The Heartbeat
Each heartbeat starts with an electrical impulse from the sinoatrial node, a small group of cells in your right atrium. The impulse is carried by the Purkinje fibers, located in the inner ventricular walls of the heart. The electrical signal causes your right and left atria to contract and fills the relaxed ventricles with blood. The electrical impulse in a normal, healthy heart follows a precise pathway across the heart. From the sinoatrial node, it travels to the atrioventricular node at the center of your heart and from there to your ventricles, causing them to contract and discharge blood throughout your body. The cells then recharge (repolarize) in preparation for the next heartbeat.
Electrocardiograms (EKGs)
An electrocardiogram, abbreviated as EKG or ECG, is a simple test that detects and records the electrical activity of your heart. EKGs are used to detect and locate the source of heart problems.
Each phase of a single heartbeat is the result of electrical activity that generates a distinctive electrical pattern on the EKG:
- Atrial contractions (both right and left) show up as the P wave.
- Ventricular contractions (both right and left) show as a series of 3 waves, Q-R-S, known as the QRS complex.
- The third and last wave is the T wave. This reflects the electrical activity produced when the ventricles are repolarizing.
EKGs can help reveal a number of heart problems, including:
- Heart attack
- Lack of blood flow
- A heart that does not pump forcefully enough
- A heart that is beating irregularly, too fast, or too slow
- Diseased heart valves
- Enlarged heart
- Birth defects of the heart
Heart Arrhythmias
Heart arrhythmias occur when the electrical impulses that coordinate the heartbeat don't function properly. For example, a scar from a heart attack may cause the electrical impulse to short circuit around it and veer from the normal electrical pathway. Arrhythmias may cause the heart to beat too quickly, too slowly, or irregularly.
Most arrhythmias are harmless and happen fairly frequently. You've probably experienced occasional irregular heartbeats when you felt your heart skip a beat, flutter, or race. But sometimes these irregular heartbeats can be problematic, even life threatening.
Types of Arrhythmia
Arrhythmias are classified by the speed of the heart rate they create and by where they originate.
- Tachycardia is a rapid heart rate (more than 100 beats per minute)
- Bradycardia is a slow heart rate (fewer than 60 beats per minute)
Tachycardias
Tachycardias in the atria:
- Atrial fibrillation- Fibrillation is the uncontrolled twitching of the heart muscle fibers. In atrial fibrillation, the heart beats so fast (up to 350-600 beats per minute) that it essentially quivers, rather than producing a single contraction. Atrial fibrillation can last from a few minutes to an hour or more. It is more common in older people and is seldom life threatening, but if it becomes chronic it can cause more serious problems, like stroke.
- Atrial flutter- Similar to atrial fibrillation, but the beats are more organized and the condition may respond to some forms of treatment.
- Supraventricular tachycardia (SVT)- This term refers to many forms of tachycardia that originate above the ventricles. SVTs cause a burst of rapid heartbeats (160-200 beats per minute) that can last from seconds to hours. They usually occur in young people and are not life threatening.
Tachycardias in the ventricles:
- Ventricular fibrillation (VF)- About 300,000 Americans die every year from sudden cardiac death, believed to be the result of VF. In it, the normal electrical impulse becomes chaotic and the heart quivers instead of pumping blood. Blood supply to vital organs, including the brain, is cut off, and often consciousness is lost within seconds. Chances of survival are improved if cardiopulmonary resuscitation (CPR) is administered until the heart can be shocked back into a normal rhythm with a defibrillator. Without medical intervention, death occurs in minutes. VF is usually connected with heart disease and is often triggered by a heart attack.
- Ventricular tachycardia (VT)- Fast, regular beating of the heart. VT is often caused by short circuits around scarred areas of the heart due to a previous heart attack or a damaged ventricle. Short episodes are usually harmless, but sustained VT is a medical emergency.
Bradycardias
A resting heart rate of below 60 beats per minute doesn't always signal a problem, if you're physically fit and have an efficient heart capable of pumping an adequate supply of blood to your body at that rate. But if you have a slow heart rate and your body isn't getting enough blood, you may have a bradycardia, such as:
- Sick sinus syndrome- This can be caused by an improperly functioning sinoatrial node, or an impulse block near the sinoatrial node.
- Conduction block- A block in or near the AV node or along the impulse pathways to your ventricles. This may be an early sign of heart problems.
Premature Heartbeats
A premature heartbeat is actually an extra heartbeat between two normal beats. They are often caused by stimulants such as caffeine, cold remedies, and some asthma medications. They are usually not of serious concern, but they can trigger a longer-lasting arrhythmia, especially in people with heart disease.