In 1901, a German scientist developed the electrokardiogram, or EKG. The English translation-- electrocardiogram, or ECG--represents the same measuring device physicians routinely use to record the normal and abnormal electrical activity or "rhythm" of the heart. As the heart beats, each beat is preceded by an electrical impulse that begins in the upper-right side of the heart and is transmitted by special nerve cells into the bottom right and left portion of the heart. The recurrent electrical impulse pattern stimulates the heart's muscle cells to contract, effectively generating the needed force to pump blood throughout the entire body. Galen S. Wagner of Duke University's Department of Internal Medicine says that abnormal rhythms, also called arrhythmias, could indicate enlargement of the heart, insufficient blood flow, electrical blocks, death of heart muscle and even abnormal levels of ions such as potassium or calcium.
Tachycardia and Bradycardia
The heart rate is one of the most basic measurements taken during an EKG. A normal heart beats about 70 times per minute, although a healthy range falls between 60 to 100 beats per minute. Tachycardia and bradycardia refer to when the heart beats outside the "normal" range. According to the American Heart Association, tachycardia ranges from 100 to 200 beats per minute, while bradycardia refers to a heart rate of 60 or fewer beats per minute. In certain situations and for certain populations, both rates can indicate a life-threatening condition such as insufficient blood flow to the body, brain or heart. This can result in dizziness, unconciousness or a heart attack. For some healthy individuals such as athletes, a resting heart rate below 60 is common and heart rates during vigorous exercise may elevate above 200 beats per minute.
Premature Heart Beats
As the electrical impulse that stimulates the heart's muscle cells moves from the top of the heart to the lower portion of the heart, the electrical impulse will reach the four chambers of the heart. Each chamber then responds to the electrical stimulus by contracting sequentially from the top chambers, called the right and left atria, quickly followed by the bottom chambers, called the right and left ventricles. Occasionally, however, a contraction occurs out of the normal sequence. If the abnormal beat occurs in one of the atria, the term premature atrial contraction is used, also known as PAC. If it happens in the right or left ventricle, the result is a premature ventricular contraction, or PVC. Dale Dubin, author of "Rapid Interpretation of EKGs," suggests that irritation of certain nerve cells stimulates premature contractions and may be caused by too much adrenaline or by other chemicals, such as caffeine, amphetamines or cocaine, that mimic the effects of adrenaline. Unless they occur frequently during a short period of time, PACs and PVCs are generally considered to be harmless.
Electrical Blockages
In some cases, the conduction pathway that an electrical impulse normally travels becomes blocked. This can cause the heart to beat slightly out of sequence, which subsequently shows up on the EKG reading. Because blockages occur at different points along the conduction pathway, names for each type of blockage are assigned based on the location where the blockage occurs. The most common types of blockages include atrio-ventricular blocks and right and left bundle branch blocks, each named for its location along the conduction pathway. While all electrical blockages should be adequately noted by a physician, left bundle branch blocks of the left ventricle present the most serious health threat because of the importance of the left ventricle in pumping blood to the rest of the body.
References
- "Marriott's Practical Electocardiography"; Galen S. Wagner, MD, 2001
- American Heart Association: Tachycardia
- American Heart Association: Bradycardia
- "Rapid Interpretation of EKG's"; Dale Dubin, MD, 2000
