High defibrillation threshold: the science, signs and solutions

Abstract

Defibrillation threshold (DFT) testing has traditionally been an integral part of implantable cardioverter defibrillator (ICD) implantation. With the increasing number of patients receiving ICDs, physicians are encountering high DFT more often than before. Tackling the problem of high DFT, warrants an in-depth understanding of the science of defibrillation including the key electrophysiological concepts and the underlying molecular mechanisms. Numerous factors have been implicated in the causation of high DFT. Due consideration to the past medical history, pharmacotherapy, laboratory data and cardiac imaging, help in assessing the pre-procedural risk for occurrence of high DFT. Drugs, procedural changes, type and location of ICD lead system are some of the key players in predicting DFT during implantation. In the event of encountering an unacceptably high DFT, we recommend to follow a step-wise algorithm. Ruling out procedural complications like pneumothorax and tamponade is imperative before embarking on a search for potentially reversible clinical or metabolic derangements. Finally, if these attempts fail, the electrophysiologist must choose from a wide range of options for device adjustment and system modification. Although this review article is meant to be a treatise on the science, signs and solutions for high DFT, it is bound by limitations of space and scope of the article.

Keywords: DFT; Defibrillation threshold testing; implantable cardioverter defibrillator.

Figure 1

Flowchart describing the interplay of electrical, electrophysiological, molecular and anatomical factors that favours high Defibrillation threshold (DFT). Key electrical parameters that influence the DFT are voltage and the duration it is being applied. The device related factors are the capacitance of the device and the impedance of the coil-tissue composite. The shock voltage - duration graph shows the relationship of the capacitance and voltage in relationship to the transmembrane response (TMR). 'Wasted energy' is the component of the delivered energy which is counterproductive when the duration of application is beyond the peak TMR, particularly with high capacitance energy devices. (Note the inverse relationship of the initial voltage and the capacitance of the ICD). ICD's (implantable cardioverter defibrillator) programmable features, if not appropriately programmed will alter the shock vector and thus can influence the DFT. Antiarrhythmics and other drugs can directly and indirectly affect the DFT. Cardiac pathology like MI or medications can affect the ionic mechanisms responsible for the membrane stability. This can increase the arrhythmogenic potential and can influence the DFT. Several mechanisms are still investigational or has conflicting study results and hence marked with '?'. Other pathophysiological and anthropometric factors are also included for completion. (LVEF-left ventricular ejection fraction, ULV-upper limit of vulnerability, BMI-body mass index, CTR-cardiothoracic ratio, MI-myocardial infarction, CHF- congestive heart failure, ∆Vm- change in transmembrane potential, VF- ventricular fibrillation & APD - action potential duration). Cutaway view of the ICD: Image reproduced with permission from St Jude medical. Inc.