Causes and Risk Factors of Cardiac Arrhythmias

The cardiac electrical system controls the heart rate and makes sure that the various chambers of the heart contract in a completely coordinated way. These tasks are complex and intricate—for instance, when the heart’s electrical signal travels from the atria to the ventricles, a difference in timing of just a tenth of a second can make a big impact on the efficiency of the heartbeat.

With a physiologic system that requires this level of precision and complexity, there are bound to be any number of ways it can be disrupted or disordered. So it should not be a surprise that cardiac arrhythmias can result from a host of underlying causes.

This means, among other things, that a critical step in evaluating and treating a person who has a cardiac arrhythmia is to identify, as precisely as possible, the underlying cause of the heart rhythm problem. Eliminating or mitigating that cause is often the best way to treat the arrhythmia.

The causes of cardiac arrhythmias can be grouped into several categories. Here is a brief description of the major categories of the problems that often cause arrhythmias, listed (roughly) from the most to the least common.

The autonomic nervous system, in general, controls the bodily functions that we normally don’t consciously think about, like breathing, sweating, and the heart rate.

Bradycardias (slow heart rhythms) and tachycardias (fast heart rhythms) often occur as a result of overstimulation of (respectively) vagal tone, or of sympathetic tone. Vagal overstimulation, and bradycardia, may result from (for instance), an episode of vomiting, severe constipation, or urinary obstruction. Excess sympathetic tone (too much adrenaline) from acute stress or sudden fright may cause rather impressive tachycardia.

The trick to treating arrhythmias due to autonomic imbalance is to get rid of the cause of the excessive vagal or sympathetic tone. These arrhythmias go away when the autonomic imbalance is resolved.

Any type of heart disease can affect the heart’s electrical system and cause cardiac arrhythmias. The arrhythmias that result from heart disease can span the entire gamut of cardiac arrhythmias—from entirely benign premature atrial complexes to extremely malignant ventricular fibrillation.

However, structural heart disease is the most common cause of truly dangerous rhythm disturbances. The cardiac disorders that most commonly produce life-threatening heart rhythm problems are coronary artery disease, cardiomyopathy (heart muscle disease), and heart valve disease.

In fact, the risk of sudden death is sufficiently high in people who have had heart attacks or heart failure that one of the chief goals of the physician when treating these patients should be to take the steps necessary to substantially reduce that risk.

Especially in people who may have an underlying propensity to develop cardiac arrhythmias (for instance, because of underlying heart disease or a genetic issue), various drugs can be the triggering factor that makes an arrhythmia actually occur. The list of drugs that can trigger arrhythmias is very large.

The kinds of drugs that are most commonly associated with cardiac arrhythmias include:

• Digoxin.
• Antiarrythmic drugs, especially quinidine, disopyramide, procainamide, sotalol, and dofetilide.
• Cocaine.
• Alcohol, especially after binge drinking.
• Antibiotics, including erythromycin, azithromycin, clarithromycin, and ciprofloxacin.
• Non-sedating antihistamines, such as terfenadine and astemizole.
• Psychotropic drugs, especially haloperidol, thorazine, and methadone. 

Since 2000, researchers have identified several genetic mutations that now explain many previously mysterious cardiac arrhythmias. Here is a list of the most common cardiac arrhythmias now known to be genetically mediated.

As research progresses, it is certain that other arrhythmias will be added to this list:

• Long QT syndrome.
• Brugada syndrome.
• Some forms of heart block and bundle branch block.
• Sick sinus syndrome in young people.
• Certain types of atrial fibrillation.
• Certain types of ventricular tachycardia. 

For reasons that are not clear, aging is itself associated with a form of diffuse fibrosis (scarring) within the cardiac muscle that may lead to sick sinus syndrome, heart block or atrial fibrillation. The cardiac fibrosis of aging is the most common reason for elderly people to require a pacemaker.

Various disorders of serum electrolytes, and of the acidity of the blood, can trigger cardiac arrhythmias. These disorders are most commonly seen in people who have kidney disease, diabetes, are taking certain drugs (especially diuretics), are dehydrated, or are acutely ill.

The electrolyte and metabolic disorders that most prominently cause arrhythmias include:

• Hypokalemia (low potassium levels).
• Hyperkalemia (high potassium levels).
• Hypomagnesemia (low magnesium levels).
• Hypocalcemia (low calcium levels).
• Acidosis (blood too acidic).
• Alkalosis (blood too alkaline).

Cardiac arrhythmias are quite common in people who are undergoing general anesthesia. While most of these arrhythmias are benign and easily managed, some can become dangerous and difficult to treat.

Anesthesia is associated with cardiac arrhythmias for several reasons, including :

• The anesthetic agents themselves.
• Electrolyte and metabolic disorders that may occur during anesthesia.
• Fluctuations in blood pressure that may occur during anesthesia.
• Autonomic imbalances during anesthesia. 
• Changes in blood pressure during anesthesia.
• Cardiovascular damage during surgery, especially heart surgery.

Cardiac trauma from any chest injury, or as a result of cardiac surgery, can produce arrhythmias of almost any type.

“Idiopathic” is the medical term for, “We don’t know what caused it.” A cardiac arrhythmia is deemed to be idiopathic if, after a thorough investigation, the underlying cause remains unknown.

In recent years, many arrhythmias that used to be classified as idiopathic are now known to be genetic in origin. 

The best way to reduce your risk of developing cardiac arrhythmias is to do everything you can to avoid heart disease. This is for a simple reason: the vast majority of cardiac arrhythmias that are dangerous or disruptive to a person’s life result from heart disease that may be largely preventable.

Minimizing your risk for heart disease can be a challenge for many people, but it is worth the effort—not only to reduce the risk of arrhythmias but (even more importantly) to reduce the risk of the heart disease that causes the arrhythmias in the first place.

The chief lifestyle factors that can lower your risk of heart disease include:

• Not smoking, or stopping if you do.
• Managing elevated cholesterol and/or triglycerides.
• Getting plenty of exercise.
• Avoiding or treating hypertension.
• Managing diabetes.
• Avoiding obesity, or losing weight.

While more and more arrhythmias are now known to be genetically mediated, the genetics of cardiac arrhythmias tends to be quite complex. Genetic testing for cardiac arrhythmias is not generally recommended because translating the results of testing into practical advice is usually not possible.

However, for close family members of people who have had long QT syndrome, Brugada syndrome, or arrhythmias associated with hypertrophic cardiomyopathy, genetic testing may be of value in deciding whether prophylactic treatment should be considered.