Rhythm of the Month February 2011
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These two rhythms are similar but different.
The question is how are they different?
Both are sinus rhythms originating in the sinus node, the normal pacemaker of the heart. And, they both have PQRST wave forms missing. When there is an interruption in the rhythm (missing wave forms), a pause is created and you should ask yourself, what is the cause of the pause?
Let’s first take a closer look at Figure 1. The pacemaker cells of the sinus node fail to initiate an electrical impulse for one or more beats. Anytime the SA node fails to initiate an impulse, the AV node or ventricles should assume responsibility of pacing the heart; this is known as an escape pacemaker site. If there is no escape rhythm present, then you will see absent PQRST complexes on the EKG or no electrical activity. In figure 1, one or more cardiac cycles are missing.
Now take a look at Figure 2. Again there are missing wave forms without an escape beat or rhythm. In this example, the pacemaker cells within the SA node initiate an impulse but it is blocked as it exits the SA node. Because impulses are generated at regular intervals, the rhythm of the SA node is not affected. If you marched the P waves with calipers (yes I am a huge believer in caliper usage) from left to right, you will notice the rhythm stays right on target as you go across the strip. The impulse is blocked as it exits the SA node. The atria are not activated, resulting in the PQRST complex as missing. The SA node, not realizing an impulse was blocked, continues to fire at its’ regular rate, thus the beat following the pause, occurs right on time.
Interpretation: Figure 1 - Sinus Rhythm with Sinus Arrest
Figure 2 - Sinus Rhythm with Sino-atrio Exit Block
