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Electrocardiogram
From Citizendium, the Citizens' Compendium
ECG stands for electrocardiogram. It is also known as EKG, either to reflect the German or Dutch Elektrokardiogramm, or to avoid confusion with EEG for electroencephalogram. While there had been earlier measurements of the electric activity of the heart, the Dutch physician Willem Einthoven developed the first medically useful device in 1903, for which he received the 1924 Nobel Prize in Physiology or Medicine.
The ECG refers to the small voltages (~1mv) found on the skin as a result of electrical activity of the heart. These electrical actions trigger various electrical and muscular activity in the heart, so a combined display of the ECG, along with another modality, such as ultrasonography (i.e., echocardiography, which could present yet another ambiguity for the abbreviation ECG), will show the ECG actions slightly preceding muscular action.
ECG electrodes usually come as 3,5 or 10 lead. These are:
- 3lead - Left ARM or LA, Right Arm or RA, and Left Leg or LL
- 5lead - additional leads of Right Leg or RL and V for Chest
- 10lead - additional leads of V1-V6 replacing the V lead, with each V lead being placed between the ribs
The health and function of the heart can be measured by the shape of the ECG waveform. Typical heart problems are leaking valves and blocked coronary arteries.
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The basic waveform
Alhough the appearance of the waveform, even in normal individuals, should be different on the various leads, the repeating waveform has several common points of inflection or "waves", as well as some components that are present only in disease:
- P wave: caused by the discharge of the sinoatrial node
- QRS complex: associated with ventricular activity
- Q: may or may not be present, and if present, may or may not indicate disease. In combination with other indications of myocardial infarction (MI), there can be useful clinical distinctions between MIs where a Q wave is present, and non-Q-wave MIs.
- R: caused by the first electrical signal arriving, via the AV node and Purkinke fibers, at the ventricles.
- S: caused by the completion of depolarization of the ventricles after they contract
- T: repolarization of the ventricles
The "segments", or times between these above points also are clinically significant. In particular, the ST interval, which is normally at baseline, tends to be elevated in the presence of MI and depressed in the presence of myocardial ischemia. By examining the degree of elevation or depression from the different leads, which essentially are different angles of view of the heart, it is possible to localize the damage.
- In the presence of symptoms suggestive of heart disease, MI and myocardial infarction cannot be excluded if the ECG is normal. It is possible to have severe heart disease and a normal ECG. Other tests are needed for a conclusive diagnosis.
Electrolyte effects on the ECG
Calcium
Hypercalcemia
- Shortening of the QT interval
Hypocalcemia
- Prolongation of the QT interval
Potassium
Hyperkalemia
- Peaked T waves
- PR interval lengthens
- QRS duration increases
Hypokalemia
- Depression of the ST segment
- Decrease in the amplitude of the T wave
- U waves, especially in the lateral precordial leads V4-V6
