Rule: A Heart Rate > 200 rules out the SA nodeThe
atrial rate is the rate of P waves, which is sometimes different that the
ventricular rate, which is the rate of QRS complexes. Please pay close attention if the rules are talking about atrial or ventricular rates and try to understand the logic.
In adults, the SA node normally does not pace faster than 200 beats per minute. Therefore, an atrial or ventricular rate > 200 rules out the SA node. However, a re-entry circuit in the atria could pace the atria at a rate as fast as 300 or even faster. Therefore, an
atrial rate > 200 does NOT rule out the atria.
Rule: A Ventricular Rate > 200 rules out the SA node or AtriaThe AV node is special. It protects your ventricles from going too fast. When your ventricles beat too fast, they do not have time to fill and ventricles that are underfilled cannot eject very much blood, so this is a big problem.
We are designed with a safety mechanism: the AV node. It has slow conduction and a long refractory time. Therefore, in adults, any rhythm that is from atria that is faster than 200 cannot be fully conducted through the AV node. Many of the beats will be blocked because they will hit the AV node when the AV node is refractory and unable to conduct. Therefore, the ventricular rate will be lower than 200.
Therefore, a ventricular rate > 200 has a very low probability of originating from either the SA node or atria because the AV node would be protective against this scenario.
Exceptions to these Rules:
There are 2 exceptions. In babies and children, especially very young children, this rule does not apply. Second, in a rare condition called Wolff-Parkinson-White (WPW syndrome), this rule can be broken.
Why can the AV node or Ventricles go so fast?
If we have ruled out the SA node and the atria, then the 2 options we have left are the AV node and the ventricles. When the origin is INSIDE the AV node, then the normal rules of slow AV node conduction and long refractory period no longer apply. For this reason, the AV node is possible of generating a ventricular rate > 200 and this can happen if there is a "re-entry" circuit that develops inside the AV node.
If a re-entry circuit within the ventricles, then the ventricles could also generate a very fast heart rate as well. The AV node or ventricles each could generate ventricular rates > 200 bpm.
We can combine 2 rules now and make a new rule:- Ventricular rate > 200 rules out SA node and atria
- Narrow QRS rules out ventricles (previous lesson)
- Therefore:
Ventricular rate > 200 + narrow QRS = AV node (junctional)
This rhythm is most likely coming from the AV node. Remember that there are exceptions and all rules will have exceptions.Summary:- an atrial rate > 200 is not likely to be the SA node
- a ventricular rate > 200 is not likely to be the SA node or atria
- a narrow QRS and ventricular rate > 200 is most likely the AV node (junctional)
- exceptions:
