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The normal ECG and the 12-lead system

Created: 28/8/2006
Updated: 9/8/2006

The standard electrocardiogram (ECG) is a representation of the heart's electrical activity. It consists of recordings from each of the 12 electrodes on the body surface. The use of 12 recording leads is a convention and has little logical or scientific basis.

The basic ECG waveform

The basic ECG waveform

The basic ECG waveform consists of three recognisable deflections. These were named as "P wave", "QRS complex" and "T wave" by Einthoven.

The P wave represents the spread of electrical activation (depolarisation) through the atrial myocardium. Normally, it is a smooth, rounded deflection preceding the QRS complex.

The QRS complex represents the spread of electrical activation through the ventricular myocardium. It is usually (not always) the largest deflection on the ECG and is "spiky" in shape.

Deflections resulting from electrical activation of the ventricles are called QRS complexes, irrespective of whether they start with a positive (above the baseline) or a negative (below the baseline) deflection and whether they have one or more recognisable deflections within them.

The various components of the QRS complex however, are named on the basis of the following convention :

a) The first positive wave (above the baseline) is called r or R
b) Any second positive wave is called r' or R'
c) A negative wave that follows an r or R wave is called an s or S wave
d) A negative wave that precede an r or R wave is called a q or Q wave
e) An entirely negative wave is called a qs or QS wave
f) LARGE DEFLECTIONS are named with an appropriate CAPITAL letter and small waves with an appropriate small (lower case) letter.

The T wave represents electrical recovery (repolarisation) of the ventricular myocardium. It is a broad, rounded wave following the QRS complex.

The U wave may be due to slow replolarisation of the papillary muscles.  Some causes include: Bradycardia, hypokalaemia and digoxin.

Note: For normal calibration, see article on "A systematic approach to ECG interpretation".

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