Francais | English | Espanõl

From Wikipedia, the free encyclopedia

For other meanings of ECG, see ECG (disambiguation)

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 the screening and diagnosis of cardiovascular diseases.

The electrocardiogram does not assess the contractility of the heart.

The baseline voltage of the electrocardiogram is known as the isoelectric line.

Contents 1 Uses 2 Lead placement 3 Normal ECG 3.1 Axis 3.2 P wave 3.3 QRS 3.4 T wave 3.5 U Wave 4 ECG measures 4.1 QT interval 4.2 PR interval 5 History 6 Representation in culture 7 See also 8 References 9 External links

[edit] Uses

The ECG has a wide array of uses:

• Determine whether the heart is performing normally or suffering from abnormalities (eg. extra or skipped heartbeats - cardiac arrhythmia).
• May indicate acute or previous damage to heart muscle (heart attacks/myocardial infarction) or ischaemia of heart muscle (angina).
• Can be used for detecting potassium, calcium, magnesium and other electrolyte disturbances.
• Allows the detection of conduction abnormalities (heart blocks and in bundle branch blocks).
• As a screening tool for ischaemic heart disease during an exercise tolerance test.
• Can provide information on the physical condition of the heart (eg: left ventricular hypertrophy, mitral stenosis).
• Can suggest non-cardiac disease (e.g. pulmonary embolism, hypothermia).

[edit] 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 (V_1-6). Leads Readings

Therefore, there are twelve leads in total. Each, by their nature, record information from particular parts of the heart:

• The inferior leads (leads II, III and aVF) look at electrical activity from the vantage point of the inferior region (wall) of the heart. This is the apex of the left ventricle.
• The lateral leads (I, aVL, V₅ and V₆) look at the electrical activity from the vantage point of the lateral wall of the heart, which is the lateral wall of the left ventricle.
• The anterior leads, V₁ through V₆, and represent the anterior wall of the heart, or the frontal wall of the left ventricle.
• aVR is rarely used for diagnostic information, but indicates if the ECG leads were placed correctly on the patient.

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).

[edit] Normal ECG

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 normally visible in 50 to 75% of ECGs.

[edit] Axis

The frontal plane axis is the general direction of the depolarization wavefront through the heart, also known as the mean electrical vector. It is usually directed to the bottom left (normal axis: -30^o to +90^o).

• Left axis deviation may indicate left anterior fascicular block or Q waves from inferior MI. Right axis deviation may indicate left posterior fascicular block, Q waves from high lateral MI, or right ventricular strain. Right ventricular strain may result from pulmonary hypertension. In the setting of right bundle branch block, right or left axis deviation in the frontal plane may indicate bifascicular block.
• It also can diagnose dextrocardia or a reversal of the direction in which the heart faces, but this condition is very rare and often has already been diagnosed by something else (such as a chest X-ray).

[edit] P wave

The P wave is the electrical signature of the current that causes atrial depolarization. Both the left and right atria contract simultaneously. Its relationship to QRS complexes determines the presence of a heart block.

• Irregular or absent P waves may indicate arrhythmia.
• The shape of the P waves may indicate atrial problems.

[edit] 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.

• Very wide and deep Q waves do not have a septal origin, but indicate myocardial infarction that involves the full depth of the myocardium and has left a scar.

The R and S waves indicate the spread of the action potential along the ventricular myocardium itself.

• Abnormalities in the QRS complex may indicate bundle branch block (when wide), ventricular origin of tachycardia, ventricular hypertrophy or other ventricular abnormalities.
• The complexes are often small in pericarditis or pericardial effusion.

[edit] 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.

• Inverted (also described as negative) T waves can be a sign of disease, although an inverted T wave is normal in V₁ (and V₂-V₃ in African-Americans/Afro-Caribbeans).
• T wave abnormalities may indicate electrolyte disturbance, such as hyperkalemia or hypokalemia.
• The earliest electrocardiographic finding resulting from acute myocardial infarction is the hyperacute T wave.

The ST segment connects the QRS complex and the T wave.

• This segment ordinarily lasts about 0.08 second and is usually level with the PR segment. Upward or downward displacement may indicate damage to the cardiac muscle or strain on the ventricles. It can be depressed in ischemia and elevated in myocardial infarction or Brugada syndrome, and upslopes in digoxin use.

