85. Pacemakers and Defibrilators- Review of CT Surgery

Velu Balasubramanian and Tony Panos

This chapter is a revision and update of that included in previous editions of the TSRA Review written by Giovanni Piovesana (2nd edition) and Keshava Rajagopal (1st edition). 

Definition and Function

A pacemaker (PM) is a temporary or permanent system used to electrically pace the heart in the setting of inadequacy of native rate and/or rhythm. An automatic implantable cardioverter-defibrillator (AICD) is a device implanted to electrically shock the heart out of unstable cardiac rhythms.

The basic structure of PM and AICD includes:

  • Pulse generator (implantable or external). “Can,” a metal shell (usually titanium) or an external box containing the power source (battery), the circuitry, and the outlets for the pacing leads.
  • Leads. Insulated wires (usually by silicone or polyurethane) connecting the generator to the heart. Unipolar leads have a platinum cathode at the tip and the generator functions as anode, whereas bipolar leads have both anode and cathode at the lead tip, separated by a distance of about 1 cm. Leads can be fixed to the heart passively (tines or talons) or actively (screw). Typically, they are placed via the endovascular route but occasionally they need to be placed on the epicardial surface surgically. Defibrillator leads consist of high voltage conductor coils.

Pacing threshold [measured in Volts (V)] and the pulse width [in milliseconds(ms)] is defined as the minimum amount of energy needed to be delivered in order to obtain an electrical response from the heart (“capture”). Ventricular thresholds should ideally be <0.7 V to obtain an R-wave amplitude >5 mV.

Sensing threshold is defined as the minimum myocardial voltage amplitude measured in milliVolts (mV), required to be detected by the pacemaker as a P or R wave. Range for a normal pacemaker is 0.4-10mV for the atria, and 0.8-20mV for the ventricles.

For intramyocardial placed electrodes, 15 minutes should be allowed after placement of the electrode before threshold testing, to permit acute myocardial changes caused by electrode entry into the myocardium to subside.

Pacing modes

Pacing modes are indicated according to a 5-letter international NBG code (Revised 2002 NASPE/BPEG Generic Code), referring to the chamber paced, chamber sensed, type of response when sensed, programmability, and multisite pacing or anti-tachyarrhythmia function (for combined PM-AICD), respectively (Table 85-1). The first three letters are usually the most important.

Table 85-1. NBG (NASPE/BPEG Generic) Pacemaker Codes

Pacing: Chamber(s) paced by direct stimulation if no intrinsic cardiac electrical activity sensed: Atrial (A), Ventricular (V), Dual (D), or none (O).

Sensing: Chamber(s)’ intrinsic electrical activity detected by the pacemaker: Atrial (A), Ventricular (V), Dual (D), or none (O).

Response: Device response to sensing intrinsic electrical activity (pacing algorithm): Triggered (T), Inhibited (I), Dual (D), or none (O).

  • T mode. Sensed intrinsic activity triggers pacemaker stimulus (used to avoid inhibition of the pacemaker).
    • I mode. Sensed intrinsic activity inhibits the pacemaker.
    • D mode. For AV sequential pacing. Detected intrinsic ventricular activity inhibits the pacemaker, while detected intrinsic atrial activity inhibits the atrial pacemaker stimulus and triggers a ventricular stimulus (with a programmable AV delay).

Rate modulation programmability: None (O), communicating (C), simple programmable (P), multiprogrammable (M), rate modulation (R). Permanent pacemakers are usually set as R, with the rate self-adjusting depending on different sensors (motion, temperature, etc.) to compensate for the metabolic demands of the patient.

Temporary epicardial pacing wires placed after cardiac surgery used to be fixed rate: either A-paced (AOOO), V-paced (VOOO) or A-V sequentially paced (DOOO) based on the presence, type and rate of intrinsic rhythm, and AV node conduction function. However, the newer external pacemakers can be programmed as AAI(O), VVI(O), DDD(O) and occasionally DVI(O). Permanent pacemakers implanted postoperatively for persistent complete heart block are usually DDD(R) (“full-service pacemakers”) or VVI(R) in the setting of atrial fibrillation with complete heart block.

AICD devices detect tachyarrhythmias, deliver the therapy and analyze the success of the therapy. Different zones of detection can be programmed (e.g., V-tach, fast V-tach, V-fib). Based on the type of arrhythmia detected, the therapy is delivered as either high-energy defibrillation shocks, low-energy synchronized cardioversion, or anti-tachycardia pacing.

