PRINCIPLE OF ELECTROCARDIOGRAPHY

P wave is produced by atrial depolarization and is about 3mm in height with a duration of up to 0.1sec. It is upright in all the leads except in a VR where the P is inverted. The p wave is also inverted in a nodal rhythm where the impulse travels in the reverse direction. In that case, the inverted P is seen if the respective ventricular complex is absent and this wave remains absent when the cardiac impulse is not generated at the Sino Atrial node. The p wave is tall in right atrial hypertrophy and is found to be replaced by a fibrillary wave in atrial fibrillation and by a saw tooth in atrial flutter.

Significance of normal P wave:

1. The impulse originates at the SAN
2. The impulse travel normally along with the atria.
3. The atria are normal.

PR Interval is the time taken by the impulse to reach the ventricular musculature from the SAN including the nodal delay. It’s from the start of the P wave to the start of the ventricular complex. It is measured by counting the number of small squares of the graph in between those two points and multiplying by 0.04. Now let’s say there are 5 small squares, so the PR Interval becomes 5×0.04 = 0.20 sec. The normal value is 0.12 to 0.2 s. If it is greater than 0.22 sec, then the condition is called partial heart block. The PR Interval gradually increases in subsequent beats in the Wenckebach phenomenon. It becomes abnormally short in the case of WPW syndrome (Wolff- Parkinson -White syndrome is caused due to extra electrical connection in the heart which causes the heart to beat abnormally fast for periods of time.) due to the passage of the impulse from atria to the ventricle through abnormal path bypassing the Atrioventricular junction.

The QRS complex is produced due to ventricular depolarization. A normal QRS complex indicates normal ventricular health and normal depolarization. Q wave is produced due to septal depolarization as the vector is directed away from these positions. If a Q wave of whatever small size is found where it should not be found or the presence of a large Q greater than 5 mm where it is normally found, both indicate a pathological condition of Q. Abnormal Q usually signifies Myocardial infarction.

QRS complex with R and S of equal amplitude is called Transition Complex. The normal QRS pattern may change as in bundle branch block it becomes of RSR or M pattern.

QRS complex in limb leads is used to study the mean electrical axis of the heart.

ST segment is the isoelectric segment between the end of the S wave, and the start of the T wave. It is isoelectric since the absence of vector as the whole ventricle has been completely depolarized. If the ventricular musculature is affected by the disease or ischemic, then the whole of it will not depolarize at the same time and will result in a difference in potential during this period. As such the ST segment will not remain isoelectric. ST depression is seen during ischemia and ST elevation is seen during Myocardial infarction. T is inverted is found during ischemia some time after infarction and also during severe hyperkalemia.

UTILITIES OF ECG

1. Helps in diagnosing cardiac ailments like arrythmias, ischemia, blockage, infarction.
2. Helps to follow up during the above treatment and after treatment.
3. Helps to follow up during some cases of electrolyte balance.
4. Helps to study the relationship between mechanical and electrical activities of the heart.

Stress ECG

This is used to confirm the suspected causes of ischemia where no significant abnormalities are observed at rest. In this test, ECG is recorded while the person runs on the Treadmill. Here the depressed ST confirms the diagnosis. The ECG may also be recorded continuously for 24 hours called Holter Monitoring which is useful for the occasional abnormalities.