An electrocardiogram, or ECG, is a recording of the heart’s electrical activity as a graph over a period of time. Learning Objectives Key Points
An electrocardiogram (ECG or EKG) is a recording of the heart’s electrical activity as a graph over a period of time, as detected by electrodes attached to the outer surface of the skin and recorded by a device external to the body. The graph can show the heart’s rate and rhythm. It can also detect enlargement of the heart, decreased blood flow, or the presence of current or past heart attacks. ECGs are the primary clinical tool to measure electrical and mechanical performance of the heart. The ECG works by detecting and amplifying tiny electrical changes on the skin that occur during heart muscle depolarization. The output for the ECG forms a graph that shows several different waves, each corresponding to a different electrical and mechanical event within the heart. Changes in these waves are used to identify problems with the different phases of heart activity. ECG: Illustration of a patient undergoing a 12-lead ECG.
Normal Systole ECG: The U wave is not visible in all ECGs. The first wave on an ECG is the P wave, indicating atrial depolarization in which the atria contract (atrial systole ). The P wave is the first wave on the ECG because the action potential for the heart is generated in the sinoatrial (SA) node, located on the atria, which sends action potentials directly through Bachmann’s bundle to depolarize the atrial muscle cells. Increased or decreased P waves can indicate problems with the potassium ion concentration in the body that will alter nerve activity. A missing P wave indicates atrial fibrillation, a cardiac arrhythmia in which the heart beats irregularly, preventing efficient ventricular diastole. This is generally not fatal on its own.
The QRS complex refers to the combination of the Q, R, and S waves, and indicates ventricular depolarization and contraction (ventricular systole). The Q and S waves are downward waves while the R wave, an upward wave, is the most prominent feature of an ECG. The QRS complex represents action potentials moving from the AV node, through the bundle of His and left and right branches and Purkinje fibers into the ventricular muscle tissue. Abnormalities in the QRS complex may indicate cardiac hypertrophy or myocardial infarctions.
Animation of a Normal ECG Wave: The red lines represent the movement of the electrical signal through the heart. The T Wave indicates ventricular repolarization, in which the ventricles relax following depolarization and contraction. The ST segment refers to the gap (flat or slightly upcurved line) between the S wave and the T wave, and represents the time between ventricular depolarization and repolarization. An elevated ST segment is the classic indicator for myocardial infarctions, though missing or downward sloping sloping ST segments may indicate myocardial ischemia. Following the T wave is the U wave, which represents repolarization of the Purkinje fibers. It is not always visible on an ECG because it is a very small wave in comparison to the others.
When ECG output shows no identifiable P waves, QRS complexes, or T waves, it imdicates ventricular fibrillation, a severe arrhythmia. During ventricular fibrillation, the heart beats extremely fast and irregularly and can no longer pump blood, acting as a mass of quivering, disorganized muscle movements. Ventricular fibrillation will cause sudden cardiac death within minutes unless electrical resuscitation (with an AED) is performed immediately. It generally occurs with myocardial infarcations and heart failure, and is thought to be caused by action potentials that re-enter the AV nodes from the muscle tissue and induce rapid, irregular, weak contractions of the heart that fail to pump blood. The QRS complex is the combination of three of the graphical deflections seen on a typical electrocardiogram (ECG or EKG). It is usually the central and most visually obvious part of the tracing. It corresponds to the depolarization of the right and left ventricles of the heart and contraction of the large ventricular muscles.
In adults, the QRS complex normally lasts 80 to 100 ms; in children it may be shorter. The Q, R, and S waves occur in rapid succession, do not all appear in all leads, and reflect a single event and thus are usually considered together. A Q wave is any downward deflection immediately following the P wave. An R wave follows as an upward deflection, and the S wave is any downward deflection after the R wave. The T wave follows the S wave, and in some cases, an additional U wave follows the T wave. To measure the QRS interval start at the end of the PR interval (or beginning of the Q wave) to the end of the S wave. Normally this interval is 0.08 to 0.10 seconds. When the duration is longer it is considered a wide QRS complex. Depolarization of the heart ventricles occurs almost simultaneously, via the bundle of His and Purkinje fibers. If they are working efficiently, the QRS complex duration in adults is 80 to 110 ms.[1] Any abnormality of conduction takes longer and causes "widened" QRS complexes. In bundle branch block, there can be an abnormal second upward deflection within the QRS complex. In this case, such a second upward deflection is referred to as R′ (pronounced "R prime"). This would be described as an RSR′ pattern. Ventricles contain more muscle mass than the atria. Therefore, the QRS complex is considerably larger than the P wave. The QRS complex is often used to determine the axis of the electrocardiogram, although it is also possible to determine a separate P wave axis. The duration, amplitude, and morphology of the QRS complex are useful in diagnosing cardiac arrhythmias, conduction abnormalities, ventricular hypertrophy, myocardial infarction, electrolyte derangements, and other disease states. High frequency analysis of the QRS complex may be useful for detection of coronary artery disease during an exercise stress test.[2] Schematic representation of the QRS complex.
