Electrocardiogram

The electrocardiogram (ECG) is a recording of the electrical activity of the heart. It is the vector sum of the depolarization and repolarization potentials of all myocardial cells . At the body surface these generate potential differences of about 1 mV, and the fluctuations of these potentials create the familiar P-QRS-T pattern. At rest the intracellular voltage of the myocardium is polarized at -90 mV compared with that of the extracellular space. This diastolic voltage difference occurs because of the high intracellular potassium concentration, which is maintained by the sodium-potassium pump despite the free membrane permeability to potassium. Depolarization of cardiac cells occurs when there is a sudden increase in the permeability of the membrane to sodium. Sodium rushes into the cell and the negative resting voltage is lost (phase 0). The depolarization of a myocardial cell causes the depolarization of adjacent cells and, in the healthy heart, the entire myocardium is depolarized in a coordinated fashion. During repolarization, cellular electrolyte balance is slowly restored (phases 1, 2 and 3). Slow diastolic depolarization (phase 4) follows until the threshold potential is reached. Another action potential then follows.

I· ECG Waveform

i. The first deflection is caused by atrial depolarization, and it is a low-amplitude slow deflection called a P wave.

ii. The QRS complex reflects ventricular activation or depolarization and is sharper and larger in amplitude than the P wave. An initial downward deflection is called the Q wave. An initial upward deflection is called an R wave. The S wave is the last part of ventricular activation.

iii. The T wave is another slow and low-amplitude deflection that results from ventricular repolarization

iv. The PR interval is the length of time from the start of the P wave to the start of the QRS complex. It is the time taken for activation to pass from the sinus node, through the atrium, AV node and the His-Purkinje system to the ventricle

v. The PR interval is the length of time from the start of the P wave to the start of the QRS complex. It is the time taken for activation to pass from the sinus node, through the atrium, AV node and the His-Purkinje system to the ventricle

vi. The QT interval extends from the start of the QRS complex to the end of the T wave. This interval represents the time taken to depolarize and repolarize the ventricular myocardium. QT interval varies greatly with heart rate and is often represented as a corrected QT interval (or QTc) for a given heart rate

vii. The QT interval extends from the start of the QRS complex to the end of the T wave. This interval represents the time taken to depolarize and repolarize the ventricular myocardium. QT interval varies greatly with heart rate and is often represented as a corrected QT interval (or QTc) for a given heart rate

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