It depends.

Please provide further information like 1. right or left phrenic nerve, 2. ICD and lead brand name3, ICD and lead model.4. If it is possible, Chest X-ray AP and lateral were helpful 

Couls be due to higher atrial pressure during asynchronous (VVI) pacing- causing pressure on the nerve. Thats only hypothesis, 

One possibility could be that that RV lead was accidentally inserted into coronary sinus and therefore pacing epicardially - phrenic nerve stimulation. Chest X - ray especially lateral would answer this hypothesis. Otherwise lead dislocation / perforation is the second option. X-ray AP and LAT should again provide the answer.

Chest X-ray AP and lateral were performed. everything was normal. ventricular catheter was in the RV apex, no lead dislocation or perforation were found.

The first hypothesis is to eliminate a portion of the probe in the left pericardium with phrenic nerve stimulation of the left. Do you have  modification of Hight voltage impedance?

Has the ICD given any high energy therapy?That could have partially damaged the nerve, although one would expect to have permanent palsy if this was the case.

Perhaps the electrode stimulates the phrenic nerve because of an anatomic variant of the nerve's traject, if the RV electrode is the right position.

This is a rather unusual situation. Anomalous course of the phrenic nerve is one explanation. In that case, I would presume the patient is in A pace, V sense during DDD mode. 

Few days ago the patient came back to the emergency room for palpitations. ICD control showed normal parameters except for low atrial sensing (1,4 mV, previous control 3,6 mV). 

RX evalutation showed atrial lead floating in the RV..