Ventricular defibrillators

Demand pacemakers are very low current devices, requiring only 25-50 jiW for sensing and 60-100 pW for stimulation. In contrast, implanted ventricular defibrillators (Fig. 1.3) must be able to deliver short electric pulses of 25-40 J (e.g. 2 A at 2 V for 10 s) which can shock the heart into normal rhythm, and hence require a much higher rate battery. The most common system is a lithium-silver vanadium oxide cell with a liquid-organic based electrolyte. More than 80 000 such units have been implanted. Implanted drug delivery devices also use lithium primary batteries, as do neurostimulators and bone growth stimulators. 

Optimal shock intensity for defibrillation is also controversial because in adult patients stronger shocks have a modestly higher success rate, but probably are more likely to damage the myocardium. Accordingly, experts and users take the position that a weaker first shock should be used for cardioversion, with gradual increase until success occurs. However, for the emergency situation of ventricular defibrillation, the best sequence (strength of first shock and extent of increases with subsequent shocks) has not yet been determined. 

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Zhou X, Daubert JP, Wolf PD, Smith WM, Ideker RE. Epicardial mapping of ventricular defibrillation with monophasic and biphasic shocks in dogs. Cite Res 1993 72 145-60. 

Jones DL, Sohla A, Bourland JD, Tacker WA, Kallok MJ, Klein GJ. Internal ventricular defibrillation with sequential pulse countershock in pigs comparison with single pulses and effects of pulse separation. Pacing Clin Electrophysiol 1987 10 497-502. 

Zhang Y, Boddicker KA, Davies LR, Jones JL, Kerber RE. Surgical open-chest ventricular defibrillation triphasic waveforms are superior to biphasic waveforms. Pacing Clin Electrophysiol 2004 27 941-8. 

Chattipakom N, Banville L Gray RA, Ideker RE. Effects of shock strengths on ventricular defibrillation failure. Cardiovasc Res 2004 61 39-44. 

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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. 

The ICD generator communicates with the heart through a ventricular defibrillator + atrial and LV electrode or "lead." The leads are connected to the ICD via a "header." The header provides holes for insertion of all aspects of the ventricular defibrillator lead and, if also needed, an atrial and LV lead in specific ICD models. As in the pacemaker, set screws in the header may be tightened to fix the leads in place or loosened to allow their removal. 

A contemporary ventricular defibrillator lead consists of multiple internally separated metal wires that are externally encased in silicone rubber or polyurethane insulation. This allows the lead to function similarly to a standard pacemaker lead, i.e. transmission of electrical pacing/sensing signals between the heart and generator, but also structurally provides a separate pathway that participates in the delivery of shocks. This pathway includes what are commonly referred to as the lead "coil(s)." Shocks are delivered across the heart between the coil(s) and potentially the ICD can. Each element of the lead has its own pin that may connect to the ICD header. 

A pacing impedance level of about ventricular defibrillator lead. 

As an SVT causes signal input below the atrio-ventricular (AV) node at progressively more rapid rates one of the bundle branches may cease conducting temporarily due to its refractory period being reached. What can happen then is the R wave morphology recorded by the ventricular defibrillator lead may change due to the alteration in ventricular activation. 

Management Solution Decrease ventricular sensitivity without compromising VF detection. Rarely the ventricular defibrillator lead might even be repositioned to minimize the T wave amplitude on the VEGM. 

H 1. 5-French atrial pacemaker lead I 2. 7-French atrial pacemaker lead B 3. Coronary sinus left ventricular lead F 4. Endocardial rate/sensing lead G 5. Epicardial defibrillator patch D 6. Right ventricular defibrillator lead C 7. Screw-in right ventricular pacemaker lead A 8. Surface ECG lead... 

Contact of a ventricular defibrillator and pacemaker lead near/close to their tips can produce that may be sensed as events. 

A ventricular defibrillator lead tip placed at a distance away from the RV apex can result in an elevated. ... 

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