• WHAT IS CARDIAC ELECTROPHYSIOLOGY?
• WHAT IS ATRIAL FIBRILLATION?
• WHAT IS BRADYCARDIA?
• WHAT IS SUDDEN CARDIAC ARREST?
• WHAT ARE IMPLANTABLE DEFIBRILLATORS?
• WHAT IS RADIOFREQUENCY ABLATION?

WHAT IS CARDIAC ELECTROPHYSIOLOGY?

• Sinus Node
• Heart Pump
• Heart’s Electrical System
• Diseases of the Electrical System

Cardiac electrophysiology is a subspecialty of cardiology which deals with the study of the heart’s electrical system. The term “electrophysiology study” or “EP study” applies to any procedure that requires the insertion of an electrode catheters into the heart to make electrical measurements. Electrode catheters are long, flexible wires that allow electrical measurements and stimulation of the heart muscle and its electrical system.

Electrophysiology studies may be done to diagnose electrical abnormalities (see Heart Rhythm Problems and Conditions), to access the heart for treatment or correction of certain conditions, such as Pacemaker Implantation, ICD Insertion, or Cardiac Ablation.

Sinus Node (Natural Pacemaker)

The heart’s electrical system controls the rhythmic contractions that keep the blood pumping and circulating throughout your body. These electrical impulses originate in the sinus node, a group of specialized cells that acts as the heart’s natural pacemaker.
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Heart Pump

The heart is a muscular pump that serves as the driver for blood circulation. The heart has four chambers. The upper chambers are called the right and left atrium, and the lower chambers are called the right and left ventricle.

Blood from all parts of the body drains into the right atrium, passes through a valve and reaches the right ventricle. The right ventricle contracts with each heartbeat and blood is pushed into the lungs, where it exchanges carbon dioxide, with oxygen, and returns oxygenated blood via the pulmonary veins to the left atrium. During relaxation of the left ventricle, the blood passes from the left atrium to the left ventricle where, with each heartbeat, it is ejected through the aortic valve into the aorta. From the aorta, blood flows through the circulatory system to nourish the organs and tissues of the body.

Heart’s Electrical System

The heart’s electrical system:

• causes the heart to beat.
• controls the heart rate (the number of beats per minute)
• has special pathways (conduction pathways) that carry the electrical signals throughout the lower heart chambers (ventricles) for each heartbeat

A healthy heart beats steadily and rhythmically at a rate of about 60 to 100 beats per minute when at rest (normal sinus rhythm). During strenuous exercise, the heart can increase the amount of blood it pumps up to four times the amount it pumps at rest–within only a matter of seconds.

When heart cells in the upper heart chambers (atria) receive an electrical signal, they contract (pump) and then relax. The blood from the atria is pumped into the relaxed lower heart chambers (ventricles) and then the ventricles pump blood to the body.

In a healthy heart, each heart beat begins in the heart’s sinus node (the heart’s natural pacemaker), which is located in the right atrium. The electrical signal from the sinus node (sinoatrial or SA node) starts an electrical chain reaction that spreads across both atria. This causes the atria to contract and pump blood into the ventricles.

This electrical chain reaction continues from the atria through an area between the atria and ventricles called the atrioventricular node (AV node or AV junction). The AV node connects to conduction pathways that relay the signal to the ventricles. The AV node acts as an electrical gateway to the ventricles. The conduction pathways deliver the signals to the ventricles and the ventricles pump blood to the body.

Diseases of the Electrical System

An irregularity in the heart’s electrical system is called an arrhythmia, or heart rhythm disorder. Rhythm disorders can cause the heart to beat too slowly (Bradycardia) or too fast (Tachyarrhythmia).

WHAT IS ATRIAL FIBRILLATION?

Atrial fibrillation (AF) is the most commonly diagnosed arrhythmia, which is characterized by fast and irregular heart rhythms. It is caused by electrical abnormalities located in the pulmonary veins of the left atrium.

What are the symptoms of Atrial Fibrillation?

Individuals experience the symptoms of atrial fibrillation differently. Some who have this condition may not feel any symptoms, and their condition will go unnoticed until it is detected by a physician during a routine examination. Others may experience only minor symptoms, while some are sensitive to the slightest sensation. Symptoms commonly reported include shortness of breath, lightheadedness, palpitations, and/or chest discomfort.

Medical Treatment for AF

The initial treatment for atrial rhythm disorders is antiarrhythmic drug therapy. These drugs can slow the conduction of rapid atrial fibrillation and/or convert atrial fibrillation to a normal sinus rhythm. Drugs, however, do not cure heart rhythm disorders, and they are not effective in all patients. They also require that a patient maintain a very strict schedule of follow-up care with his or her physician. Many patients with atrial fibrillation require a blood thinner, such as Warfarin, to prevent the formation of blood clots, to prevent stroke.

