The effect of polarity of the initial phase of a biphasic shock waveform on the defibrillation threshold of pectorally implanted defibrillators.

The effect of initial phase polarity on the DFT of two pectorally implanted biphasic ICDs was tested in a randomized, prospective manner at the time of implantation. Twenty-two consecutive patients with VT-VF who received either the Medtronic PCD 7219C Jewel device (10 patients) or PCD 7219D Jewel device (12 patients) were studied. DFT testing was performed in a standard step-down manner. Both initial phase polarities--initial defibrillation current flowing from active can/SVC coil +/- subcutaneous patch) to the RV coil (RV coil to active can/SVC (RV+)--were tested in random order. The mean DFT achieved with RV+ compared with RV-was lower for the 7219C patient group (6.6 +/- 3.1 vs 10.8 +/- 5.5 J; P = 0.007). A similar trend was observed for the 7219D group, though the difference did not reach statistical significance (12.0 +/ 4.0 vs 16.3 +/- 7.3 J; P = 0.07). Seven of the 10 patients in the 7219C group had a lower DFT with RV+, while the initial phase polarity made no difference in 3. In the 7219D group, 7 patients had a lower DFT using RV+, 2 patients had a lower DFT using RV-, and the initial phase polarity made no difference in 3. In conclusion, this study demonstrates that changing the polarity of the initial phase of a biphasic shock waveform can have a significant impact on the DFT achieved at the time of ICD implantation.