ARRHYTHMIA'S

The usual rhythm of heartbeats, established by the SA node, is called normal sinus rhythm. The term arrhythmia (a-RITH-me￣-a) or dysrhythmia refers to an abnormal rhythm as a result of a defect in the conduction system of the heart. The heart may beat irregularly, too quickly, or too slowly. Symptoms include chest pain, shortness of breath, lightheadedness, dizziness, and fainting. Arrhythmias may be caused by factors that stimulate the heart such as stress, caffeine, alcohol, nicotine, cocaine, and certain drugs that contain caffeine or other stimulants. Arrhythmias may also be caused by a congenital defect, coronary artery disease, myocardial infarction, hypertension, defective heart valves, rheumatic heart disease, hyperthyroidism, and potassium deficiency.

Arrhythmias are categorized by their speed, rhythm, and origination of the problem. Bradycardia (bra¯d-e-KAR-de¯-a; brady- slow) refers to a slow heart rate (below 50 beats per minute); tachycardia (tak-i-KAR-de¯-a; tachy- quick) refers to a rapid heart rate (over 100 beats per minute); and fibrillation (fi-bri-LA ¯ -shun) refers to rapid, uncoordinated heartbeats. Arrhythmias that begin in the atria are called supraventricular or atrial arrhythmias; those that originate in the ventricles are called ventricular arrhythmias.

Ø Supraventricular tachycardia (SVT) is a rapid but regular heart rate (160–200 beats) per minute that originates in the atria. The episodes begin and end suddenly and may last from a few minutes to many hours. SVTs can sometimes be stopped by maneuvers that stimulate the vagus (X) nerve and decrease heart rate.

Ø Atrial flutter consists of rapid, regular atrial contractions (240–360 beats/min) accompanied by an atrioventricular (AV) block in which some of the nerve impulses from the SA node are not conducted through the AV node.

Ø Atrial fibrillation (AF) is a common arrhythmia, affecting mostly older adults, in which contraction of the atrial fibers is asynchronous (not in unison) so that atrial pumping ceases altogether. The atria may beat 300–600 beats/min. The ventricles may also speed up, resulting in a rapid heartbeat (up to 160 beats/min).

Ø Heart block is an arrhythmia that occurs when the electrical pathways between the atria and ventricles are blocked, slowing the transmission of nerve impulses. The most common site of blockage is the atrioventricular node, a condition called atrioventricular (AV) block.

Ø Atrial premature contraction (APC) is a heartbeat that occurs earlier than expected and briefly interrupts the normal heart rhythm. It often causes a sensation of a skipped heartbeat followed by a more forceful heartbeat. APCs originate in the atrial myocardium and are common in healthy individuals.

Ø Ventricular premature contraction. Another form of arrhythmia arises when an ectopic focus (ek-TOP-ik), a region of the heart other than the conduction system, becomes more excitable than normal and causes an occasional abnormal action potential to occur.

Ø Ventricular tachycardia (VT or V-tach) is an arrhythmia that originates in the ventricles and is characterized by 4 or more ventricular premature contractions. It causes the ventricles to beat too fast (at least 120 beats/min).

Ø Ventricular fibrillation (VF or V-fib) is the most deadly arrhythmia, in which contractions of the ventricular fibers are completely asynchronous so that the ventricles quiver rather than contract in a coordinated way. As a result, ventricular pumping stops, blood ejection ceases, and circulatory failure and death occur unless there is immediate medical intervention. During ventricular fibrillation, the ECG has no clearly defined P waves, QRS complexes, or T waves.