Cardiac implantable electronic devices

Cardiac implantable electronic devices (CIEDs) are battery-powered medical devices that help control and/or monitor arrhythmias in patients with conduction disorders or heart failure. CIEDs include artificial cardiac pacemakers, implantable cardioverter defibrillators (ICDs), cardiac resynchronization therapy (CRT) devices (biventricular pacemakers), and left ventricular assist devices for controlling arrhythmias, as well as implantable cardiac monitors (loop recorders) for monitoring. This article focuses primarily on pacemakers stimulating the myocardium for antibradycardia pacing, ICDs overstimulating or defibrillating for antitachycardia pacing, and biventricular pacemakers for cardiac resynchronization therapy between mismatched left and right ventricular contractions.

• Description: CIEDs are insertable battery-powered medical devices that help control and/or monitor arrhythmias in patients with conduction disorders or heart failure
• Types of CIED
  • For controlling arrhythmias
    • Antibradycardia pacing: artificial cardiac pacemakers
    • Antitachycardia pacing: ICDs
    • Cardiac resynchronization therapy for ventricular dyssynchrony (e.g., due to left bundle branch block and/or heart failure): biventricular pacemakers
  • For monitoring arrhythmias: implantable cardiac monitors (loop recorders)
  • For end-stage heart failure: left ventricular assist devices

Technical information for CIEDs

• Structure
  • A metal case (typically titanium) contains a battery-powered pulse generator with a computer
    • ∼ 5 cm across, although newer, much smaller versions exist
    • The battery life depends heavily on the working mode and underlying disease: Newer models last between 5 and 15 years.
  • Leads with electrodes connect the device to the myocardium.
    • The proximal end of the lead is connected to the CIED unit.
    • The distal end of the lead is implanted into the right atrium and/or the right ventricle.
    • Biventricular pacemaker: another lead is placed into the left ventricle via the coronary sinus
    • The distal end is fixed to the myocardium with a small screw or anchor to prevent lead displacement.

• Basic functions
  • Electrodes detect the heart's electrical activity and transmit it to the device impulse generator unit.
  • The impulse generator unit identifies arrhythmias.
  • The electrodes then transmit electric impulses to the myocardium, which results in contraction of the respective heart chamber (atrium and/or ventricle).
• Implantation
  • In most cases, the generator is implanted on the patient's nondominant side between the pectoral muscle and the muscle fascia.
  • Electrodes are inserted into the right cardiac cavity, usually transvenously, via the subclavian and brachiocephalic veins.

Comparison of pacemakers, ICDs, and CRTs

Antibradycardia pacing can be achieved using an artificial cardiac pacemaker, which is capable of monitoring the cardiac rhythm and delivering electrical impulses to maintain an adequate heart rate (e.g., in severe bradycardia) or synchronize the ventricles (e.g., in atrioventricular block) when needed. While there are various types of pacemakers, most modern pacemakers have multiple functions and can be combined with a cardioverter-defibrillator (i.e., an ICD) .

• Structure and implantation: See “Technical information for CIEDs.”
• Types of pacemakers
  • Single-chamber pacemaker: one electrode lead in the right atrium or ventricle
  • Dual-chamber pacemaker: two electrode leads, one in the right atrium and one in the right ventricle
  • Triple-chamber pacemaker (also known as a biventricular pacemaker or CRT-P): electrode leads in the right atrium, right ventricle, and left ventricle (via the coronary sinus) for CRT
• Indications
  • Symptomatic bradycardia (syncope, Stokes-Adams syndrome, cardiac insufficiency)
  • Sick sinus syndrome
  • Third-degree AV block, second-degree AV block, Mobitz II
  • Carotid sinus syndrome
  • Sinoatrial block
  • Atrial fibrillation that presents as bradycardia
  • Transient emergency pacemaker (myocardial infarction, digitalis toxicity)

Antitachycardia pacing can be achieved via cardioversion, including mechanical cardioversion (precordial thump), pharmacological cardioversion (antiarrhythmic drugs), or electrical cardioversion (ICD). Modern ICDs are capable of multiple forms of pacing, as well as electrical cardioversion, both synchronized or unsynchronized (commonly known as defibrillation).

