Pacemaker/Implantable Cardioverter Defibrillator Insertion

Overview

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The Heart's Electrical Conduction System

The heart is, in the simplest terms, a pump made up of muscle tissue. Like all pumps, the heart requires a source of energy in order to function. The heart's pumping energy comes from an indwelling electrical conduction system.

An electrical stimulus is generated by the sinus node (also called the sinoatrial node, or SA node), which is a small mass of specialized tissue located in the right atrium (right upper chamber) of the heart. The sinus node generates an electrical stimulus periodically (60 to 100 times per minute under normal conditions). This electrical stimulus travels down through the conduction pathways (similar to the way electricity flows through power lines from the power plant to your house) and causes the heart's chambers to contract and pump out blood. The right and left atria (the two upper chambers of the heart) are stimulated first and contract a short period of time before the right and left ventricles (the two lower chambers of the heart). The electrical impulse travels from the sinus node to the atrioventricular (AV) node, where it stops for a very short period, then continues down the conduction pathways via the bundle of His into the ventricles. The bundle of His divides into right and left pathways to provide electrical stimulation to both ventricles.

What is an EKG?

This electrical activity of the heart is measured by an electrocardiogram (EKG or ECG). By placing electrodes at specific locations on the body (chest, arms, and legs), a graphic representation, or tracing, of the electrical activity can be obtained. Changes in an EKG from the normal tracing can indicate one or more of several heart-related conditions.

Many conditions can cause changes in the EKG pattern, including, but not limited to the following:

• ischemia - decreased blood flow to the heart muscle due to clogged or partially clogged arteries.
  
  
• heart attack (Also called myocardial infarction, or MI.) - damage to the heart muscle due to insufficient blood supply.
  
  
• conduction disorder - a dysfunction in the heart's electrical conduction system, which can make the heart beat too fast, too slow, or at an uneven rate.
  
  
• electrolyte disturbance - an imbalance in the level of electrolytes, or chemicals, in the blood, such as potassium, magnesium, or calcium.
  
  
• pericarditis - an inflammation or infection of the sac which surrounds the heart.
  
  
• valve disease - malfunction of one or more of the heart valves may cause an obstruction of the blood flow within the heart.
  
  
• enlarged heart - a condition caused by various factors, such as valve disorders, high blood pressure, congestive heart failure, conduction disturbances, etc.

When a problem develops with the heart's rhythm, there may or may not be any symptoms. One of the most common heart dysrhythmias (or arrhythmias) is premature ventricular beats, or PVCs. PVCs are just what they sound like: the ventricles beat sooner than they should. This means that the ventricle is contracting and pumping out blood before the atrium above it has completely pumped its blood volume into the ventricle. Most of the time, PVCs are harmless. In fact, almost everyone has them at one time or another. However, if PVCs occur too frequently (more than several times per minute), the heart is unable to pump an adequate volume of blood to the body, which can cause symptoms such as weakness, fatigue, palpitations, or low blood pressure.

Other types of dysrhythmias can have similar effects. Dysrhythmias occur when the heart beats too fast, too slow, or with an irregular rhythm. But whether the heart is beating too fast, too slow, or too irregularly, the effects are often the same as one or more of those described above. Some additional examples of dysrhythmias include:

• atrial fibrillation - occurs when the atria beat irregularly and too fast.
  
  
• ventricular fibrillation - occurs when the ventricles beat irregularly and too fast.
  
  
• bradycardia - occurs when the heart beats too slow.
  
  
• tachycardia - occurs when the heart beats too fast.
  
  
• heart block - occurs when the electrical signal is delayed after leaving the SA node; there are several types of heart blocks, and each one has a distinctive EKG tracing.

Dysrhythmias are diagnosed by methods such as EKG, Holter monitoring, signal-average EKG, or electrophysiological studies. Dysrhythmias cause problems because the heart is unable to pump an adequate amount of blood to the body. If the heart rate is too slow, the blood is pumped too slowly. If the heart rate is too fast or too irregular, the heart chambers are unable to fill up with enough blood to pump out with each beat. When the body does not receive enough blood, symptoms such as fatigue, dizziness, fainting, and/or chest pain may occur. These symptoms may be treated with medication or procedures such as a cardiac ablation (removal of a location in the heart that is causing a dysrhythmia by freezing or radiofrequency).

Another method of treatment is to insert a pacemaker (to signal the heart to beat when the heartbeat is too slow), or an implantable cardioverter defibrillator (also called an ICD; senses when the heart is beating too fast and delivers an electrical shock to slow the heart down). Newer ICDs can also be programmed to function as pacemakers in addition to the cardioversion and defibrillation functions.

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