Orthodromic AV Reentry Tachycardia

What is the ECG rhythm interpretation?

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We will take the long lead II and analyze the highlighted section of the strip.

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First, this is a regular, narrow complex, short RP tachycardia. 

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This (first) beat is conducted with a normal PRI and QRS morphology.

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The second beat is conducted with a short PRI, wide QRS and a delta wave (red arrowhead). This is a pre-excited beat.  

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Accessory Pathway

The normal conduction of impulse is through the AV node.  In a very few, there is an extra pathway outside of the AV node (accessory pathway or AP)  that connects the atrium to the ventricle. This  accessory pathway can transmit impulse antegrade (atrium to ventricle) or retrograde (ventricle to atrium). During antegrade conduction (as seen in the diagram), the ventricle is activated by an impulse transmitted to the AV node (continuous arrow) and the AP (broken arrow). An impulse transmitted to the AP can activate a portion of the myocardium earlier than the impulse transmitted through the AV node. This preexcitation creates the delta wave. The rest of the myocardium is activated by the impulse transmitted through the AV node. The widened QRS complex is due to the fusion between the impulse transmitted through the AP and the AV junction. The PR interval is affected by the degree of preexcitation. In fully preexcited beats, the PRI is the full P wave duration.
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The third beat is a premature atrial beat or complex (PAC). The impulse is blocked at the AP because it is refractory but was transmitted in the AV node. Thus, creating a normal looking QRS.
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In the 4th beat, the impulse was able to travel retrograde (ventricle to atrium) because the AP has recovered. This created an inverted P wave and a short RP. It then travelled antegrade by way of the AV node creating the normal QRS.
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The 5th beat is the repetition of the circuit (ventricle - AP - AV node or orthodromic).
Here are the diagrams all together.

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#144

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