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282 P R I N C I P L E S A N D P R A C T I C E O F C R I T I C A L C A R E

procedure and have all necessary preparatory tasks including patients with and without non sustained
explained to them. VT. 99,100 In these contexts patients may receive pure ICDs,
or ICDs coupled with cardiac resynchronisation therapy
The cardioversion team should include a minimum of
one medical officer, skilled in emergency rhythm man- capabilities to also combat their heart failure (CRT-D
agement and airway management including intubation, devices).
and two critical care nurses, who usually prepare the The modern ICD features both antibradycardia and anti-
patient and equipment, assist in sedation, perform the tachycardia capabilities. As antibradycardia devices they
cardioversion, document events and manage aftercare. possess all the characteristics of standard dual chamber
Often there is a cardiologist and anaesthetist present for pacemakers, increasingly in the DDD mode. However, if
the separate roles. All team members should confirm there is no history of bradycardia then they may be pro-
readiness, confirm synchronisation selection, and correct grammed at low base pacing rates (e.g. 40/min). If there
defibrillator energy settings (in joules). The patient is is significant heart failure the antibradycardia arm may
sedated (e.g. midazolam) or anaesthetised (e.g. propo- be provided as biventricular pacing (to achieve cardiac
fol), preoxygenated on 100% oxygen delivered by bag resynchronisation). Antitachycardia features are those
and mask, and cardioverted under ECG and oximetry therapies provided to treat ventricular tachyarrhythmias
monitoring. Electrical safety, and ensuring that all per- and include antitachycardia pacing (ATP), also termed
sonnel are clear of the bed, is the primary responsibility overdrive pacing, as well as cardioversion (for VT) and
of the nurse delivering cardioversion, whether via paddles defibrillation (for very fast VT or VF).
or hands-free electrodes.
Devices are inserted in a similar fashion to the pacemaker
After the procedure the patient should be closely moni- (see above on permanent pacemakers). However, ICDs
tored for return to wakefulness, airway protection capa- are most commonly positioned in the left subclavian/
bility, effective respiration and gas exchange, rhythm pectoral location, leaving the right side available for
stability, blood pressure, and for any changes in neuro- conventional placement of external defibrillator paddles
logical status or peripheral pulses. Pain and inflamma- should they ever become necessary. Atrial and ventricular
tion at cardioversion discharge sites may be lessened by leads are placed transvenously via the left subclavian vein.
application of topical ibuprofen 5% cream 2 hours before Atrial leads are normal atrial pacing leads, but the ven-
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elective DCR, where this is feasible. Energy require- tricular ICD lead differs from a standard pacing lead. ICD
ments for reversion of atrial tachycardia or flutter may be leads are slightly larger and carry the normal ventricular
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as little as to 50 J. The 2010 recommendations of the pacing circuitry, as well as coils encircling the lead that
European Resuscitation Council are for initial shocks at emit the high energy shock discharges. Single coil systems
70–120 J (biphasic) for atrial flutter, and 120–150 J for have one coil positioned on the lead at the level of the
cardioversion of atrial fibrillation and ventricular tachy- right ventricular cavity, and shocks travel from this coil to
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cardia. In any of the arrhythmias, if initial shocks are the metal casing of the ICD. Dual coil leads feature this
unsuccessful, repeat attempts at higher energy settings same right ventricular coil as well as a second coil in the
(up to 360 J) may be undertaken. Prior to discharge, superior vena cava. In these systems, shocks can be con-
patients and their families should be informed of the figured to travel from the RV coil to the superior vena cava
potential for post-procedural chest wall discomfort and (SVC) coil, from the RV coil to the ICD, or from the RV
topical and oral analgesic advice provided. Relevant coil to both the SVC coil and the ICD. Configurations can
contact information in the event of redevelopment of impact significantly on the defibrillation threshold, and
arrhythmia symptoms or other health concerns should changes to the shock vector may be undertaken for
also be provided. patients with high defibrillation thresholds.
All modern ICDs provide biphasic shock waveforms only.
IMPLANTABLE CARDIOVERTER Arrhythmia detection and classification usually requires
DEFIBRILLATORS only a few seconds, and charging to maximum joules
Implantable cardioverter defibrillators (ICDs) may be in a new device takes up to 10 seconds. As the battery
implanted for survivors of sudden cardiac death (SCD) declines charge time may increase to 15–20 seconds or
or haemodynamically significant, potentially lethal, ven- longer. Maximum energy delivery capabilities vary
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tricular arrhythmias. They have been repeatedly demon- between manufacturers but are all in the range of 30–40
strated in large clinical trials to provide significantly J. Typically, shocks for ventricular fibrillation are pro-
improved survival compared with conventional or phar- vided at the maximum available capability of the device,
macological treatment. 95-97 This ‘secondary prevention’ but for ventricular tachycardia, lower ‘cardioversion’
application of ICDs dominated the early indications for shocks may be attempted first (e.g. 15–25 J). If initial
devices, with trial meta analysis demonstrating a mean shocks are unsuccessful, devices are usually programmed
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27% mortality reduction compared to antiarrhythmics. to increase to maximum joules for subsequent shocks.
However, more recently indications have expanded to Defibrillation thresholds may be measured at the time of
‘primary prevention’ in patients without prior cardiac implantation of the ICD. It is desirable that a 10 J safety
arrest, as it has become clear that heart failure patients margin exists, i.e. for a device that can deliver 30 J, it is
with ejection fractions <30% (including both ischaemic preferred that successful defibrillation can be achieved
and non-ischaemic cardiomyopathies) have a high risk at 20 J or less so that there can be confidence that the
of sudden cardiac death due to ventricular arrhythmias, device will revert clinical arrhythmias, and to cover any