Catheter Ablation for Atrial Fibrillation – Current Indications

Atrial Fibrillation is triggered by a single (or multiple) focus of muscle firing in the heart. The fibrillation is also maintained by a few rotors located in the atria. In most patients, these sites have been mapped to and around the pulmonary veins (vessels which empty blood from the lungs to the left side of the heart). These venous structures have sleeves of atrial tissue extending from the heart (left atrium) for variable distances into the main branch or its tributaries. This musculature when diseased or damaged is capable of generating ectopic complexes, or repetitive activity at very rapid rates. In susceptible patients, this may lead to and maintains atrial fibrillation. Atrial musculature around the pulmonary veins has parallel fiber orientation and scarring of this tissue allows for rotor to form and maintain atrial fibrillation.

Elimination of the triggers and rotors can lead to reduction or elimination of Atrial Fibrillation. Catheter ablation techniques have been designed and successfully applied to a variety of patients with AF targeting the pulmonary vein musculature and tissue surrounding it. These muscle sleeves/tissue have discrete and limited connections to the atria, which are vulnerable to catheter ablation. A mapping catheter and an ablation catheter are passed into the left atrium using a trans-septal puncture. Wide area circumferential ablation around the pulmonary veins is performed using radiofrequency energy. The pulmonary veins are disconnected from the Atria which results in complete isolation of the pulmonary vein musculature and abnormal tissue around them from the left atrium. This can be achieved in about 99% of targeted veins. The pulmonary veins continue to serve as conduits of oxygenated blood to the left atrium.

This procedure is successful in approximately 85% of patients, who will then require no further medical therapy. An additional 10% of patients may respond to antiarrhythmic drugs that were previous ineffective, or have substantial reduction in AF events. About 5-10% of patients have no response, presumably due to alternate triggers in the heart. Most patients benefit from a second procedure if the first was unsuccessful. The procedure has a small risk of about 1%. Fortunately, serious complications are infrequent. The potential complications include pulmonary vein stenosis (narrowing of the opening of the pulmonary veins due to aggressive scar tissue formation), small risk of stroke and risk of cardiac perforation requiring catheter drainage or surgery. Risk of catastrophic complications (heart attack, esophageal perforation and death) is extremely small.

Ideal candidates include patients who have symptomatic atrial fibrillation without major structural heart disease. Most patients with persistent or chronic AF are also candidates for ablation. Catheter ablation is an effective tool offering a curative treatment of atrial fibrillation.

Device Therapy for AF

In a subset of patients with paroxysmal (intermittent) or persistent atrial fibrillation, devices can be used to either prevent atrial fibrillation or convert it to a normal sinus rhythm. These devices include pacemakers and implantable cardioverter defibrillators (ICD).

WHAT IS BRADYCARDIA?

When the heart rate is too slow, (usually less than 60 beats per minute), not enough oxygen is pumped to the body. The lack of oxygen causes symptoms such as dizziness, extreme tiredness, shortness of breath, or fainting. Bradycardia is when a heart rate is too slow and symptoms occur.

The heart can beat too slowly for several reasons.

• Sick Sinus Syndrome (or Sinus Node Dysfunction) is when the heart’s natural pacemaker (sinus node) sends electrical signals too slowly.
• Heart Block is when the electrical signal is blocked before reaching the lower chambers of the heart (the ventricles).

What is Sinus Node Dysfunction?

Sinus Node Dysfunction (or Sick Sinus Syndrome) is a specific kind of slow heart rate (bradycardia). With sinus node dysfunction, the heart’s natural pacemaker (sinus node) either

• Does not maintain a heart rate that is needed by the body, or
• Cannot increase the heart rate when more oxygen is needed, such as when exercising.

When the sinus node does not work properly, other heart tissues may begin a heartbeat. However, the rate may be inconsistent or too slow for normal activities. With a heart rate that is too slow, not enough oxygen is pumped to the body. The lack of oxygen causes symptoms such as dizziness, extreme tiredness, shortness of breath, or fainting.

Common causes for sinus node dysfunction include congenital heart defects, illnesses, cardiac drugs, the natural aging process, or scar tissue from a heart attack. Sometimes the exact cause is unknown.

Pacemakers relieve the symptoms of sinus node dysfunction by beginning a heartbeat at the rate needed to meet the oxygen needs of the body.

What is Heart Block?