Normal Q waves, when present, represent depolarization of the interventricular septum. For this reason, they are referred to as septal Q waves and can be appreciated in the lateral leads I, aVL, V5 and V6. Pathologic Q waves occur when the electrical signal passes through stunned or scarred heart muscle; as such, they are usually markers of previous myocardial infarctions, with subsequent fibrosis. A pathologic Q wave is defined as having a deflection amplitude of 25% or more of the subsequent R wave, or being > 0.04 s (40 ms) in width and > 2 mm in amplitude. However, diagnosis requires the presence of this pattern in more than one corresponding lead. R wave progressionLooking at the precordial leads, the R wave usually progresses from showing an rS-type complex in V1 with an increasing R and a decreasing S wave when moving toward the left side. There is usually a qR-type of complex in V5 and V6, with the R-wave amplitude usually taller in V5 than in V6. It is normal to have a narrow QS and rSr' patterns in V1, and this is also the case for qRs and R patterns in V5 and V6. The transition zone is where the QRS complex changes from predominantly negative to predominantly positive (R/S ratio becoming >1), and this usually occurs at V3 or V4. It is normal to have the transition zone at V2 (called "early transition") and at V5 (called "delayed transition").[11] In biomedical engineering, the maximum amplitude in the R wave is usually called "R peak amplitude", or just "R peak".[12][13] Accurate R peak detection is essential in signal processing equipment for heart rate measurement and it is the main feature used for arrhythmia detection.[14][15] The definition of poor R wave progression (PRWP) varies in the literature. It may be defined, for example, as R wave of less than 2–4 mm in leads V3 or V4 and/or presence of a reversed R wave progression, which is defined as R in V4 < R in V3 or R in V3 < R in V2 or R in V2 < R in V1, or any combination of these.[11] Poor R wave progression is commonly attributed to anterior myocardial infarction, but it may also be caused by left bundle branch block, Wolff–Parkinson–White syndrome, right and left ventricular hypertrophy, or a faulty ECG recording technique.[11] R wave peak time (RWPT)R wave peak time represents the time from the onset of QRS complex to the peak of R wave, which is usually measured in aVL and V5 or V6 leads.[16] R-peak time for right ventricle is measured from leads V1 or V2, where upper range of normal is 35 ms. R wave peak time for left ventricle is measured from lead V5 or V6 and 45 ms is the upper range of normal.[7] R wave peak time is considered to be prolonged if it's more than 45 ms. J-pointThe point where the QRS complex meets the ST segment is the J-point. The J-point is easy to identify when the ST segment is horizontal and forms a sharp angle with the last part of the QRS complex. However, when the ST segment is sloped or the QRS complex is wide, the two features do not form a sharp angle and the location of the J-point is less clear. There is no consensus on the precise location of the J-point in these circumstances.[17] Two possible definitions are:
Various QRS complexes with nomenclature. Not every QRS complex contains a Q wave, an R wave, and an S wave. By convention, any combination of these waves can be referred to as a QRS complex. However, correct interpretation of difficult ECGs requires exact labeling of the various waves. Some authors use lowercase and capital letters, depending on the relative size of each wave. For example, an Rs complex would be positively deflected, while an rS complex would be negatively deflected. If both complexes were labeled RS, it would be impossible to appreciate this distinction without viewing the actual ECG. Monomorphic or polymorphicMonomorphic refers to all QRS waves in a single lead being similar in shape. Polymorphic means that the QRS change from complex to complex.[19] These terms are used in the description of ventricular tachycardia. A common algorithm used for QRS complex detection is the Pan-Tompkins[20] algorithm (or method); another is based on the Hilbert transform.[21][22][23][24] Numerous other algorithms have been proposed and investigated.[25]
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