Ep Defined | Getting Started | Working in the EP Lab
Right Atrium | Right Ventricle | Left Atrium | Left Ventricule | Cardiac Conduction | Cardiac Cell Properties | Action Potential | Sympathetic or Not | Med Page
Electrograms Defined | Recording Modes | Electrode Spacing | Filters | EGM Interpretation | Arrhythmia Analysis
The Physical Lab | Tools of the Trade
Setting Up | Catheter Placement | Baseline Measurement | SNRT | Conduction Study | Arrhythmia Induction | Pacing Protocols | Ablation | Tilt Table | Secrets to Success
Bradycardia | Atrial Tach | Atrial Flutter | Atrial Fibrillation | AVNRT | AVRT | Ventricular Tachycardia
Surface ECG's | Intracardiac Questions | Med Challenge | Advanced

EP Fundamentals - Arrhythmia Mechanisms

Why too fast or too slow...

          Once you are reasonably comfortable with the procedural aspects of working in the EP lab, you can begin to explore the mechanisms of arrhythmias. This is, after all, the reason we are here. In order to advance your understanding of electrophysiology testing, it is necessary to learn a little about the problems that can occur with the electrical conduction system of the heart. This section contains a simple look at some of the rhythm abnormalities that you may encounter in the lab.

          We can classify the abnormal rhythms into two groups, bradycardias and tachycardias. Bradycardias occur when the rhythm is slower than the normal defined limits. When a bradycardia is caused by non-physiologic reasons, it is termed a bradyarrhythmia. Tachycardia indicates a rhythm that is faster than the defined normal. Tachy arrhythmias refer to tachycardias caused by non-physiologic reasons.

Physiologic vs Nonphysiologic

          Before we go any further, let us look at the difference between physiologic and non-physiologic causes. Physiologic sources of tachycardias or bradycardias are those that are in response to normal physiological mechanisms within the body. An examples of a rhythm that falls into this category is a bradycardia that is caused by advanced physical conditioning. Many athletes have a strong cardiac muscle that pumps a significantly higher volume of blood with each heart beat than the average persons heart does. Because the stroke volume is elevated, the number of heart beats in each minute the athlete needs to maintain an adequate cardiac output at rest is lower than what is usually considered normal. On a similar note, tachycardia caused by a sudden scare is also considered to be a normal physiologic response. The increase in heart rate is due to the quick release of epinepherine into the blood stream.

          However, if the rhythm is outside normal limits due to an abnormality of the conduction system, this is considered to be non-physiologic. An example of this would be a patient who experiences heart block due to ischemia or infarct after an inferior MI. The tissue is damaged and the normal signal is not conducted. A tachycardia that is caused by an irritable focus after an MI would also be considered to be non-physiologic. If the damaged tissue were not present, the rhythm would stay within normal limits.

The Underlying Cause

          The field of diagnostic cardiac electrophysiology came about as scientists and physicians sought to investigate the underlying causes of abnormal heart rhythms. Effictive treatment of a problem is tied to an understanding of what causes it. If the exact mechanism of each arrhythmia could be determined, then a successful therapy could be applied. In order for us to fully comprehend what is done in the EP Lab, we too must understand these arrhythmia mechanisms.

          As mentioned above, there are two primary classifications of abnormal rhythms, bradycardia and tachycardia. Each of these groups can be broken down into a limited number of causes. The next few pages discuss the different mechanisms of tachycardic arrhythmias and the bradycardic dysrhythmias.

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