A biventricular pacemaker is a type of implantable pacemaker designed to treat heart failure. This type of therapy becomes necessary when the two lower chambers of the heart, the ventricles, do not pump together. A biventricular pacemaker helps both ventricles to contract more efficiently. This type of pacing is also called cardiac resynchronization therapy (CRT).

Biventricular pacemakers are relatively new. Previous pacemakers were used to pace only the right side of the heart. They were usually used when the heart beat too slowly. By contrast, biventricular pacemakers pace both the right and left sides of the heart. They do not increase heart rate, but rather stimulate the left and right ventricles simultaneously. This helps the left ventricle pump blood more efficiently.

Biventricular pacemakers may be combined with an implantable cardioverter defibrillator (ICD). This is for heart failure patients felt to be at high risk of sudden cardiac death due to some types of abnormal heart rhythms (arrhythmias).

Heart failure is a chronic condition that affects more than 5 million Americans. Heart failure describes a situation in which the heart fails to pump enough blood to meet the body’s needs. It often occurs in patients whose hearts have been weakened or damaged by a heart attack or other conditions. Patients may experience breathlessness, fluid buildup in the limbs and severe fatigue.

When the heart is working properly, both of its lower chambers (ventricles) pump at the same time and in sync with the pumping of the two upper chambers (atria). This pumping is stimulated by an electrical charge that originates in a portion of the right atria called the sinoatrial node, or SA node. The impulse first spreads through the atria, stimulating them to contract. Then it passes through the atrioventricular node, or AV node, and moves into the ventricles through two specialized paths called the left and right bundle branches. Under normal circumstances, the bundle branches stimulate both ventricles to contract at the same time.

Up to 40 percent of heart failure patients, however, have disturbances in the conduction of electrical impulses to the ventricles (e.g., bundle branch block or intraventricular conduction delay). As a result, the left and right ventricles are activated at different times. When this happens, the left ventricle and the right ventricle do not contract simultaneously. In addition, different segments within the left ventricle may contact at different times resulting in dissipation of function and  a reduction of the heart’s efficiency as a pump. The heart typically responds by beating faster and dilating. This results in a vicious cycle of further dilation, constriction of the vessels in the body, salt and water retention, and further worsening of heart failure.

Patients who have heart failure and who may be candidates for biventricular pacing can be diagnosed with an electrocardiogram, which will demonstrate whether they have a bundle branch block or intraventricular conduction delay. These conduction delays do not respond to antiarrhythmics or other drugs.

A biventricular pacemaker can help by sending tiny electrical signals simultaneously to the left ventricle and to the right ventricle. By stimulating both ventricles (biventricular pacing), the device makes the walls of the right and left ventricles pump together again. In addition, activation of the left ventricle simultaneously from different sites results in improving the efficiency of the heart and increases its output. The heart is thus resynchronized, pumping more efficiently while causing less wear and tear on the heart muscle itself. This is why biventricular pacing is also referred to as cardiac resynchronization therapy (CRT).

Like a pacemaker or defibrillator, the biventricular pacemaker consists of three parts:

The generator is a small box, usually about 2 inches wide and 3 ounces in weight. Some generators are even smaller, measuring 1 inch in diameter and weighing about half an ounce. They are battery-powered, and most use lithium batteries that last for five to 10 years. When the battery runs out, the entire generator is replaced. The generator is responsible for generating the electric impulses that pace the heartbeat.

Attached to the generator are leads, or wires, which carry the electrical impulses from the generator.

At the tip of each lead is a tiny device called an electrode that delivers the necessary electrical impulses to the heart. Thus, the electric impulses are created by the generator, carried by the leads and delivered by the electrodes to the heart.

There are three main differences between a biventricular pacemaker and a standard pacemaker:

• The biventricular pacemaker has an additional wire (lead) that paces the left ventricle in addition to the standard pacemaker leads, which pace the right ventricle and right atrium.
  
  
• Biventricular pacemakers are specifically designed to treat heart failure, whereas pacemakers treat cardiac conditions that involve bradycardia (an abnormally slow heart rate).
  
  
• Standard pacemakers have been in use for nearly 40 years. Biventricular pacemakers began receiving FDA approval in 2001. The combined biventricular pacemaker/ICD was FDA approved in 2002.