Implantable cardiac defibrillators

Regarding implantable cardiac defibrillators, which one of the following statements is TRUE?

a) An implantable defibrillator must be turned off before surgery involving diathermy

b) If the indifferent grounding pad is greater than 15 cm from the defibrillator, the risk from unipolar diathermy electrocautery is eliminated

c) An internal cardioversion shock of two joules will cause painful skeletal and diaphragmatic contraction in the awake patient

d) External cardiac pacing is contraindicated in the presence of implantable defibrillator leads

e) In approaching 90% of cases, a functioning implantable defibrillator will successfully terminate a malignant arrhythmia within 15 seconds

Answer: c

Explanation

An implantable cardiac defibrillator (ICD) consists of a pulse generator and leads embedded in the endocardium and responsible for detection of a malignant tachyarrhythmia and delivery of the shock to terminate it. This function may be its only responsibility or it may complement other pacemaking functions. If the unit functions as a pacemaker as well, turning it off prior to surgery may result in reversion to the intrinsic bradyarrhythmia with deleterious consequences. Instead the unit should be reprogrammed to a safer, fixed VVI mode. If it functions solely as a defibrillator, it may be turned off pre-operatively. In this circumstance, pharmacological therapy and external pacing pads should be available for use in the event of malignant tachyarrhythmia. External pads should be placed at least 10 cm from the pulse generator and leads. Unipolar electrocautery may be used but the indifferent pad should be as far as possible (at least 15 cm) from components of the implanted system. The surgeon should be advised to use low-energy, short bursts to minimise electromagnetic interference or induction. Alternatives such as bipolar or ligation techniques should be considered

where possible. An internal shock of two joules will cause the symptoms described in the awake patient. For asynchronous defibrillation of ventricular fibrillation energies of 10 to 40 joules may be used. Implantable cardiac defibrillator batteries may contain 20 000 joules of energy. The efficacy of ICDs is higher than quoted in Option (e), approaching 100% in 5 to 15 seconds.

Reference

Bukhari A, Gars S, Mehta Y. Anaesthetic management of patients with implantable

cardioverter defibrillator. Ann Card Anaesth 2005; 8(1): 61