with this condition require an artificial pacemaker for the rest of his life, although normal rhythms sometimes return after recovering the underlying cause, such as a heart attack.
Sick sinus syndrome
The sinus node disease encompasses a wide variety of abnormalities of the functioning of the natural pacemaker.
This syndrome can produce a persistent slow heartbeat (bradycardia) or a complete blockage between the pacemaker and the atria (sinus arrest), in which case the impulse from the pacemaker does not cause contraction of the atria. When this happens, usually comes into operation an emergency pacemaker located downstream, either in the atrium or the ventricle.
An important subtype of this disease is the bradycardia-tachycardia syndrome, in which rapid atrial rhythms, including atrial fibrillation or flutter, alternating with long periods of slow heart rhythms. All kinds of sick sinus syndrome are particularly common in the elderly.
Symptoms and Diagnosis
Many types of sick sinus syndrome do not cause symptoms, although the cause persistently low heart rates, often weak and tired. When the frequency becomes very slow, even fading occurs. Often, rapid heart rates are perceived as palpitations.
A slow pulse, especially if it is irregular or no pulse with large variations coincide with changes in the activity of the subject, leads to suspect sick sinus syndrome. The characteristic abnormalities of the electrocardiogram (ECG), particularly if they register for a period of 24 hours and are considered along with the symptoms that accompany them generally provide confirmation of the diagnosis.
Treatment
People who have symptoms usually are placed an artificial pacemaker. These pacemakers are used to accelerate the heart rate rather than to decrease. In cases with occasional periods of rapid frequency, may also be necessary administrarción drugs. Therefore, usually the best treatment involves the implantation of a pacemaker with administration of an antiarrhythmic drug, as a beta-blocker or verapamil.
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