How to program pulse duration or tilt in implantable cardioverter defibrillators

Implantable intravascular defibrillator: evaluation of defibrillation waveforms with inferior vena cava electrode system

BACKGROUND

A percutaneously placed, totally intravascular defibrillator has been developed that shocks via a right ventricular (RV) single-coil and titanium electrodes in the superior vena cava (SVC) and the inferior vena cava (IVC). This study evaluated the defibrillation threshold (DFT) with this electrode configuration to determine the effect of different biphasic waveform tilts and second-phase durations as well as the contribution of the IVC electrode.

METHODS

Eight Bluetick hounds (wt = 30-40 kg) were anesthetized and the RV coil (first-phase anode) was placed in the RV apex. The intravascular defibrillator (PICD®, Model no. IIDM-G, InnerPulse Inc., Research Triangle Park, NC, USA) was positioned such that the titanium electrodes were in the SVC and IVC . Ventricular fibrillation was electrically induced and a Bayesian up-down technique was employed to determine DFT with two configurations: RV to SVC + IVC and RV to SVC. Three waveform tilts (65%, 50%, and 42%) and two second-phase durations (equal to the first phase [balanced] and truncated at 3 ms [unbalanced]) were randomly tested. The source capacitance of the defibrillator was 120 μF for all waveforms.

RESULTS

DFT with the IVC electrode was significantly lower than without the IVC electrode for all waveforms tested (527 ± 9.3 V [standard error], 14.5 J vs 591 ± 7.4 V, 18.5 J, P < 0.001). Neither waveform tilt nor second-phase duration significantly changed the DFT.

CONCLUSION

In canines, a totally intravascular implantable defibrillator with electrodes in the RV apex, SVC, and IVC had a DFT similar to that of standard nonthoracotomy lead systems. No significant effect was noted with changes in tilt or with balanced or unbalanced waveforms.

From defibrillation theory to clinical implications

BACKGROUND

Our defibrillation theory claims that the mean voltage threshold is a hyperbolic function of pulse duration and that voltages below rheobase should be avoided as being counterproductive. Truncation of the pulse just at rheobase level yields minimal stored energy thresholds. To verify or falsify this theory, animal experiments were carried out.

MATERIAL AND METHODS

In two animal experiments, 212 defibrillation thresholds in 22 swine were determined with different biphasic pulses of which 92 were optimally truncated in phase 1. Step-up test procedure was used with the first successful shock defined as "threshold."

RESULTS

Experimental proof is gained that truncation according to "rheobase condition" shows lowest stored energy. A ranking order of stored energy thresholds demonstrates that (1) lower output capacitances reduce needed energy, and (2) pulse durations shorter or longer than optimal increase needed energy. The voltage-pulse-content threshold is linearly correlated with pulse duration.

CONCLUSIONS

Truncation above or below rheobase increases the stored energy threshold. Voltage averaged during pulse duration is a hyperbolic function of pulse duration. The stored energy is reduced with decreasing output capacitance. The experimental results do not only fully verify our theory, they also suggest clinical implications: (1) the current usage of the "constant tilt concept" in implantable cardioverter defibrillator (ICD) should be abandoned in favor of "optimal truncation concept," (2) an algorithm developed for calculating optimal truncation proved to be useful so that incorporation into ICD for automatic adjustment is recommended, and (3) the output capacitance should be reduced from about 100 microF to 60 to 70 microF.

Parameters characterizing implantable defibrillator output: a proposal

AIMS

Recently, a discussion was carried out in Heart Rhythm on the specifications that could characterize implantable defibrillators. It is the intention of this paper to participate in this discussion on defibrillation characteristics and to give recommendations on how this problem could be solved. Theoretical considerations and results There are different defibrillation theories, all finding that the defibrillation's efficacy depends on the time constant RC which is output capacitance C times load resistance R. Efficacy decreases with increasing RC. This means that (i) the knowledge of C is of paramount importance, (ii) the energy is 'devalued' with increasing RC and that those parameter settings such as tilt or pulse duration should be adjusted to the time constant, and (iii) the energy values given without further specification are not meaningful. As there is always a voltage drop across an internal resistance within the ICD, the measured voltage across the output differs from the capacitor voltage and is reduced which determines the efficiency of the device. From the data given by Thammanomai et al., one can determine the parameters maximum voltage, capacitance, internal resistance, and tilt. These parameters are adequate and necessary to describe an ICD device and to derive the effective energy for device comparison. Discussion The 'high output devices' with their high nominal energy are reduced in their effective energies to a degree that they are comparable to the best 'standard output devices'. They do not offer that superiority which is promised by the nominal energy. Moreover, if the tilt is fixed and larger than optimal, the energy requirements are still higher or the effective energy will further drop. The term 'delivered energy' is not used by us because the delivered energy increases with increasing tilt. However, today's tilts are too large as judged by theories, which means that high delivered energies can be worse than lower ones. The delivered energy is, therefore, not a meaningful parameter in judging ICDs.

CONCLUSION

ICD devices should be characterized by: (i) voltage, (ii) capacitance, (iii) tilt or pulse duration (if not programmable), and (iv) internal resistance. All other parameters can be derived from them by simple calculations. Introduction of a 'devaluation factor' characterizes the decreasing efficacy with increasing time constant and renders the output characteristics transparent and comparable.