# Para-Hisian Pacing

## Use

- Distinguish retrograde accessory pathway (AP) conduction from retrograde AV nodal (AVN) conduction.
- Useful for assessing retrograde conduction when SVT not inducible in patient with WPW or concealed septal AP, and post-ablation to exclude continued retrograde AP conduction.
- Most useful for detecting paraseptal pathways.

## Technique

- Pace ventricle near His bundle or right bundle (HB-RB), using catheter with closely spaced electrodes (2 mm).
- Pace at high output (10 mA and 2 to 6 ms pulse width).
- Withdraw catheter towards HB-RB until there is shortening of the QRS duration, indicating His or RB capture.
- Decrease pacing output until loss of HB-RB capture, indicated by widening of QRS.
- Measure stimulus to local A (S-A), as well as His to A (H-A) if possible.
- Assess atrial activation sequence for changes using multiple atrial recordings (e.g. coronary sinus, high right atrium).

## Results

- Loss of HB-RB capture with change in atrial activation sequence indicates combined retrograde AP and AVN conduction.
- Loss of HB-RB capture with no change in atrial activation sequence indicates exclusive retrograde AP or AVN conduction.
  - Loss of HB-RB capture without change in S-A interval indicates exclusive retrograde AP conduction. H-A shortens as its activation is now delayed due to retrograde activation via remote inputs near apex of RV.
  - Loss of HB-RB capture with increase in S-A interval indicates exclusive retrograde AVN conduction. H-A interval is unchanged.
- Note slowly conducting AP or distant AP can give false AVN only response.

## Reference

Hirao K, Otomo K, Wang X, et al. Para-Hisian pacing. A new method for differentiating retrograde conduction over an accessory AV pathway from conduction over the AV node. Circulation 1996; 94: 1027-1035. http://circ.ahajournals.org/content/94/5/1027.full