ECG
What Is an Electrocardiogram (ECG, EKG)?
Picture of the basic anatomy of the heart
The electrocardiogram (ECG or EKG) is a diagnostic tool that is routinely used to assess the electrical and muscular functions of the heart. While it is a relatively simple test to perform, the interpretation of the ECG tracing requires significant amounts of training. Numerous textbooks are devoted to the subject.
The heart is a two stage electrical pump and the heart’s electrical activity can be measured by electrodes placed on the skin. The electrocardiogram can measure the rate and rhythm of the heartbeat, as well as provide indirect evidence of blood flow to the heart muscle.
A standardized system has been developed for the electrode placement for a routine ECG. Ten electrodes are needed to produce 12 electrical views of the heart. An electrode lead, or patch, is placed on each arm and leg and six are placed across the chest wall. The signals received from each electrode are recorded. The printed view of these recordings is the electrocardiogram.
By comparison, a heart monitor requires only three electrode leads – one each on the right arm, left arm, and left chest. It only measures the rate and rhythm of the heartbeat. This kind of monitoring does not constitute a complete ECG.
Heart Function, ECG, and ECG Wave Strips
Electrode leads on the chest wall are able to detect electrical impulses that are generated by the heart. Multiple leads provide many electrical views of the heart. By interpreting the tracing, the physician can learn about the heart rate and rhythm as well as blood flow to the ventricles (indirectly).
The rate refers to how fast the heart beats. Normally, the SA node generates an electrical impulse 50-100 times per minute. Bradycardia (brady=slow+cardia=heart) describes a heart rate of less than 50 beats per minute. Tachycardia (tachy=fast+cardia=heart) describes a heart rate faster than 100 beats per minute. Rhythm refers to the type of heartbeat. Normally, the heartbeats in a sinus rhythm with each electrical impulse generated by the SA node resulting in a ventricular contraction, or heartbeat. There are a variety of abnormal electrical rhythms, some are normal variants and some are potentially dangerous. Some electrical rhythms do not generate a heartbeat and are the cause of sudden death. Rhythm strip showing a normal 12-lead ECG. Click to view a larger image.
Examples of heart rhythms include:
Normal sinus rhythm
Sinus tachycardia
Sinus bradycardia
Atrial fibrillation
Atrial flutter
Ventricular tachycardia
Ventricular fibrillation
There can also be delays in transmission of the electrical impulse anywhere in the system, including the SA node, the atria, the AV node, or in the ventricles. Some aberrant impulses cause normal variants of the heart rhythm and others can be potentially life-threatening. Some examples include:
1st degree AV block
2nd degree AV block, type I (Wenckebach)
2nd degree AV block, type II
3rd degree AV block or complete heart block
Right bundle branch block
Left bundle branch block
There can also be short circuits that can lead to abnormal electrical pathways in the heart causing abnormalities of rate and rhythm. Wolfe-Parkinson-White (WPW) syndrome is a condition where an abnormal accessory pathway at the AV node can cause tachycardia.
The ECG tracing can also provide information about whether the heart muscle cells are conducting electricity appropriately. By analyzing the shape of the electrical waves, the physician may be able to determine if there is decreased blood flow to parts of the heart muscle. The presence of an acute blockage associated with a myocardial infarction or heart attack can be determined as well. That’s one of the reasons that an ECG is done as soon as possible when a patient presents with chest pain.
Anatomy of the Heart
The heart has four chambers — the right and left atrium and the right and left ventricle.The right side of the heart collects blood from the body and pumps it to the lungs while the left side of the heart receives blood from the lungs and pumps it to the body.
Blood flows through the body in the following way:
Oxygen-rich blood from the lungs enters the left atrium through the pulmonary veins.
Blood then flows into the left ventricle where it is pumped into the aorta and is distributed to the rest of the body. This blood supplies organ and cells with oxygen and nutrients necessary for metabolism.
Blood that returns to the heart is depleted of oxygen and carries carbon dioxide, the waste product of metabolism. The blood enters the right atrium though the vena cava, where it is collected and pumped to the right ventricle.
The right ventricle then pumps blood through the pulmonary artery to the lungs where carbon dioxide is stripped off, oxygen is replaced, and the cycle begins again.
Picture of heart anatomy blood flow
Like any muscle, the heart requires oxygen and nutrients to function. Oxygen and nutrients are supplied by arteries that originate from the aorta. These vessels branch out to supply all the regions of the heart with oxygen rich blood.
Electrically, the heart can be divided into upper and lower chambers. An electrical impulse is generated in the upper chambers of the heart that causes the atria to squeeze and push blood into the ventricles. There is a short delay to allow the ventricles to fill. The ventricles then contract to pump blood to the body and the lungs.
Conducting system of the heart: SA means sinoatrial node. AV means atrioventricular node. RB and LB mean right and left bundle, respectively, and are the nerves that spread the electric impulse from the AV node into the ventricles.
The heart has its own automatic pacemaker called the sinaoatrial, or SA node, located in the right atrium. The SA node acts independently of the brain to generate electricity for the heart to beat.
Normally, the impulse generated by the SA node runs through the heart’s electrical grid and signals the muscle cells in the atria to beat simultaneously, allowing for a coordinated squeeze of the heart. Contraction of the atria pushes blood into the ventricles.
The electrical signal that was generated in the SA node travels to a junction box between the atria and ventricles (the AV node) where it is delayed for a few milliseconds to allow the ventricles to fill.
The electrical signal then travels through the ventricles, stimulating those heart muscle cells to contract. Ventricular contraction pumps blood to the body (from the left ventricle) and the lungs (from the right ventricle).
There is a short pause to allow blood to return to the heart and fill before the electrical cycle repeats itself for the next heartbeat.
