What are arrhythmias? |

Causes and Symptoms

The heart

is a unique muscle with characteristics of each of the other two types of muscle, skeletal and smooth. Like skeletal muscle
cells, myocardial cells are striated and contract following the all-or-none principle, but individual cells can contract independently. Like smooth muscle, the heart is controlled by the autonomic nervous system but able to contract independently because it has its own unique conduction system, composed of the sinoatrial (S-A) node; the interatrial and internodal conduction tracts; the atrioventricular (A-V) junction, including the A-V node and bundle of His; the right and left bundle branches; and the Purkinje fibers.

The heart delivers blood to all tissues needing nutrients and oxygen. The ability to contract effectively twenty-four hours a day, seven days a week for years requires a system with good internal control and many backup systems. Blood flow through the heart needs to move progressively through the right atrium and ventricle to the lungs in order to oxygenate the blood and through the left atrium and ventricle and out to the body to deliver the nutrients and oxygen. Control of heart rate and rhythm by the autonomic nervous system and conduction tissues creates effective pumping.

When something goes awry with the electrical impulses controlling this pumping, the condition is called an "arrhythmia" and is defined by where the problem originates. Arrhythmias range from benign to life threatening, depending on which portion of the heart is involved. The description of arrhythmias follows the conduction system, from S-A node or sinus arrhythmias to independent ventricular myocardial cells producing ectopic (extra) beats, called "premature ventricular contractions (PVCs)." These may be occasional single irregular beats of little consequence or a series of irregular beats that compromise regular blood flow. The determination of what type of arrhythmia is present is made by interpreting the results of one or more tests, which may include an electrocardiogram (ECG or EKG), exercise stress test, or electrophysiological test.

Two types of arrhythmias may occur: impulse propagation, in which an area of the heart or conduction tissue blocks the passage of an impulse, or impulse initiation, in which an area of the heart or conduction tissue becomes excited and creates one or more extra beats. They may occur because of something acute and reversible, such as inadequate oxygenation (myocardial infarction, or heart attack), or a chronic condition, possibly from damage to the myocardial tissue or the conduction system.

Treatment and Therapy

Depending on the diagnosis, treatments range from nothing to hospitalization. An example of a benign arrhythmia is sinus bradycardia, a normal rhythm but a very slow rate, less than sixty beats per minute. Athletes often have well-conditioned cardiovascular systems that pump more blood each beat than do those of nonathletes; thus, their heart rates are slower when the same amount of blood needs to be delivered.

The other extreme, ventricular fibrillation, is characterized by ventricular cells contracting wildly. Blood flow is totally disrupted, and if the arrhythmia is not controlled by defibrillation (the application of electricity), then sudden death will occur. In between these extremes would be someone who reacts to caffeine. Premature atrial contractions (PACs), isolated early beats coming from irritated cells in the atrium, may occur when a person is under stress or has taken stimulants. The obvious treatment is to remove the stress or the stimulant in order to return the heart to normal.

The treatment of arrhythmia may depend on whether it has an impact on blood flow. It is important to keep blood moving smoothly and progressively through the heart to maintain blood flow and oxygenation of body tissues. Additionally, when blood flow is turbulent, the cells suspended in the blood bump into one another, and some are damaged. When this happens, blood clots can form. If an abnormal number of clots occur, a greater chance exists that they could also block a blood vessel in the brain, heart, or lungs, which can be life threatening.

The decision to treat the arrhythmia or to try to prevent complications, such as blood clots, is a question for patients and physicians. Medications can be used to reduce clotting, but this requires long-term treatment with possible side effects. Surgical procedures—such as radiofrequency ablation, which destroys an abnormal electrical pathway causing an arrhythmia, or the Maze procedure, which forms new pathways using scar tissue—may result in permanent cure, avoiding the need for lifelong medication. Knowing the risks and benefits of each treatment helps the patient decide.

If damaged myocardium is the cause of the arrhythmia, then the physician will determine the extent of the damage and offer treatment options, ranging from medications to surgery. The medication may interrupt or keep the arrhythmia from occurring, or it may help the weakened muscle pump blood, which will increase the flow of oxygen and help reduce the chance for further arrhythmias.

If the conduction system is not functioning, then a pacemaker may be inserted and attached to the heart to create a regular rhythm. If the problem is ventricular fibrillation that cannot be controlled with medication, an internal defibrillator may be placed to shock the heart back to a normal rhythm. At the extreme, a heart transplant has the ability to correct an arrhythmia, but this is not usually the reason that a transplant occurs. A transplanted heart has its internal conduction system intact and so should be able to beat on its own.

Perspective and Prospects

The longer one lives, the greater the chance that the heart muscle or conduction system will be damaged. Therefore, it is essential that pharmaceutical companies continue to search for effective drugs and that surgeons continue to refine the techniques required to best treat arrhythmias. Pacemakers have advanced dramatically since the early 1980s, from bulky instruments with nonrechargeable batteries to sophisticated, miniature combination machines that can adjust the heart rate depending on need and, in the extreme, defibrillate with a shock to convert a problem arrhythmia. A concerted effort by emergency medical personnel and technicians has created automatic external defibrillators (AEDs) that anyone can use to allow those people suffering from fibrillation to be saved. Such devices are necessary because cardiopulmonary resuscitation (CPR) is effective in only about 20 percent of cases, often because fibrillation persists during CPR and continues to interrupt blood flow.

Bibliography

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