• Structure and implantation: ICDs consist of a subcutaneous generator connected to 1–3 lead wires, which are placed in the myocardium. ^[1] See “Technical information for CIEDs.”
• Types of ICDs
  • Several variations exist: may include a single lead (right ventricle), two leads (right atrium and right ventricle), or three leads (right atrium and both ventricles)
  • Transvenous ICD: Leads for single-chamber or dual-chamber transvenous ICDs are usually implanted in the right side of the heart via the axillary or subclavian vein.
    • Advantages: capable of antitachycardia pacing, cardioversion, and pacing in bradycardia
    • Disadvantages: in addition to potentially difficult venous access, more susceptible to severe complications, including pneumothorax, cardiac tamponade, and higher risk of infection
  • Subcutaneous ICD: Sensing electrodes and shocking coils are positioned subcutaneously.
    • Advantages: less risk of complications than transvenous lead implantation
    • Disadvantages: not capable of antitachycardia pacing or bradycardia pacing
  • Biventricular ICD (ICD combined with a biventricular pacemaker): also called CRT-D; see “Cardiac resynchronization therapy.”
• Function: synchronized and/or unsynchronized cardioversion
  • Synchronized cardioversion: A low-energy shock is delivered at or just after the peak of the R wave of the QRS complex.
    • Indications: tachycardia with evidence of hemodynamic instability (e.g., hypotension, altered mental status, chest pain, other signs of shock) and a palpable pulse
  • Unsynchronized cardioversion (defibrillation): A high-energy shock is delivered to the precordium but not timed with any part of the cardiac cycle.
    • Indications: electrical activity is too rapid for synchronized cardioversion (ventricular fibrillation, pulseless ventricular tachycardia) or synchronized cardioversion is not possible (e.g., polymorphic ventricular tachyarrhythmia)
  • Typically more effective than pharmacological cardioversion with antiarrhythmic drugs
• Indications ^[1]
  • Primary prophylaxis
    • Congestive heart failure with certain criteria (see “Implantable cardioverter defibrillator” in “Congestive heart failure”)
    • Increased risk of life-threatening cardiac arrhythmias
      • Arrhythmogenic right ventricular cardiomyopathy
      • Hypertrophic obstructive cardiomyopathy
      • Brugada syndrome
      • Long QT syndrome
  • Secondary prophylaxis: history of sudden cardiac arrest, ventricular flutter, or ventricular fibrillation

Cardiac resynchronization therapy (CRT) aims to synchronize left- and right-ventricular contractions using a biventricular pacemaker, which improves cardiac function.

• Structure and implantation: A left ventricular electrode is inserted transvenously into the coronary sinus (via the vena cava and right side of the heart) to innervate the posterior wall of the left ventricle. See “Technical information for CIEDs.”
• Types of CRT: A biventricular pacemaker has leads in the right atrium, right ventricle, and left ventricle (via the coronary sinus), which allows pacing in a coordinated manner.
  
  • System types
    • CRT-P: comprises a biventricular pacemaker; used for resynchronization and pacing
    • CRT-D: comprises a biventricular pacemaker and an ICD; used for resynchronization, pacing, and defibrillation
• Function: In CRT, electrodes stimulate the right and left ventricle at the same time to achieve synchronous contractions, resulting in:
  • Reduction of secondary mitral regurgitation
  • Reversal of ventricular remodeling
  • Improved ventricular function ^[2][3]
• Indications: Pacemakers that only stimulate the right ventricle can cause asynchronous left ventricular contractions. As this form of iatrogenic left bundle branch block can lead to hemodynamic problems if the LVEF is limited, CRT is preferred in patients with low LVEF or preexisting LBBB.
  • Left bundle branch block
  • Dilated cardiomyopathy
  • Congestive heart failure with EF < 35% and QRS duration of > 150 ms or require pacing (see “Cardiac resynchronization therapy” in “Congestive heart failure” for more details)

X-rays should be conducted to control for lead displacement or complications such as myocardial perforation, pneumothorax, or disconnection between the device and leads.

Proper CIED positioning

• Single-chamber pacemaker or ICD : one electrode lead in the right atrium or right ventricle
  
• Dual-chamber pacemaker or ICD : two electrode leads, one in the right atrium and one in the ventricle
  
• Triple-chamber pacemaker (biventricular pacemaker) : three electrode leads, one in the right atrium, one in the right ventricle, and one in the left ventricle via the coronary sinus
  

In contrast to pacemaker leads, ICD leads have thick shock coils that are visible on the radiograph.

Lead displacement

• Placement complications
  • Pneumothorax
  • Myocardial perforation, cardiac tamponade
  • Infection
  • Lead displacement or disconnection
    
• Pacemaker syndrome
  • May occur with VVI pacemakers
  • Causes include:
    • Atrioventricular dyssynchrony or retrograde atrial excitation that leads to atrial contraction against a closed AV valve
    • Interventricular dyssynchrony
  • Clinical features: hypotension, dizziness, dyspnea, palpitations, and syncope
• Pacemaker-induced reentry tachycardia: excitation of the atrium as a result of retrograde conduction via the AV node, which prematurely restimulates the ventricle
• Pacemaker malfunction is associated with symptoms ranging from bradycardia to asystole; causes include:
  • Electric fields (short-wave, high-frequency): produced during diathermy and interfere with pacemaker functionality
  • Magnetic fields, e.g., product by MRI
  • Displacement of pacemaker electrodes, or battery or cable defect: Tingling sensations in the arm may indicate displaced electrodes.

We list the most important complications. The selection is not exhaustive.

• Interference with MRI: Despite many CIEDs now being MRI safe, an electrophysiologist or the device manufacturer should be consulted before scanning.
• Surgical patients: Consider consulting an electrophysiologist regarding temporary CIED deactivation, as surgery involving diathermy can trigger the device to stimulate the heart unnecessarily. ^[4]
• Patient education
  • Driving advice
  • Education about possible interactions during medical procedures and with electronic devices ^[5][6]
• Discussions about device deactivation during end-of-life care
• Provision of a cardiac device wallet card and medical alert bracelet
• Regular follow-up is essential.

Reference: ^[1]