Subcutaneous ICD (S-ICD)

Advantages:

Avoids complications of transvenous leads:

Risks at the time of insertion – cardiac perforation, pericardial effusion, cardiac tamponade, hemothorax, pneumothorax

Delayed risks over the lifetime of the device – intravascular lead infection, lead failure

Components:

Pulse generator and shocking lead The pulse generator is implanted in a subcutaneous pocket in the left lateral,

mid-axillary thoracic position. The subcutaneous lead, which toward its terminal end contains an 8-cm

shocking coil electrode, is tunnelled from the pulse generator to a position along the left parasternal margin

There are proximal and distal sensing electrodes within the lead that flank the 8-cm shocking coil.

The distal electrode sits just below the sternal notch, and the proximal electrode lies just above the xiphoid process.

The cardiac rhythm is detected via a wide bipole between the two sensing electrodes or between one of the sensing electrodes and the pulse generator.

The device delivers an 80-Joule shock for defibrillation of ventricular tachyarrhythmias including monomorphic ventricular tachycardia (VT), polymorphic VT, and ventricular fibrillation (VF).

If VT or VF persists following the initial shock, the device will reverse polarity between the electrodes and deliver subsequent shocks.

The S-ICD will deliver a maximum of five shocks for a single episode of a ventricular arrhythmia.

If more than 3.5 seconds of asystole occurs following a shock, the S-ICD can deliver 30 seconds of demand pacing at a rate of 50 beats per minute.

During an event, the S-ICD will store the electrocardiogram (ECG) tracing for subsequent review

Patient Selection:S-ICDs are generally considered in the following situations:

●Younger patients due to the expected longevity of the implanted leads and a desire to avoid chronic transvenous leads, for example, in patients with hypertrophic cardiomyopathy, congenital cardiomyopathies, or inherited channelopathies.

●Candidates for an ICD without a current or anticipated need for pacing

●Patients at high risk for bacteraemia, such as patients on haemodialysis or with chronic indwelling endovascular catheters.

●Patients with challenging vascular access or prior complications with TV-ICDs

Complications:

Inappropriate shocks; incident range 4-16%; in most cases due to oversensing of T waves but can also be caused by sensing of myopotentials from chest muscle activity;

o A preimplantation ECG screening tool has also been developed to minimize the number of patients at risk for inappropriate shock due to T-wave oversensing errors, e.g. patients with large and/or late T-waves relative to the QRS

pocket haematoma (incidence rate 1-5%) pocket infection (incidence rate 1-10%); infections requiring device

explantation are rare lead dislodgement or migration (incidence rates 3-11%); typically, thought to

result from vigorous physical activity occurring without adequate fixation of the parasternal lead and requires reoperation to reposition the lead. In most patients, suture sleeves are now used to anchor the proximal segment of the parasternal lead.

skin erosion, premature battery depletion (battery usually lasts 5 years), or explantation due to need for anti-tachycardia/bradycardia pacing or a new indication for resynchronization therapy

Transvenous ICD (TV-ICD)

Indications

secondary prevention in patients with prior sustained ventricular tachycardia (VT), ventricular fibrillation (VF), or resuscitated sudden cardiac death (SCD) thought to be due to VT/VF (where a completely reversible cause cannot be identified)

secondary prevention in patients with episodes of spontaneous sustained VT in the presence of heart disease (valvular, ischemic, hypertrophic, dilated, or infiltrative cardiomyopathies)

primary prevention in patients at increased risk of life-threatening VT/VF who have optimal medical management (including use of beta blockers and ACE inhibitors)

o Patients with a prior myocardial infarction (at least 40 days ago) and left ventricular ejection fraction ≤30 percent.

o Patients with a cardiomyopathy, New York Heart Association functional class II to III, and left ventricular ejection fraction ≤35 percent.

o Patients with syncope who have structural heart disease and inducible sustained VT or VT going to VF on electrophysiology study

o Select patients with certain underlying disorders who are deemed to be at high risk for life-threatening VT/VF: congenital long QT, HOCM, Brugada, arrhythmogenic right ventricular cardiomyopathy

Components

Consists of a pulse generator, pacing and sensing electrodes, defibrillation electrodes

Contemporary ICD systems use a "coil" of wire that extends along the ventricular lead as the primary defibrillation electrode, so a single lead can accomplish all pacing, sensing and defibrillation functions.

the metal housing of the ICD serves as one of the shocking electrodes

most commonly implanted on the left side in the pectoral region either subcutaneously or sub-muscularly with one to three leads placed transvenously via the axillary, subclavian or cephalic vein

if a patient with an ICD develops VT, the ICD will first employ anti-tachycardia pacing (delivery of short bursts (e.g., eight beats) of rapid ventricular pacing to terminate VT); this is painless

if this fails, a synchronised shock will be delivered (i.e. cardioversion); this requires low output in the range of 10 joules or less (and often less than 2 joules)

for VF or very rapid VT (HR > 200bpm) the device will deliver a shock initially at a lower energy and then at the maximum output of the ICD (30-35 joules); this is painful

all TV-ICD’s have pacing capabilities which is often necessary to treat post shock bradycardia