Parahisian Pacing – Quick Review
As discussed in the previous post, parahisian pacing is performed by pacing the HIS catheter at high output. A narrower QRS complex is observed with HIS capture, the pacing mA will then be decreased until a wide complex is observed.
Interpretation – Negative Result
Today, let’s look at how to interpret the results.
When interpreting, it is best to display a narrow complex and a wide complex beat on the same review screen (consecutive beats). The narrow QRS complex is due to capture of the His (& local RV tissue) as seen here. In this example, the Stim-to-A is 93 ms representing the retrograde conduction due to direct His capture.
Next, the mA is decreased until only ventricular capture is observed which results in a wider QRS complex, and the same Stim-to-A measurement is made. Now the measurement is much longer due to the impulse having to travel through the ventricular myocardium and then up to the atrium via the normal conduction system instead of having the direct route from the His capture beat.
This is an example of a negative finding, or no evidence of an accessory pathway.
Interpretation – Positive Result
Now let’s look at a positive example. On a different patient the narrow QRS complex with His capture and the wide complex after the loss of His capture (decreased the mA) is observed in consecutive beats.
The Stim-to-A on the narrow beat is 114 ms. After the mA is decrease and only ventricular capture is observed, the measurement is repeated and again is 114 ms. This demonstrates evidence of an accessory pathway. If there were no pathway the impulse would have had to travel through the ventricular myocardium to reach the distal conduction system to be able to travel back up to the atrium… this takes a longer period.
So…
Narrow QRS Stim-to-A < Wide QRS Stim-to-A = no evidence of an accessory pathway / negative result
Narrow QRS Stim-to-A = Wide QRS Stim-to-A = this IS evidence of an accessory pathway
Why do we say evidence of an accessory pathway?
Think about an accessory pathway located on the left side of the heart. After His capture is lost the impulse may be able to enter the distal conduction system and travel up to the atrium via the normal conduction system faster or at the same time as the time it takes for the impulse to travel through the ventricle all the way over to the left lateral pathway. So, there may be a pathway, but we just are not able to prove it with this maneuver.
For more information and many practice examples, please refer to our text Understanding EP: A Comprehensive Guide.
