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 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:
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.
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.
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:
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, a radiologist 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.
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.
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.
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:
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.
Usually, VFib has a very short time frame of profound signs and symptoms, for example:
AFib can produce a number of symptoms that can occur over prolonged periods, for example:
