Parahisian Pacing: Getting the Basics Right

Answer: Parahisian Pacing manoeuvre with a result suggestive of the presence of an accessory pathway.

Parahisian pacing is a method of determining whether or not a septal accessory pathway is present. In the above example we can see:

• Evidence of His bundle capture (QRS width 140ms) and non-His bundle capture (QRS width 190ms). (Note: Although this is not <120ms, it still suggests His bundle capture since the QRS is significantly narrower than the next beat.)

• VA time is too long to suspect direct atrial capture (which can usually be suspected when the VA time is <60ms).

• VA conduction time does not change with His bundle capture Vs no His bundle capture. This manoeuvre suggests that VA conduction is NOT dependent on the His/AV Node and that, therefore, an Accessory Pathway may indeed be heavily suspected to exist. The below diagram demonstrates how an accessory pathway responds to Parahisian pacing.

Common pitfall of Parahisian Pacing – Atrial Capture

• High output pacing near the Tricuspid Annulus risks direct stimulation of atrial tissue, as well as ventricular or His bundle tissue. Direct atrial capture invalidates the test results, and will inevitably lead to misdiagnosing the presence of an accessory pathway. An example of atrial capture is shown below.

Excluding atrial capture:

• In principle, the shorter the SA time, the more concerned you should be that direct atrial capture has occurred. The authors below have reported that Stim-PCS <64ms, or Stim-HRA <85ms is strongly suggestive of atrial capture. You can see the table below for individual sensitivity & specificities of various SA times for detecting atrial capture.

If atrial capture is suspected, the catheter should be repositioned, or the pacing output reduced in order to avoid atrial capture.

My Two Cents:

• Parahisian Pacing is a readily available manoeuvre for determining the presence of an accessory. A change in VA time with loss of His capture indicates that VA time is dependent on the His-Bundle, suggesting that an accessory pathway is NOT present.
• No change in VA time indicates that VA time is not dependent on the His-bundle and therefore an accessory pathway is likely present.
• Inadvertent atrial capture can result in misdiagnosis and can be presumed when Stim-PCS <64ms or Stim-HRA <85ms.

This is discussed in more detail in EP in Practice Program 2 (SVT).

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Cheers

Mitch & CPP Team