Achieving sufficient safety margins with fixed duration waveforms and the use of multiple time constants

There are several options to achieve a sufficient safety margin in a patient with a high defibrillation threshold (DFT), with varying and typically modest success. Programming fixed (millisecond) durations of both phases of a biphasic waveform in an implantable cardioverter defibrillator (ICD) has demonstrated utility.

We established an informal multisite registry of ICD implanting facilities. Each facility agreed to attempt the use of fixed duration waveforms whenever there was an inadequate safety margin with tilt-based waveforms. A 3.5-ms-based fixed duration shock was tried first. If that failed to achieve a 10-J safety margin then a 2-ms-based shock was used. We also tabulated an HEDFT (high estimate DFT) as precise DFTs were not determined.

Sixteen patients (15 M, 1 F) were entered into the registry (age 58.2 +/- 17.9 years) with ejection fractions of .30 +/-.11. Superior vena cava coils were used in 7 patients according to physician preference. The tilt-based HEDFTs were 35.4 +/- 3.2 J delivered and 35.8 +/- 3.3 J stored energy. The 3.5-ms based shocks were evaluated on 14 patients and the HEDFT fell to 23.4 +/- 6.3 J delivered (P < 0.0001) and 26.2 +/- 6.9 J stored energy (P < 0.0001). The 2-ms-based fixed duration shocks were then evaluated on 6 patients and the delivered energy HEDFT was 22.2 +/- 5.8 J (P = 0.001 vs. tilt-based shocks) while the stored energy HEDFT was 27.9 +/- 6.4 J (P = 0.01 vs. tilt-based shocks). Using the better of the two fixed duration waveforms, the mean safety margin was improved from -1.2 +/- 1.9 J to 9.5 +/- 5.9 J (P < 0.00001). Multivariate predictors of the safety margin improvement were the absence of the Superior Vena Cava (SVC) coil and absence of Ventricular fibrillation (VF) presentation. Four patients still required lead repositioning after the use of the fixed duration waveforms. No additional leads were implanted.

The use of a selection of directly programmed fixed duration biphasic shocks had a striking impact on the HEDFT for these difficult patients. Adequate safety margins were obtained for 12 of 16 patients with no lead manipulation or other approaches.