ECG Level 2 Tutorial: Origins of Activity
Tutorial: Origins of Activity
This module will teach how to determine the origins (pacemaker location) of the electrical activity.
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Tutorial: Origins of Activity Narrow QRS
Times Practiced
Cases Completed
1h 24m
Total Time spent
1m 24s
Average Time
Narrow QRS
Rule: A narrow QRS rules out the ventricles

A wide QRS is created by slow conduction through the ventricles. If the origin is in the ventricles, then spread of the electricity is from myocyte to myocyte instead of being conducted through the fast purkinje fibers. Myocyte to myocyte conduction is slow. Slow = wide on the ECG. Therefore, a narrow QRS cannot originate from the ventricle.

Here are 2 examples of a wide QRS complex tachycardia that have the origin of electrical activity in the ventricle:

The reverse rule is not true. A wide QRS does not rule out SA node, atria, or AV node.

Why not? Slow conduction through diseased purkinje fibers will produce a wide QRS. The SA node, atria, and AV node all use the purkinje fibers to get the electrical signal down to the ventricles. If the purkinje system is diseased, conduction to the ventricles will be slow and the QRS will be wide. 

Sometimes, with a fast ECG we have difficulty telling the difference between the SA node, the atria, and the AV node. If the QRS is narrow, then we know it is NOT the ventricles, but it could still be one of the other 3 options. In this situation, we use the term "supraventricular" or SVT to describe any origin that is "above" the ventricles. Here are some examples of narrow QRS complex tachycardias that all fit into the category of SVT:

  • a narrow QRS rules out the ventricle
  • a wide QRS could have one of 4 origins:
    • ventricular
    • SA node, atrial, or AV node with diseased purkinje fibers
  • a very rare origin for wide QRS:
    • AVRT due to WPW  (we didn't teach you this one yet ... is advanced level topic)
    • AVRT = atrio-ventricular re-entry tachycardia  (different than AVNRT)