[Untitled]

Background: The waveform tilt of bipha- sic shocks yielding the lowest defibrillation threshold (DFT) is not well defined. Some evidence indicates that tilts less than 65% may improve DFTs. Methods: In 57 patients undergoing ICD implanta- tion, DFTs were determined with truncated exponen- tial biphasic waveform tilts at 65%/65% and at 42%/42%. An external defibrillator with custom software was used for testing. The effective capacitance of the defibril- lator was 132-µF for both waveforms. DFTs were de- termined using a binary search method starting with 12 Joules (J). Patients were randomly assigned to ini- tial testing with either one of the two tilts. Thirty patients (Group 1) were tested with a two electrode (active can to RV coil, or SVC coil to RV coil) and 27 patients (Group 2) were tested with a three electrode system (subcutaneous patch or active can + SVC coil to RV coil). Results: Groups 1 and 2 did not differ in age, ejection fraction or antiarrhythmic medications. Group 1 deliv- ered energy DFTs were 10.1 ± 5.5 J with the 65%/65% tilt and 10.1 ± 5.9 J for the 42%/42% tilt (p = 0.92). In group 2 the average DFT for the 65%/65% tilt was 8.4 ± 5.7 J and for the 42%/42% tilt was 8.1 ± 5.3 J (p = 0.70). There were no significant differences in DFTs for either group. The system impedance for Group 1 was 64 ± 12 ohms and for Group 2 was 39 ± 6 ohms (p < 0.0001). Conclusions: We found no differences in DFTs bet- ween 65%/65% tilt and 42%/42% tilt using either 2- or 3-electrode defibrillation systems. Further research is needed to optimize waveforms in order to minimize DFTs, which will result in smaller ICDs and/or greater safety margins for defibrillation.