Each individual heartbeat is actually a collection of several muscle movements spurred into action by electrical impulses. The first electrical signal comes from the heart's own natural pacemaker, the sinoatrial node, comprised of electrically active cells and located in the upper right heart chamber. This node sends a steady stream of electrical signals along a pathway through the heart's upper chambers. The signals then travel to the electrical bridge-the atrioventricular node- between the upper and lower chambers and, finally, move to the lower chambers. A problem at any point in the electrical pathway can wreak havoc with the regular beating of a heart. Luckily, an artificial pacemaker - a small, battery-operated device- can take over the role of the heart's own electrical system, if necessary. What is a pacemaker? Although it weighs just about an ounce, a pacemaker contains a powerful battery, electronic circuits and computer memory that together generate electronic signals. The signals, or pacing pulses, are carried along thin, insulated wires, or leads, to the heart muscle. The signals cause the heart muscle to begin the contractions that cause a heartbeat. A pacemaker is implanted just below the collarbone in a procedure that takes about two hours. It is programmed to stimulate the heart at a pre-determined rate, and settings can be adjusted at any time. Routine evaluation, sometimes even via telephone, ensures the pacemaker is working properly and monitors battery life, which generally runs from five to ten years. When is a pacemaker needed? The most common reason for a pacemaker is a heartbeat that slows to an unhealthy rate, or bradycardia. A pacemaker resets the heart rate to an appropriate pace; ensuring adequate blood and oxygen are delivered to the brain and other parts of the body.
Take Note
- Magnetic Resonance Imaging (MRI) test employ powerful magnets to create images. If you are scheduled for an MRI test, you should make certain your doctor knows you have a pacemaker. - Electronic security systems, such as those common in airports, can pose problems if exposure is prolonged. Passing through such systems, even several times, does not pose a threat. However, patients with pacemakers should avoid the "wand" used in some security checks. - Microwave ovens, cellular phones and electric blankets do not, as myth might have it, affect pacemaker functioning. Cardiac Resynchronization Therapy More than 22 million people worldwide suffer from congestive heart failure (CHF), a potentially debilitating disease. Until recently, lifestyle changes, medication and, sometimes, heart surgery were the only treatment options. Patients with severe symptoms, however, received little, if any, relief from such approaches. To make matters worse, up to 40 percent of patients with CHF also have an arrhythmia that further reduces the heart's ability to beat properly. Cardiac resynchronization therapy (CRT) is an innovative new therapy that can relieve CHF symptoms by improving the coordination of the heart's contractions. CRT builds on the technology used in pacemakers and implantable cardioverter devices. CRT devices also can protect the patient from slow and fast heart rhythms. Overview of a heart beat The heart is comprised of four chambers: two upper atria, and two lower ventricles. An electrical system controls the synchronized pumping action of these chambers. The normal heartbeat originates in a section of the right atrium known as the sinoatrial, or SA node. The electrical signal from the sinoatrial node spreads through both atria causing them to contract and squeeze blood into the ventricles. The electrical signal then passes through an electrical bridge known as the atrioventricular or AV node. After a split second delay, the signal continues to the ventricles by way of a specialized network known as the left and right bundle branches. The bundle branches separate to the left and right ventricles, which enables the electrical signal to stimulate both ventricles simultaneously. This coordinated contraction, or squeezing, of the ventricles is necessary for optimal pumping of blood to the body and lungs.
Uncoordinated contractions When there is a delay in electrical signal transmission through the left bundle branch, this causes left bundle branch block (LBBB). Because the electrical signal to the left ventricle is delayed, the right ventricle begins to contract a fraction of a second before the left ventricle, instead of simultaneously. The result is an asynchronous or uncoordinated contraction of the ventricles and a mis-timing in the contraction pattern of the left atrium and ventricle. Other conduction abnormalities, such as right bundle branch block (RBBB), also may contribute to less efficient contraction of the heart. This further reduces the pumping ability of the already weakened heart muscle. Cardiac Resynchronization Therapy The concept behind CRT is quite simple. Resynchronization restores the normal coordinated pumping action of the ventricles by overcoming the delay in electrical conduction caused by bundle branch block. This is accomplished by means of a special type of cardiac device. These powerful, "built-in" devices have enormous potential to improve the quality of life and probably survival for patients with heart failure. The CRT device Pacemakers are typically used to prevent symptoms due to an excessively slow heartbeat. The pacemaker continuously monitors the heartbeat and, when necessary, delivers tiny, imperceptible electrical signals to stimulate the heartbeat. Most pacemakers have two electrode wires, or leads, one in the right atrium and one in the right ventricle. This ensures the pacemaker will maintain the normal coordinated pumping relationship between the upper and lower chambers of the heart. The wires that carry the electrical signals connect to an electrical pulse generator placed under the skin in the upper chest. In addition to the two leads (right atrium and right ventricle) used by a common pacemaker, the CRT device has a third lead that is positioned in a vein on the surface of the left ventricle. This allows the CRT device to simultaneously stimulate the left and right ventricles and restore a coordinated, or "synchronous," squeezing pattern. This is sometimes referred to as "bi-ventricular pacing" because both ventricles are electrically stimulated (paced) at the same time. This reduces the electrical delay and results in a more coordinated and effective heart beat.
CRT results The response to CRT can vary greatly among patients. Clinical studies involving more than 2000 patients worldwide demonstrate modest improvements in exercise tolerance, CHF severity, and quality of life in most patients. Improvement may happen quickly, but sometimes it can take several months.
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