Module title = Tutorial: Analyze the Waveforms

Lesson title = QRS Width

• less than or equal to 120 ms is narrow, which is normal
• greater than 120 ms is wide

• an electrical event that is slow will be spread out over more time and therefore will be wide
• an electrical event that is fast will be completed quickly and therefore will be narrow

• conduction from ventricular myocyte to myocyte is much slower compared to the purkinje fibers. This happens when an electrical signal STARTS in the ventricle muscle.
• diseased conducting fibers, typically called conduction delay, aberrancy, or bundle branch block.

For more details on bundle branch block and conduction delay, see the Bundle Branch Block tutorial. In brief, with bundle branch block one of the bundle branches is not conducting. Therefore, the electrical signal moves quickly down from the AV node through one bundle branch, but to get to the other side of the heart, the signal must be conducted from myocyte to myocyte, which is slow conduction. This requires extra time and causes the QRS to be wide.

Here is an example of narrow QRS:

The signal in this ECG travels through the fast purkinje fibers and rapidly depolarizes both ventricles. This occurs very quickly, so the QRS is narrow.

When you apply calipers, you can see that the QRS duration is less than 120 ms (3 little squares):


Here is an example of a wide QRS:


The calipers show the QRS duration to be 155 ms:


How about this ugly one?

That is very wide. It is virtually impossible to identify the start and finish of the QRS.

Try practicing the QRS width.