Information AboutElectrocardiography |
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| cardiology | |
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| cardiac electrophysiology | |
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An electrocardiogram ('''ECG''' or '''EKG''', abbreviated from the German ''Elektrokardiogramm'') is a graphic produced by an '''electrocardiograph''', which records the Electrical voltage in the Heart in the form of a continuous strip graph. It is the prime tool in Cardiac Electrophysiology , and has a prime function in screening and diagnosis of Cardiovascular Disease s. The electrocardiogram does not assess the Contractility of the heart. USES The ECG has a wide array of uses:
LEAD PLACEMENT An ECG is constructed by measuring Electrical Potential between various points of the body using a Galvanometer . Leads I, II and III are measured over the limbs: I is from the right to the left arm, II is from the right arm to the left leg and III is from the left arm to the left leg. From this, the imaginary point ''V'' is constructed, which is located centrally in the chest above the heart. The other nine leads are derived from potential between this point and the three limb leads (aVR, aVL and aVF) and the six precordial leads (V1-6). Leads Readings Therefore, there are twelve leads in total. Each, by their nature, record information from particular parts of the heart:
Understanding the usual and abnormal directions, or vectors, of depolarization and repolarization yields important diagnostic information. The right Ventricle has very little muscle mass. It leaves only a small imprint on the ECG, making it more difficult to diagnose than changes in the left Ventricle . The leads measure the average electrical activity generated by the summation of the action potentials of the heart at a particular moment in time. For instance, during normal atrial Systole , the summation of the electrical activity produces an electrical vector that is directed from the SA node towards the AV node, and spreads from the right Atrium to the left Atrium (since the SA node resides in the right Atrium ). This turns into the P wave on the EKG, which is upright in II, III, and aVF (since the general electrical activity is going towards those leads), and inverted in aVR (since it is going away from that lead). THE NORMAL ECG ),'' with waves, segments, and intervals labeled.]] A typical ECG tracing of a normal heartbeat consists of a P wave, a QRS Complex and a T wave. A small ''U wave'' is not normally visible. Axis The axis is the general direction of the electrical impulse through the heart. It is usually directed to the bottom left (normal axis: -30o to +90o), although it can deviate to the right in very tall people and to the left in Obesity .
P wave The P wave is the electrical signature of the current that causes Atrial Contraction . Both the left and right atria contract simultaneously. Its relationship to QRS complexes determines the presence of a Heart Block .
QRS The QRS Complex corresponds to the current that causes contraction of the Left and Right ventricles, which is much more forceful than that of the atria and involves more muscle mass, thus resulting in a greater ECG deflection. The duration of the QRS complex is normally less than or equal to 0.10 second. The Q wave, when present, represents the small horizontal (left to right) current as the action potential travels through the interventricular septum.
The R and S waves indicate contraction of the Myocardium itself.
T wave The T wave represents the repolarization of the ventricles. The QRS complex usually obscures the atrial repolarization wave so that it is not usually seen. Electrically, the cardiac muscle cells are like loaded springs. A small impulse sets them off, they depolarize and contract. Setting the spring up again is repolarization (more at Action Potential ). In most leads, the T wave is positive.
The ST segment connects the QRS complex and the T wave.
U WAVE Not always seen. Quite small. Follows T wave. Thought to represent repolarization of the papillary muscles or Purkinje fibers. Prominent U waves are most often seen in hypokalemia, but may be present in hypercalcemia, thyrotoxicosis, or exposure to digitalis, epinephrine, and Class 1A and 3 antiarrhythmics, as well as in congenital long QT syndrome and in the setting of intracranial hemorrhage. An inverted U wave may represent myocardial ischemia or left ventricular volume overload. ECG MEASURES QT interval The QT Interval is measured from the beginning of the QRS complex to the end of the T wave. A normal QT interval is usually about 0.40 seconds. The QT interval as well as the corrected QT interval are important in the diagnosis of Long QT Syndrome and Short QT Syndrome . The QT interval varies based on the heart rate, and various correction factors have been developed to correct the QT interval for the heart rate. The most commonly used method for correcting the QT interval for rate is the one formulated by Bazett and published in 1920 1. Bazett's formula is , where QTc is the QT interval corrected for rate, and RR is the interval from the onset of one QRS complex to the onset of the next QRS complex, measured in seconds. However, this formula tends to not be accurate, and over-corrects at high heart rates and under-corrects at low heart rates. PR interval The PR Interval is measured from the beginning of the P wave to the beginning of the QRS complex. It is usually 0.12 to 0.20 seconds. A prolonged PR indicates a First Degree Heart Block , while a shorting may indicate an accessory bundle that depolarizes the ventricle early, such as seen in Wolff-Parkinson-White Syndrome . HISTORY In the 19th Century it became clear that the heart generated electricity. The first to systematically approach the heart from an electrical point-of-view was Augustus Waller , working in St Mary's Hospital in Paddington , London . In 1911 he still saw little clinical application for his work. The breakthrough came when Willem Einthoven , working in Leiden , The Netherlands , used the String Galvanometer invented by him in 1901 , which was much more sensitive than the Capillary Electrometer that Waller used. Einthoven assigned the letters P, Q, R, S and T to the various deflections, and described the electrocardiographic features of a number of cardiovascular disorders. He was awarded the 1924 Nobel Prize For Physiology Or Medicine for his discovery. REPRESENTATION IN CULTURE The ECG has become so familiar to the general population that it is part of the logo of many medical organisations, representing the technical side of medicine ''vs.'' the Rod Of Asclepius or Caduceus , which are more traditional. Being an electrical representation, it signifies vitality and urgency. In various Television medical dramas, an isoelectric ECG (no cardiac electrical activity or Flatline ) is often used as a symbol of death or at least extreme medical peril. This is technically known as Asystole , a form of Cardiac Arrest with a particularly bad prognosis. Though sometimes shown on television, Defibrillation , which can be used to correct arrythmias such as ventricular fibrillation and pulseless ventricular tachycardia, cannot correct asystole. REFERENCES # Bazett HC. ''An analysis of the time-relations of electrocardiograms.'' Heart 1920;7:353-370 # Cooper JK. ''Electrocardiography 100 years ago. Origins, pioneers, and contributors.'' N Engl J Med 1986;315:461-4. PMID 3526152. SEE ALSO
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