Pacemakers and Implantable Cardioverter Defibrillators

Introduction

Cardiac implantable electronic devices fall into three broad groups, and it is worth being clear about which problem each solves, because they are frequently confused.

• Pacemakers treat a rate that is too slow. They deliver an impulse when the heart fails to generate one.
• Implantable cardioverter defibrillators (ICDs) treat rhythms that are too fast and life-threatening. They detect ventricular tachycardia and fibrillation and terminate them. All ICDs can also pace for bradycardia.
• Cardiac resynchronisation therapy (CRT) treats mechanical dyssynchrony in heart failure. By pacing both ventricles it coordinates their contraction, improving symptoms and survival.

The categories combine: a CRT-D is a resynchronisation device with defibrillator capability, and a CRT-P provides resynchronisation pacing only. Over 50,000 devices are implanted annually in the UK, so students will meet these patients regularly, and being able to identify a device on a chest X-ray and interpret a paced ECG is a realistic examination expectation.

How a pacemaker works

A conventional transvenous system consists of a pulse generator - a titanium case containing a lithium battery and circuitry, implanted subcutaneously below the left clavicle - connected to one or more leads passed through the subclavian or cephalic vein into the right heart.

• Single-chamber - one lead, in either the right atrium or the right ventricle
• Dual-chamber - two leads, one in the right atrium and one in the right ventricle, allowing atrioventricular synchrony to be preserved
• Biventricular (CRT) - three leads, adding one passed through the coronary sinus to pace the lateral wall of the left ventricle
• Leadless pacemakers - a self-contained capsule implanted directly into the right ventricle via the femoral vein, avoiding leads and a subcutaneous pocket, though currently limited to single-chamber ventricular pacing
• Subcutaneous ICD (S-ICD) - electrodes placed outside the thorax with no intravascular components, useful where venous access is difficult or infection risk is high, but unable to provide anti-tachycardia or bradycardia pacing

The pacing code

Devices are described by a standard three- to five-letter code. The first three letters are the ones to know.

Indications

Permanent pacing

• Complete (third degree) heart block - almost always, unless a reversible cause is identified
• Mobitz type II second degree AV block - even if asymptomatic, because progression is unpredictable
• Symptomatic sick sinus syndrome, including chronotropic incompetence
• Tachy-brady syndrome - to allow rate-controlling drugs to be given safely
• Symptomatic bradycardia from a cause that cannot be removed, for example essential beta-blockade after myocardial infarction
• Persistent AV block after anterior myocardial infarction
• Bifascicular or trifascicular block with syncope, or with documented intermittent complete heart block
• Recurrent syncope from carotid sinus hypersensitivity with a cardioinhibitory response
• After AV node ablation for refractory atrial fibrillation - the patient becomes pacemaker dependent

Implantable cardioverter defibrillators

An ICD is not indicated where an arrhythmia had a fully reversible cause, such as ventricular fibrillation within 48 hours of an acute infarct that has been successfully revascularised, or torsades from a drug that has been withdrawn. Nor is it appropriate where life expectancy from other causes is under a year, since it would prevent a sudden death only to permit a slower one.¹

Cardiac resynchronisation therapy

In heart failure with left bundle branch block, the left ventricular free wall contracts substantially after the septum. This dyssynchrony wastes contractile work and worsens mitral regurgitation. Biventricular pacing restores coordination.

Consider CRT in patients with symptomatic heart failure, an ejection fraction of 35% or below, and a QRS duration of 130 ms or more despite optimal medical therapy. The benefit is greatest with left bundle branch block morphology and a QRS above 150 ms, and is uncertain or absent where the QRS is narrow. Whether a defibrillator is added (CRT-D rather than CRT-P) depends on the arrhythmic risk and the aetiology.^1,2

The paced ECG

Pacing spikes are narrow, sharp vertical deflections, often only a millimetre or two tall and easily missed on a poor-quality trace.

Device malfunction

Any suspected malfunction requires device interrogation by the cardiac physiology service. In a patient who is pacemaker dependent, failure to output is a medical emergency requiring external pacing while the device is assessed.

Complications

Early (procedural)

• Pneumothorax - from subclavian puncture; a chest X-ray is performed routinely after implantation
• Pocket haematoma - commoner in anticoagulated patients
• Lead displacement - most likely in the first six weeks, before fibrosis anchors the lead. Patients are advised to avoid raising the arm above shoulder height on the implant side during this period.
• Cardiac perforation and tamponade - rare but life-threatening
• Arrhythmia during lead placement
• Diaphragmatic or pectoral stimulation - causing hiccups or twitching, corrected by reprogramming

Late

• Infection - of the pocket or, more seriously, of the leads causing device-related infective endocarditis. This almost always requires complete extraction of the entire system, since antibiotics alone rarely clear infection on a foreign body. Fever in a patient with a device should always prompt consideration of this.
• Erosion of the generator through the skin
• Lead fracture or insulation failure
• Venous obstruction or thrombosis of the subclavian vein
• Tricuspid regurgitation - from a lead crossing the valve
• Pacemaker syndrome - loss of AV synchrony in single-chamber ventricular pacing
• Pacing-induced cardiomyopathy - a high burden of right ventricular pacing creates dyssynchrony resembling LBBB and can reduce ejection fraction over years. This is why modern devices are programmed to minimise unnecessary ventricular pacing, and why conduction system pacing is increasingly used.
• Battery depletion - generators typically last 6-15 years and are replaced in a straightforward procedure that leaves the leads in place
• Psychological effects - anxiety and depression are common, particularly after ICD shocks

Practical advice for patients

Red flags

Prognosis

Permanent pacing for complete heart block restores life expectancy close to that of the general population and produces a marked improvement in symptoms and quality of life. It is among the most reliably effective interventions in cardiology.

ICDs reduce mortality substantially in appropriately selected patients, both for secondary prevention after a survived arrhythmic arrest and for primary prevention in those with significantly impaired ventricular function. The benefit depends heavily on selection: patients who are likely to die of progressive heart failure or non-cardiac disease derive little advantage, since the device converts a sudden death into a slower one.

Cardiac resynchronisation therapy improves symptoms, ejection fraction, hospitalisation rates and survival in patients with heart failure and a broad QRS, with the greatest benefit in those with left bundle branch block and a QRS above 150 ms. Around a third of patients are non-responders, which is why patient selection and careful device optimisation matter.

Generators require replacement every 6-15 years depending on the device type and pacing burden, and each replacement carries a small additional infection risk. Long-term outcomes are otherwise excellent, and the great majority of patients live entirely normal lives with their device.