1. ECG Dr Mahvash Khan MBBS, MPhil

2. The ECG is a record of the overall spread of electrical activity through the heart.

3. The electrical currents generated by cardiac muscle during depolarization and repolarization spread into the tissues around the heart and are conducted through the body fluids. A small part of this electrical activity reaches the body surface where it can be detected using recording electrodes. The record produced is an electrocardiogram or ECG.

4. An ECG is a recording of that part of the electrical activity present in body fluids from the cardiac impulse that reaches the body surface, not a direct recording of the actual electrical activity of the heart.

5. The ECG is a complex recording representing the overall spread of activity throughout the heart during depolarization and repolarization. It is not a recording of a single action potential in a single cell at a single point in time.

6. The recording represents comparisons in voltage detected by electrodes at two different points on the body surface, not the actual potential. For example, the ECG does not record a potential at all when the ventricular muscle is either completely depolarized or completely repolarized; both electrodes are viewing the same potential, so no difference in potential between the two electrodes is recorded.

7. The exact pattern of electrical activity recorded from the body surface depends on the orientation of the recording electrodes.

8. ECG records routinely consist of 12 conventional electrode systems, or leads. The 12 different leads each record electrical activity in the heart from different locations—six different electrical arrangements from the limbs and six chest leads at various sites around the heart. To provide a common basis for comparison and for recognizing deviations from normal, the same 12 leads are routinely used in all ECG recordings

9. Different parts of the ECG record can be correlated to specific cardiac events.

11. Firing of the SA node does not generate enough electrical activity to reach the body surface so no wave is recorded for SA nodal depolarization. Therefore the first recorded wave, the P wave, occurs when the impulse or wave of depolarization spreads across the atria.

12. In a normal ECG, no separate wave for atrial repolarization is visible. The electrical activity associated with atrial repolarization normally occurs simultaneously with ventricular depolarization and is masked by the QRS complex.

13. The P wave is much smaller than the QRS complex, because the atria have a much smaller muscle mass than the ventricles and consequently generate less electrical activity.

14. At the following three points in time no net current flow is taking place in the heart musculature so the ECG remains at baseline

15. During the AV nodal delay. This delay is represented by the interval of time between the end of P and the onset of QRS complex. This segment of the ECG is known as the PR segment. Current is flowing through the AV node but the magnitude is too small for the ECG electrodes to detect.

16. The ECG can be used to diagnose abnormal heart rates, arrhythmias and damage of heart muscle.

17. When the ventricles are completely depolarized and the cardiac contractile cells are undergoing the plateau phase of their action potential before they repolarize, represented by the ST segment.

18. When the heart muscle is completely repolarized and at rest and ventricular filling is taking place after the T wave and before the next P wave. This period is called the TP interval.