Novel automated score mapping of diaphragmatic compound motor action potential for the early detection of phrenic nerve injury during cryoablation

The proximity of the phrenic nerve to right-sided pulmonary veins makes it susceptible to injury during cryoballoon ablation (CBA) for atrial fibrillation (AF). Although there are no specific preprocedural predictors for phrenic nerve injury (PNI), monitoring of diaphragmatic compound motor action potentials (CMAP) has been used for the early detection of PNI. A 30% drop in CMAP amplitude has been shown in animal and clinical studies as the threshold for the prevention of PNI The aims of this study were to assess the relationship of change in CMAP amplitude with change in score map and then use the automated score map prospectively for early detection of PNI during CBA In the first half of the study, we paced the phrenic nerve at 10,8,6,4,2 milliamperes or till loss of capture in 20 pts. The relationship between the change in amplitude of CMAP with a corresponding change in the score map was studied by linear regression. 30% drop in CMAP amplitude was associated with a score map of 89%(R2 0.77). In the second half of the study, we used a score map threshold of 90% and studied 84 pts. Ablation was stopped if the score map of CMAP dipped below 90% consistently for >4 beats to take respiratory variation into account A total of 174 cryo applications were done for the right-sided veins in 84 pts. A consistent score-map >98% was obtained in all pts before the beginning of the CBA. Ablation had to be stopped in 13 pts due to a drop in the score map or CMAP amplitude or decreased diaphragm contraction. 5 pts had transient PNI. Change in score map was noticeable before change in CMAP amplitude or decrease in diaphragm contraction. In 8 pts drop in the score map was attributed to catheter movement which was noted by the change in catheter position on the 3D mapping system. Score-map of CMAP returned to >98% with catheter adjustment or within 5 minutes of ablation termination along with improvement of CMAP amplitude in all cases. No pts were noted to have persistent PNI at the end of the procedure Beat-to-beat changes in CMAP amplitude can be noted with respiratory variation or catheter movement making it difficult to monitor the change in amplitude. By using an automated score rather than continuously measuring CMAP amplitude, score mapping can make monitoring changes in CMAP amplitude easier. This descriptive study demonstrates that a 90 % score-map threshold correlates with a 30% drop in CMAP amplitude and can be safely and reliably used for the early detection of PNI