Defibrillation threshold

In general, drugs that block the sodium channel and shorten the action potential tend to increase the defibrillation threshold. Drugs that prolong repolarization also tend to decrease this threshold. These changes have obvious important ramifications for patients with ICDs. 

Low doses (100-200 mg/d) of amiodarone are effective in maintaining normal sinus rhythm in patients with atrial fibrillation. The drug is effective in the prevention of recurrent ventricular tachycardia. It is not associated with an increase in mortality in patients with coronary artery disease or heart failure. In many centers, the implanted cardioverter-defibrillator (ICD) has succeeded drug therapy as the primary treatment modality for ventricular tachycardia, but amiodarone may be used for ventricular tachycardia as adjuvant therapy to decrease the frequency of uncomfortable cardioverter-defibrillator discharges. The drug increases the pacing and defibrillation threshold and these devices require retesting after a maintenance dose has been achieved. 

The effects of amiodarone appeared to be related to exacerbations of ventricular tachycardia and an increased defibrillation threshold. 

Lidocaine has been shown to increase VF threshold in both the CPR and non-CPR settings. Furthermore, it benefits the defibrillation threshold or the amount of energy required to convert VF to a more stable rhythm. Although controversial, some studies show that lidocaine has a detrimental effect on defibrillation threshold. These conflicting results may be related to drug interactions with fidocaine and the agents used for anesthesia. ... 

Fig. 8.11 Percent probability for successful defibrillation versus shock energy. The measured safety margin is the difference between the maximum output of the device and the measured DPT. The actual safety margin is the difference between the maximum output of the device and the energy required for consistent defibrillation success. ( upper comer, approximately the energy with 95% success rate.) (From Singer I, Lang D. Defibrillation threshold clinical utility and therapeutic implications. PACE 1992 15 932-949, with permission.)...

Saksena S. Defibrillation thresholds and perioperative mortality associated with endocardial and epicardial defibrillation lead systems. The PCD investigators and participating institutions. Pacing Clin Electrophysiol 1993 16 202-7. 

Singer I, Lang D. Defibrillation threshold clinical utility and therapeutic implications. Pacing Clin Electrophysiol 1992 15 932-49. 

Marinchak RA, Friehling TD, Kline RA, Stohler J, Kowey PR. Effect of antiarrhythmic drugs on defibrillation threshold case report of an adverse effect of mexUetine and review of the Uteratnre. Pacing Clin Electrophysiol 1988 11 7-12. 

Huang J, Skinner JL, Rogers JM, Smith WM, Hohnan WL, Ideker RE. The effects of acute and chronic amiodarone on activation patterns and defibrillation threshold during ventricnlar fibrillation in dogs. J Am Coll Cardiol 2002 40 375-83. 

Shepard RK, DeGroot PJ, Pacifico A, Wood MA, Ellenbogen KA. Prospective randomized comparison of 65%/65% versus 42%/42% tilt biphasic waveform on defibrillation thresholds in humans. J Interv Card Electrophysiol 2003 8 221-5. 

Gurevitz OT, Friedman PA, Glikson M, Trusty JM, Ballman KV, Rosales AG, Hayes DL, Hammill SC, Swerdlow CD. Discrepancies between the upper limit of vulnerability and defibrillation threshold prevalence and clinical predictors. J Cardiovasc Electrophysiol 2003 14 728-32. 

Is any other chest radiographic abnormality potentially related For a recent implantation, be certain there is no pneumothorax or hemopneumothorax. For the patient with an implantable cardioverter-defibrillator who has a change in defibrillation thresholds, whether acute or chronic, remember that a pneumothorax can be responsible for alterations in thresholds. 

Fig. 18.23 Posteroanterior (A) and lateral (B) chest radiographs from a patient with an implantable cardioverter-defibrillator. Unacceptable defibrillation thresholds necessitated placement of a subcutaneous array.

Ventricular stimulation by NIPS is done using the same protocols that are employed for standard invasive electrophysiologic testing (46). If ventricular tachycardia is induced, the effectiveness of ATP protocols is assessed. At the end of the study the ability of the ICD to detect and treat ventricular fibrillation can be evaluated. Ventricular fibrillation can usually be induced by ultrarapid ventricular pacing (30-50 stimuli/s) or an appropriately timed low-energy shock on the T wave. The defibrillation threshold can be evaluated using the methods outlined in Chap. 8. 

Venditti FJ, Martin DT, Vassolas G, and Bowen S. Rise in chronic defibrillation thresholds in nonthoractomy implantable defibrillator. Circulation 1994 89 216-223. 

Schwartzman D, Callans DJ, Gottlieb CD, Heo J, and Marchlinski FE. Early postoperative rise in defibrillation threshold in patients with nonthoracotomy defibrillation lead systems attenuation with biphasic shock waveforms. J Cardiovasc Electrophysiol 1996 7 483-493. 

Olsovsky MR, Pelini MA, Shorofsky SR, and Gold MR. Temporal stabihty of defibrillation thresholds with an active pectoral lead system. J Cardiovasc Electrophysiol 1998 9 240-244. 

Gold MR, Khalighi K, Kavesh NG, Daly B, Peters RW, and Shorofsky SR. Clinical predictors of transvenous biphasic defibrillation thresholds. Am J Cardiol 1997 79 1623-1627. 

Stepheson EA, Cecchin F, Walsh EP, and Berul CL Utility of routine follow-up defibrillator threshold testing in congenial heart disease and pediatric populations. J Cardiovasc Electrophysiol 2005 16 69-73. 

Hohnloser SH, Dorian P, Roberts R, Gent M, Israel CW,Fain E, Champagne J, and Connolly SJ. Effect of amiodarone and sotalol on ventricular defibrillation threshold the optimal pharmacological therapy in cardioverter defibrillator patients (OPTIC) trial. Circulation 2006 114 104-109. 

Systemic reactions to local anesthetics including cardiac arrhythmia, bradycardia, cardiovascular collapse, increased defibrillator threshold, and heart block can occur. 

As suggested earlier the proximal coil in a dual coil lead is optimally positioned in the SVC. Some patients hearts may be so large that the proximal coil would not wind up there. One way to manage this situation, when a dual coil system is initially desired, is to rather utilize a single coil lead, but adding a lead called an "SVC coil" to the system. This is a separate lead that is connected to the proximal shocking coil port of the ICD and is independently positioned to float in the SVC. In other patients who develop an elevated defibrillation threshold (DFT) after initial implant of only a single coil lead, the SVC coil may also be subsequently added. 

The biphasic defibrillator delivers two currents of electricity and lowers the defibrillation threshold of the heart muscle, making it possible to successfully deflbrillate VF with smaller amounts of energy. 

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