How to Read ECG/EKG, Part 2, Animation

(USMLE topics, cardiology). This part covers PR interval, QRS complex and ST segment. See part 1 for more. This is an update of a previously published video.

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The PR interval is measured from the start of P wave to the start of QRS complex and reflects the conduction through the AV node.
A longer than normal PR interval signifies an abnormal delay in the AV node, or an AV block. A consistent long PR interval of more than 5 small squares constitutes first-degree heart block. A progressive prolongation of PR interval followed by a P wave WITHOUT a QRS complex is the hallmark of second-degree AV block type I.
A shorter than normal PR interval, of less than 3 small squares, signifies that the ventricles depolarize too early. There are 2 scenarios for this to happen:
-Pre-excitation syndrome: presence of an accessory pathway bypassing the AV node.
-AV nodal (junctional) rhythm: Non-sinus rhythm initiated from around the AV node area instead of the SA node. In this case, P waves are either absent or inverted in the inferior leads.
The QRS complex represents depolarization of the ventricles. A normal QRS complex is narrow, between 70 and 100 miliseconds. A wider QRS complex, resulting from an abnormally slow ventricular depolarization, may be caused by:
-A ventricular rhythm: rhythm originated from ectopic sites in the ventricles.
Or
-An impaired conduction within the ventricles in conditions such as bundle branch block, hyperkalemia or sodium-channel blockade.
A QRS complex wider than three small squares despite sinus rhythm is the hallmark of bundle branch block. When bundle branch block is suspected, check leads V1 and V6 for characteristic patterns of the QRS complex.
The ST segment extends from the end of S wave to the start of T wave. A normal ST segment is mostly flat and level with the baseline. Elevation of more than two small squares in the chest leads or one small square in the limb leads, indicates the possibility of myocardial infarction. The infarction may be localized based on the leads with ST elevation.
There is usually a reciprocal ST depression in the electrically opposite leads. For example, ST elevation in leads I and aVL typically produces ST depression in lead III.
Pericarditis causes a characteristic “saddleback” ST segment elevation and PR segment depression in all leads except aVR and V1, where the reverse - ST depression and PR elevation – are seen.
ST depression is diagnostic of ischemia. ST depression may be of various morphology and may be seen in a variable number of leads, and therefore cannot be used to localize the lesion.