 |
| ECG - Electrocardiography |
|
|
| ECG is a graphic display or recording of the electrical variations produced by heart muscle during a cardiac cycle. The activity of the heart create an electrical field which is conducted through the surrounding body tissues to the surface of the body. |
| |
 |
| |
| Two electrodes are placed on the surface of the body at a distance and the potential difference between the points is measured. The sequence of events during contraction and relaxation of the heart muscle are graphically presented as voltage displacement on the Y-axis with time on the X-axis. This tracing or recording is called an electrocardiogram. |
| |
 |
| |
| Each ECG has 5 consecutive waves namely three upward deflections and two downward waves. They are named as PQRST. |
| |
| The P Wave represents the first upward deflection of the ECG. It takes place when the impulse spreads over the atrial chambers. Its average duration is about 0.1 second. It represents the depolarization of the atrial musculature. |
| |
| The QRS complex is the combined result of the repolarization of the atria and the depolarization of the ventricles. |
| |
| |
| |
 |
| |
| The sequence of events in the cardiac cycle starting with the onset of atrial systole. Part of the subsequent cycle is shown. PA, blood pressure in the pulmonary artery; RA, right atrail pressure; La, left atrid pressure. |
| |
| The T-wave is the wave of ventricular repolarization. It has a duration of 0.27 seconds. |
| |
| The shape and polarity of each of the above features varies with the location of the electrode with respect to the heart and a diagram in made based on readings taken from several electrode locations. |
| |
|
| In healthy individuals, the ECG remains reasonably constant, even though the heart rate changes with the demands of the body. |
| |
| The electrical activity of the heart shows changes under the following situations. |
| |
| a) Change in ionic environment of the heart |
| |
| b) A defect in the conduction system of the heart |
| |
| c) damage to muscle fibres due to decreased blood supply |
| |
| d) ECG is used for detecting several heart diseases. It is a non invasive |
| technique. |
| |