[edit] U Wave

The U wave is not always seen. It is quite small, and follows the T wave by definition. It is 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.

[edit] ECG measures

[edit] 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 1920¹. Bazett's Formula is <math>QTc = \frac{QT}{\sqrt {CL} }</math>, where QTc is the QT interval corrected for rate and CL is the cycle length defined as heart rate/ 60. Federica's formula is <math>QTc = \frac{QT}{\sqrt {RR} }</math>, where RR is the interval from the onset of one QRS complex to the onset of the next QRS complex, measured in seconds. However, these formula tend to be inaccurate, and over-corrects at high heart rates and under-corrects at low heart rates.

[edit] 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.

[edit] History

In 1856 Kolliker and Muller discovered the electrical activity of the heart when a frog sciatic nerve/gastrocenemius preparation fell onto an isolated frog heart and both muscles contracted synchronously.<ref>Kolliker A and Muller H. Nachweis der negativen Schwankung des Muskelstromes am naturlich sich contrahirenden Muskel. Verh Phys Med Ges 1856; 6:528–33.</ref> Alexander Muirhead attached wires to a feverish patient's wrist to obtain a record of the patient's heartbeat while studying for his DSc (in electricity) in 1872 at St Bartholomew's Hospital<ref>Ronald M. Birse, rev. Patricia E. Knowlden [1] Oxford Dictionary of National Biography 2004 (Subscription required)</ref> This activity was directly recorded and visualized using a Lippmann capillary electrometer by the British physiologist John Burdon Sanderson.<ref>Burdon Sanderson J. Experimental results relating to the rhythmical and excitatory motions of the ventricle of the frog heart. Proc Roy Soc Lond 1878; 27:410–14.</ref> 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.<ref>Waller AD. A demonstration on man of electromotive changes accompanying the heart's beat. J Physiol (Lond) 1887; 8:229–34.</ref> His electrocardiograph machine consisted of a Lippmann capillary electrometer fixed to a projector. The trace from the heartbeat was projected onto a photographic plate which was itself fixed to a toy train. [2] This allowed a heartbeat to be recorded in real time. 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.<ref>Einthoven W. Un nouveau galvanometre. Arch Neerl Sc Ex Nat 1901; 6:625</ref> 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. In 1924, he was awarded the Nobel Prize in Medicine for his discovery.

[edit] 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.

[edit] See also

• Advanced cardiac life support (ACLS)
• Electroencephalography
• Electroretinography
• Cardiac arrest
• Holter monitor
• Heart rate monitor
• SCP-ECG
• Cardiac cycle
• EKG tech

[edit] References

<references/>

1. Bazett HC. An analysis of the time-relations of electrocardiograms. Heart 1920;7:353-370
2. Cooper JK. Electrocardiography 100 years ago. Origins, pioneers, and contributors. N Engl J Med 1986;315:461-4. PMID 3526152.
3. Conrath, C.E., Opthof, T. The patient U wave Cardiovascular research 2005;67:184-6.

[edit] External links

• The whole ECG - A really basic ECG primer
• 12-lead ECG library
• Simulation tool to demonstrate and study the relation between the electric activity of the heart and the ECG
• ECG information from Children's Hospital Heart Center, Seattle.
• The EKG Club
• The EKG Club listserv at Yahoo!
• ECG simulator
• National Heart, Lung, and Blood Institute, Diseases and Conditions Indexzh-min-nan:Sim-tiān-tô͘

da:Elektrokardiogram de:Elektrokardiogramm es:Electrocardiograma eu:Elektrokardiograma fr:Électrocardiographie gl:Electrocardiograma it:Elettrocardiogramma he:אלקטרוקרדיוגרם lb:Elektrokardiogramm lt:Elektrokardiografija nl:Elektrocardiogram ja:心電図 nn:Elektrokardiografi no:Elektrokardiogram pl:Elektrokardiografia pt:Eletrocardiograma ru:Электрокардиограмма fi:EKG sv:EKG vi:Điện tâm đồ tr:Elektrokardiyografi uk:Електрокардіографія JKFGGHO