Final answer:
The likely cause of seeing a HIS signal with no near field A on the ablation catheter when looking for slow pathway is an infra-Hisian block.
Step-by-step explanation:
If you see a HIS signal with no near field A on your ablation catheter when looking for the slow pathway, the likely cause is an infra-Hisian block. In this block, there is a delay or complete blockage in the conduction of electrical signals below the His bundle in the heart. It can be recognized on an ECG as a lack of correlation between atrial activity (the P wave) and ventricular activity (the QRS complex).