Each year, 400,000 people in the United Stated die from fast or irregular heartbeat. These are types of altered heart rhythm called arrhythmia. John Thomas was almost one of them--several times.

In a crowded Boston train in December 1990, 36-year-old Thomas lost consciousness for about 8 seconds, during which his heart fibrillated. In this state, small areas of the myocardium contract in an uncoordinated fashion. As a result, the myocardium fails to contract as a whole, and blood is no longer pumped. Atrial fibrillation is not life threatening because the ventricles still pump blood, but ventricular fibrillation, which struck Thomas, is often deadly. Ventricular fibrillation can be caused by an obstructed coronary artery, toxic drug exposure, electric shock, or traumatic injury to the heart or chest wall.

Fortunately, Thomas had a device called a defibrillator surgically implanted a year earlier, after he suffered a cardiac arrest. The device sends out an electrical jolt during fibrillation, which interrupts the abnormal heart rhythm, allowing the heart to resume a normal beat. Thomas' defibrillator saves his life several times a year.

Fibrillation is cardiac chaos. An abnormally fast heartbeat, usually more than one hundred beats per minute, is called tachycardia. Increase in body temperature, nodal stimulation by sympathetic fibers, certain drugs or hormones, heart disease, excitement, exercise, anemia, or shock can all cause tachycardia.

Bradycardia means a slow heart rate, usually fewer than sixty beats per minute. Decreased body temperature, nodal stimulation by parasympathetic impulses, or certain drugs may cause bradycardia. It also may occur during sleep. Athletes sometimes have unusually slow heartbeats because their hearts have developed the ability to pump a greater than normal volume of blood with each beat. The slowest heartbeat recorded in healthy athlete was 25 beats per minute!

A premature beat occurs before it is expected in a normal series of cardiac cycles. Cardiac impulses originating from unusual (ectopic) regions of the heart probably cause a premature beat. That is, the impulses originates from a site other than the S-A node. Cardiac impulses may arise from ischemic tissues or from muscle fibers that disease or drugs irritate.

A heart chamber flutters when it contracts regularly, but very rapidly, such as 250-350 times per minute. Although normal hearts may flutter occasionally, this condition is more likely to be due to damage to the myocardium.

Any interference or block in cardiac impulse conduction may cause arrhythmia, the type varying with the location and extent of the block. Such arrhythmias are related to the fact that certain cardiac tissues other than the S-A node can function as pacemakers.

The S-A node usually initiates seventy to eighty heartbeats per minute, called a sinus rhythm. If the S-A node is damaged, impulses originating in the A-V node may travel upward into the atrial myocardium and downward into the ventricular walls, stimulating them to contract. Under the influence of the A-V node acting as a secondary pacemaker, the heart may continue to pump blood, but at a rate of forty to sixty beats per minute, called a nodal rhythm. Similarly, the Purkinje fibers can initiate cardiac impulses, contracting the heart fifteen to forty times per minute.

An artificial pacemaker can treat a disorder of the cardiac conduction system. This device includes an electrical pulse generator and a lead wire that communicates with a portion of the myocardium. The pulse generator contains a permanent battery that provides energy and a microprocessor that can sense the cardiac rhythm and signal the heart to alter its contraction rate.

An artificial pacemaker is surgically implanted beneath the patient's skin in the shoulder. An external programmer adjusts its functions from the outside. The first pacemakers, make in l958, were crude. Today, thanks to telecommunications advances, a physician can check a patient's pacemaker over the phone! In 1993, a device called a pacemaker-cardioverter-defibrillator became available, which can correct both abnormal heart rhythm and cardiac arrest.

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