ECG Level 1 Tutorial: Analyze the Waveforms
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Tutorial: Analyze the Waveforms
This module will teach the basic waveforms on ECG and a stepwise approach of how to analyze them. Practice each step and get feedback on your performance.
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Tutorial: Analyze the Waveforms QRS Width
Lessons
42
Times Practiced
1284
Cases Completed
1h 24m
Total Time spent
1m 24s
Average Time
Progress
Accuracy
Efficiency
Accuracy
Efficiency
QRS Width
The criterion to differentiate wide vs. normal QRS is 120 ms. which is 3 little squares:
  • less than or equal to 120 ms is narrow, which is normal
  • greater than 120 ms is wide

The width of any waveform on the ECG is dictated by conduction speed through the heart:
  • 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

Remember that normal conduction through the ventricles starts with the Purkinje fibers, which are fast conducting fibers and when these fibers are used, the electrical signal is very rapidly spread through both ventricles and the QRS will be narrow.

Super important causes of slow conduction (wide QRS):
  • 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 delayaberrancy, 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:
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):
Using calipers

Here is an example of a wide QRS:
Wide QRS

The calipers show the QRS duration to be 155 ms:
QRS duration is 155 msec

How about this ugly one?
Very wide QRS
That is very wide. It is virtually impossible to identify the start and finish of the QRS.

Try practicing the QRS width.