Heart Block is a specific cause of bradycardia (an irregular or abnormally slow heart rate). The slow heart rate (usually less than 60 beats per minute) occurs when a heartbeat is stopped before it makes the lower heart chambers (ventricles) pump blood to the body.

Diagnostic tests such as an electrogram (ECG) assist the doctor in seeing when heart block is causing the slow heart rhythm. When the slow heart rhythm does not happen during testing, an implantable loop recorder, may be used to clarify the diagnosis. A pacemaker relieves the symptoms of bradycardia caused by heart block by ensuring that the ventricles pump at an appropriate rate.

WHAT IS SUDDEN CARDIAC ARREST?

Sudden cardiac arrest is when the heart’s lower chambers (ventricles) suddenly develop a rapid, irregular rhythm (ventricular fibrillation) and the quivering ventricles cannot pump blood to the body. Within seconds, the person will not have a pulse and will be unconscious. Without immediate treatment, the condition is usually fatal.

Sudden cardiac arrest is not a heart attack, which is a problem with the plumbing of the heart; when blockage of one or more coronary arteries results in heart muscle damage. .

There are no prior symptoms of sudden cardiac arrest. However, these risk factors have been identified as increasing the risk of sudden cardiac arrest.

• Previous sudden cardiac arrest episode
• Previous Heart Attack
• Heart Failure
• Family history of sudden cardiac arrest
• History of heart disease or heart rhythm disorders

Treatment begins with cardiopulmonary resuscitation (CPR) {emergency measures to keep the blood flowing to the vital organs of the body}. An external defibrillator is needed to reset the heart rhythm. After the heart is in a normal rhythm, medications and an implantable defibrillator are usually prescribed.

WHAT ARE IMPLANTABLE DEFIBRILLATORS?

Some of the most serious arrhythmias that patients can experience are the rapid and prolonged arrhythmias that come from the pumping chambers. This usually occurs when these chambers have been previously damaged and scarred, such as the after a heart attack. During these arrhythmias, there is frequently a fall in blood pressure and even loss of consciousness. Unless terminated, they can lead to fatal consequences. These arrhythmias require prompt termination which can be most readily accomplished by the application of an electrical shock across the chest. Outside the hospital, this is accomplished by an ambulance team who places paddles on the chest and delivers the shock with an external defibrillator. Implantable defibrillators use the same concept. This device, being permanently available to monitor a patient’s rhythm, can automatically and within seconds, deliver life-saving electrical energy directly to the heart. Patients who are deemed high risk for the development of these dangerous arrhythmias will often be treated prophylactically with an implanted device so that they are protected without need for intervention by emergency personnel.

These devices are called implantable cardioverter defibrillators (ICD). These are implanted in a similar fashion like permanent pacemakers. Using a large vein of the arm that passes underneath the collar bone, a wire or lead can be passed into the right side of the heart. This wire can record the electrical signals from within the heart and communicate with the device when the heart has gone into a rapid, dangerous arrhythmias. This lead is connected to the device which is then buried under the skin below the collarbone. When this device detects a dangerous arrhythmia, it delivers electrical energy through the heart to reset the heart’s electrical rhythm and heart resumes its normal electrical activity. The entire process of detection and termination of this potentially fatal arrhythmia usually lasts 6-15 seconds. This device can be highly effective and often life-saving in patients who are at risk for dying from dangerous electrical heart rhythms.

WHAT IS RADIOFREQUENCY ABLATION?

An extension of the diagnostic EP Study is the catheter ablation. In a similar way, catheters are placed intravenously and advanced to several positions within the right heart. These catheters can be used, as with the EP Study, to record from and stimulate the heart. These catheters can be manipulated throughout the heart in an attempt to identify the precise location from which an arrhythmia originates. Since most arrhythmias require a specific and usually small area of the heart in order to begin or continue, localization of these key, but ablating these vulnerable sites, could lead to elimination of the arrhythmia.

If these sites are identified, a catheter is moved to this area of the heart. The tip of a specially designed catheter placed in this position can be used to deliver high frequency, or radiofrequency, energy. This energy will heat up the adjacent tissue to the point of coagulation. The amount of tissue heated, however, is quite small. But if it includes the critical area for arrhythmia formation, this tissue can be permanently made nonfunctional and thus incapable of causing an arrhythmia.

This procedure lasts somewhat longer than the typical EP Study and also often requires overnight hospital stay.

The anticipated results of the procedure depend somewhat on the nature of the arrhythmia targeted. For the most common arrhythmias, the procedural success rate by experienced operators is in the range of 90-99%. The risks of the procedure are generally small and often only related to intravenous puncture. Serious cardiac complications are uncommon, but can occur.