What Is an Electrocardiogram (ECG, EKG)?
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.
Basic Anatomy of the Heart
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 organs 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.
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.
Heart Function and the ECG
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).
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 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 heart beats 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.
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.
What Happens During an ECG?
The ECG is a relatively simple test to perform. It is non-invasive and does not hurt. Patches are placed on the skin to detect electrical impulses that the heart generates. These impulses are recorded by an ECG machine. Four patches are placed on the limbs. One is placed on each shoulder or upper arm and one on each leg. These are called the limb leads. There are six patches that are placed on the chest wall beginning just to the right of the breast bone. Patches are placed in the shape of a semi-circle ending near the left axilla (underarm). These are called the chest leads. These patches are connected to an ECG machine that records the tracings and prints them onto paper.
Newer machines also have video screens that help the technician, nurse, or doctor decide whether the quality of the tracing is adequate or whether the test should be repeated. ECG machines are also equipped with computer programs that can help interpret the ECG, although they are not completely accurate.
In certain situations, the physician may want to look at the heart from different angles after the initial ECG is done. The chest leads may then be placed across the right chest wall or on the back.
The skin should be clean and dry to prevent electrical interference to get an acceptable tracing for interpretation. Sometimes that means shaving chest hair or aggressively toweling off the skin. Shivering or tremors can interfere with the tracing and cause interference that affects the quality of the ECG tracing. Usually, the patient has to hold still for 5-10 seconds without moving to get an accurate ECG.
Reasons to Have an ECG
The ECG is used to assess heart function. Patients who complain of chest pain or shortness of breath will often have an ECG as one of the first tests to help determine if there is an acute myocardial infarction or heart attack present. Even if there is no heart attack, the ECG can help decide whether the pain is due to angina or narrowing of blood vessels to the heart muscle (atherosclerosis). It is important to realize that an initial ECG may be normal even if there is heart disease present. Serial EKGs may be needed over time to find an abnormality.
ECGs are often performed when a patient complains of lightheadedness, palpitations, or syncope (passing out) since abnormal heart rate and rhythms may affect the heart's ability to pump blood and provide the body with oxygen.
Interpreting an ECG requires a fair amount of education and experience. Numerous textbooks are devoted to ECG interpretation. The ECG is just one test to assess the heart. History and physical examination remain the cornerstones for diagnosing heart disease. The doctor-patient discussion may uncover the potential for heart problems even if the ECG is normal.
Most often, the ECG assessment includes the following:
- determination of the rate,
- assessment of the rhythm,
- evaluation of the electrical conduction patterns. Heart muscle that is irritated conducts electricity differently than heart muscle that is normal. Abnormal conduction may be apparent during ventricular contraction and during ventricular recovery.
The ECG records the heart tracing in12 leads: Six limb leads (I, II, III, AVR, AVL, AVF) and six chest leads (V1-V6).
The P wave looks at the atria. The QRS complex looks at the ventricles and the T wave evaluates the recovery stage of the ventricles while they are refilling with blood.
The time it takes for electricity to travel from the SA node to the AV node is measured by the PR interval. The QRS interval measures electrical travel time through the ventricles and the QT interval measures how long it takes for the ventricles to recover and prepare to beat again.
Basic P-QRS-T wave sequence: Strip shows a simple sequence where M equals 1.0 millivolts.
The computers imbedded in most ECG machines are able to measure the time it takes for the electrical impulse to travel from the SA node to the ventricles. These measurements can help the doctor assess heart rate and some types of heart block.
Computer programs may also try to interpret the ECG. And as artificial intelligence and programming improves, they are often correct. However, there are enough subtleties in interpretation that the human element is still a very important part of the assessment. The ECG machine is not always correct.
The decision to act upon the results of an ECG depends not only upon the ECG tracing, but also upon the clinical situation. A normal ECG does not exclude heart disease and an abnormal ECG may be the "normal" baseline for that patient.
Other ECG pictures:
Rhythm strip of a person who was cardioverted out of ventricular tachycardia by an electric shock.