Heart and blood vessel disorders

Abnormal Heart Rhythms

The heart is a muscular organ with four cham­bers designed to work efficiently, reliably, and continuously over a lifetime. The muscular walls of each chamber contract in a precise sequence, pushing along the most blood while expending the least possible energy during every heartbeat.

The contraction of the muscle fibers in the heart is controlled by an electrical discharge that flows through the heart in a precise manner along distinct pathways and at a controlled speed. The rhythmic discharge that begins each heartbeat originates in the heart's pacemaker (sinoatrial node), which lies in the wall of the right atrium. The rate of discharge is influenced by nerve im­pulses and by levels of hormones circulating through the bloodstream.

The part of the nervous system that regulates the heart rate automatically is the autonomic nervous system, which consists of the sympa­thetic and parasympathetic nervous systems. The sympathetic system speeds up the heart rate; the parasympathetic system slows it down. The sympathetic system supplies the heart with a net­work of nerves, the sympathetic plexus. The para­sympathetic system supplies the heart through a single nerve, the vagus nerve.

Heart rate is also influenced by the sympathetic system's circulating hormones—epinephrine (adrenaline) and norepinephrine (noradrenaline)—which speed up the heart rate. Thyroid hormone influences heart rate as well. With too much thyroid hormone, the heart beats too fast; with too little, it beats too slowly.

The normal heart rate at rest is usually between 60 and 100 beats per minute. However, much lower rates may be normal in young adults, par­ticularly those who are physically fit. Variations in heart rate are normal. The heart rate responds not only to exercise and inactivity but also to stim­uli such as pain and anger. Only when the heart rate is inappropriately fast (tachycardia) or slow (bradycardia) or when the electrical impulses travel in abnormal pathways is the heartbeat con­sidered to have an abnormal rhythm (arrhyth­mia). Abnormal rhythms may be regular or irreg­ular.

 

Notes:

precise sequence чёткая последовательность

precise manner определенный порядок

 

Text A. Atrial Fibrillation and Flutter

Atrial fibrillation and atrial flutter are very fast elec­trical discharge patterns that make the atria contract extremely rapidly, thus causing the ventricles to con­tract faster and less efficiently than normal.

These abnormal rhythms may occur sporadi­cally or may persist. During fibrillation or flutter, the contractions of the atria are so fast that the atrial walls simply quiver, so blood isn't pumped effectively to the ventricles. In fibrillation, the atrial rhythm is irregular, so the ventricular rhythm is also irregular; in flutter, the atrial and ventricular rhythms usually are regular. In both cases, the ventricles beat more slowly than the atria because the atrioventricular node and the bundle of His can't conduct electrical impulses at such a fast rate, and only every second to fourth impulse gets through. Still the ventricles beat too fast to fill completely. Therefore, inadequate amounts of blood are pumped out of the heart, blood pressure falls, and heart failure may occur.

The heart may go into atrial fibrillation or flut­ter with no other sign of heart disease, but more often the cause is an underlying problem, such as rheumatic heart disease, coronary artery disease, high blood pressure, alcohol abuse, or too much thyroid hormone (hyperthyroidism).

Symptoms and Diagnosis

Symptoms of atrial fibrillation or flutter depend largely on how fast the ventricles beat. A modest ventricular rate—less than about 120 beats per minute—may produce no symptoms. Higher rates cause unpleasant palpitations or chest dis­comfort. With atrial fibrillation, the person may be aware of the rhythm irregularities.

The diminished pumping ability of the heart may make the person feel weak, faint, and short of breath. Some people, especially the elderly, de­velop heart failure, chest pain, and shock.

In atrial fibrillation, the atria don't empty com­pletely into the ventricles with each beat. Over time, some blood inside the atria may stagnate and clot. Pieces of the clot may break off, pass into the left ventricle, and continue into the gen­eral circulation, where they may block a smaller artery. (Pieces of a clot that block an artery are called emboli.) Most often, the pieces of a clot break off shortly after atrial fibrillation converts to a normal rhythm, either spontaneously or with treatment. Blockage of an artery in the brain may cause a stroke. Rarely, a stroke is the first sign of atrial fibrillation.

The diagnosis of atrial fibrillation or flutter is suspected from the symptoms and confirmed by an electrocardiogram (ECG). With atrial fibrilla­tion, the pulse is irregular. With atrial flutter, the pulse is more likely to be regular but rapid.

Treatment

Treatments for atrial fibrillation and flutter are designed to control the rate at which the ventri­cles contract, treat the disorder responsible for the abnormal rhythm, and restore the normal rhythm of the heart. With atrial fibrillation, treat­ment is also usually given to prevent clots and emboli.

The first step in treating atrial fibrillation or flutter is usually to slow the ventricular rate to improve the heart's efficiency in pumping blood. Contractions of the ventricles usually can be slowed and strengthened with digoxin, a drug that slows the conduction of impulses to the ventri­cles. When digoxin alone doesn't help, giving a second drug—a beta-blocker such as propranolol or atenolol or a calcium channel blocker such as diltiazem or verapamil—usually does.

Treatment of the underlying disease rarely al­leviates atrial arrhythmias unless the disease is hyperthyroidism.

Though occasionally atrial fibrillation or flutter spontaneously reverts to a normal rhythm, more often it must be converted to normal. Sometimes such a conversion can be achieved with certain antiarrhythmic drugs. However, electric shock (cardioversion) is often the most effective ap­proach. Success by any means is less likely the longer the atria have been in their abnormal rhythm (especially after 6 months or more), the more the atria are enlarged, and the more severe the underlying heart disease has become. When conversion is successful, the risk that the arrhyth­mia will return is high, even if the person takes preventive drugs such as quinidine, procainamide, propafenone, or flecainide.

If all other treatments fail, the atrioventricular node can be destroyed by catheter ablation (de­livery of radiofrequency energy through a cathe­ter inserted in the heart). This procedure inter­rupts conduction from the fibrillating atria to the ventricles, but a permanent artificial pacemaker is required for the ventricles afterward.

The risk of developing blood clots is highest in people with atrial fibrillation who have an en­larged left atrium or who have mitral valve dis­ease. The risk that a clot will be dislodged and cause a stroke is particularly high in people who have intermittent but persistent episodes of atrial fibrillation or whose fibrillation is converted to the normal rhythm. Because anyone with atrial fibrillation is at risk of a stroke, anticoagulant therapy generally is recommended to prevent clots unless there's a specific reason not to give it, such as high blood pressure. However, antico­agulant therapy itself carries a risk of excessive bleeding that can lead to hemorrhagic stroke and other bleeding complications. Therefore, a doc­tor balances the potential benefits and risks for each person.

Notes:

atrial fibrillation фибрилляция предсердий

flutter мерцание, трепетание

faint обморок

stagnate застаиваться

clot свёртываться (о крови)

stroke инсульт, апоплексический удар

alleviate облегчать, смягчать (боль)

dislodge вытолкнуть, вытеснить

eliminate уничтожать