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What, exactly, is cardiac electrophysiology?
The first challenge in your trial by fire is to understand what cardiac electrophysiology really is and why these procedures are performed. Cardiac electrophysiology has one primary function, to study the electrical system of the heart. This is done to determine the nature and origin of abnormal cardiac rhythms known as arrhythmias, and to determine what the best possible therapy is for the given patient.
If the function of cardiac electrophysiology is to study the electrical system of the heart, understanding EP requires a basic understanding of how the heart originates and is controlled. While this is a very complex subject that is expanded upon throughout this web site, a brief explanation is provided here as a baseline from which to operate. The cells of the heart are specialized in a number of ways that set them apart from most other cells found throughout the human body. Cardiac cells have four properties that help regulate electrical activity throughout the heart:
• Automaticity: the ability to initiate an electrical signal.
• Conductivity: the ability to pass on an electrical signal to surrounding cells.
• Excitability: the ability to respond to an electrical signal.
• Refractriness: a mechanism that makes it possible for the cells to “ignore” or not respond to electrical signals.
By combining these four properties, the cells of the heart generate and regulate electrical wave fronts that are the foundation of every normal and abnormal cardiac rhythm that occurs. Our ability to understand, observe and adjust these principals and how they are applied in different cells throughout the heart is the basis of cardiac electrophysiology. Evaluation of the electrical signals within the heart is performed using two techniques, recording electrograms and controlling refractory periods using pacing.1
Electrogram recordings are a visual graph of the electrical activity of the heart displayed over time. By examining electrograms, information on the origin, strength, speed and direction of travel of the electrical activity within the heart can be determined.2
To study the electrical system of the heart, catheters are placed inside the heart by advancing them through some of the larger blood vessels. Access to the blood vessels is usually obtained by using a sheath or an introducer that allows the physician to advance and remove the catheters while preventing blood from flowing back out through the sheath. The catheters are placed into strategic locations inside the heart. The electrodes on these catheters can record the electrical signal that causes the heart muscle to contract and pump blood. By evaluating the electrical signals recorded from these catheters, a good deal of information about the function of the heart may be obtained.
The first action that is taken in an EP study is to record the patient’s baseline rhythm. In most cases, this will correlate to the rhythm the patient is normally in. Measurements are taken to help determine how long it takes the electrical signal to pass through different regions of the heart. This information will be compared to data gathered later in the study to determine if and how the function of the electrical signals inside the heart change under different conditions.
Pacing is used to “control” the conduction system of the heart by delivering signals to the heart to stimulate it to beat faster. By causing the heart to beat faster, the behavior of the myocardial tissue changes and differences in function can be recorded. Often times, abnormal electrical activation can be documented using this technique. It is also possible to trigger an abnormal heart rhythm, called an arrhythmia, using pacing. 3
Why do an EP Study?
Indications for an EP Study may include;
Documented Arrhythmia: Many patients that present to the lab do so because documentation of an arrhythmia has been acquired by the physician. Abnormal rhythms can be discovered by chance during routine examinations, or during some other form of cardiac testing such as Stress Tests. The nature of the documented arrhythmia is often well known, or at least well classified.
Suspected arrhythmia or dysrythmia with supporting evidence: Patients who have experienced a clinical event where the cause, while unknown, suggests the possibility of abnormal cardiac rhythm involvement are often recommended for an electrophysiology study. Episodes of syncope or near syncope may place a person into this category, especially if there are indications such as a bundle branch block, premature extrasystoles or palpitations.
The goals of the diagnostic EP study are;
Evaluate the physiologic properties of the different components that comprise the electrical system of the heart including assessment of the sinus node, the AV node and the His-Purkinjie network.
Determine the mechanism(s) of arrhythmia by;
a. Verifying if an accessory pathway is present.
b. Verifying if there is an area of abnormal automaticity that could contribute to a focal tachycardia.
c. Verifying if there is a reentry tachycardia present and, if so, determining the critical zone of slow conduction used by the arrhythmia.
Establish the best therapy for the patient based upon the abnormal rhythm(s) identified
Depending upon the nature of the abnormal rhythm determined by the diagnostic EP study, the patient may receive ablation therapy during the same procedure. In recent years this has become more common, especially with patients who have a known arrhythmia. Even if there is a documented abnormal rhythm, it is always advisable to perform a complete diagnostic study to establish that there are no other arrhythmias present.
Source 1: Richard Fogoros - Electrophysiologic Testing 3rd Edition, Chapter 4 – Prinicpals of the Electrophysiology Study, pg 37