For treatment of AV nodal reentrant tachycardia, atrial flutter, ventricular tachyarrhythmias, or Wolff-Parkinson-White syndrome, a non-AICD pacemaker can be used as an overdrive-pacing device.

In patients with implanted pacemakers undergoing surgery, a magnet can be applied over the device to prevent inadequate sensing of the pacemaker due to artifacts from the electrocautery. The magnet temporarily inactivates the sensing function, changing the pacing mode to AOO, VOO or DOO until removed. The rate response is switched to OFF as well. On the other hand, application of a magnet to an AICD generally inactivates the delivery of tachyarrhythmia therapy but does not inactivate sensing of the pacemaker component. AICDs can have either an auditory or vibratory confirmation of the suspension of anti-tachycardia therapy on magnet application. Alternatively, the pacemaker/AICD can be re-programmed prior to surgery to be deactivated.

Indications

Guidelines for the placement of permanent PM and AICD were developed by the AHA in collaboration with the AATS and STS. These guidelines were further modified in 2012 relative to cardiac resynchronization therapy (CRT).

Class I recommendations for permanent pacemaker

  • Sinus node dysfunction: symptomatic bradycardia (including required drug therapy), severe sinus pauses and symptomatic chronotropic incompetence
  • Atrioventricular (AV) block including:
    • After AV node catheter ablation
    • Persistent advanced second- or third- degree AV block after cardiac surgery (reported incidence up to 5.7% for aortic valve replacement, 1.8% for mitral valve replacement, and 1% for coronary artery bypass surgery)
  • Chronic bifascicular block
  • Prophylactically for ventricular tachyarrhythmias; for sustained pause dependent VT, with or without QT prolongation
  • AV block after acute MI
  • Hypersensitive carotid sinus syndrome and neurocardiogenic syncope
  • Recurrent syncope caused by spontaneously occurring carotid sinus stimulation and carotid sinus pressure that induces ventricular asystole of more than 3 seconds
  • Persistent or symptomatic bradycardia after cardiac transplantation
  • Hypertrophic Obstructive Cardiomyopathy (HOCM) with sinus node dysfunction or AV block
  • Congenital heart disease with sinus node dysfunction, advanced second or third degree AV block associated with symptomatic bradycardia, ventricular dysfunction or low cardiac output

Class I recommendations for AICD

  • Survivors of VF or hemodynamically unstable sustained VT in the absence of identified and treated causes that resolved the dysrhythmia (e.g., coronary artery disease, structural cardiac disease)
  • Structural heart disease with sustained VT (stable or unstable)
  • Sustained or inducible (during EP study) VT with associated syncope of otherwise indeterminate origin
  • Ischemic cardiomyopathy patients with left-ventricular ejection fraction (LVEF) <35% due to prior MI, a minimum of 40 days post-MI and are in NYHA functional class II or III
  • Patients with LV dysfunction due to prior MI who are a minimum of 40 days post-MI, and LVEF <30%, with New York Heart Association functional class I
  • Non-ischemic dilated cardiomyopathy with LVEF ≤35% with NYHA Class II or III CHF
  • Non-sustained VT due to prior MI and LVEF <40%, sustained VT or induced VF at EP study

Class I recommendations for CRT in patients with severe systolic heart failure

Biventricular pacing as cardiac resynchronization therapy (CRT), associated or not with implant of AICD, is indicated for patients with LVEF ≤35%, sinus rhythm, left bundle branch block with a QRS duration ≥150 ms, and NYHA class II, III, or ambulatory IV symptoms on guideline-directed optimal medical therapy.

Indications for placement of temporary pacemaker

  • Symptomatic bradyarrhythmia or bradyarrhythmia with hemodynamic compromise
  • Elevated risk of important bradyarrhythmia
  • Bradycardia that results from an acute and reversible causes (acute myocardial infarction, electrolyte disturbances, drug toxicities, Lyme disease, etc) 
  • Symptomatic sinus bradycardia, sinus pauses >3 sec, sinus bradycardia with heart rate <40 bpm
  • Ventricular asystole
  • Alternating left and right bundle branch block

Contraindications

  • Active infections: in this setting a temporary transvenous PM or AICD (e.g., life vest) with external pulse generator should be placed
  • Inadequately treated coagulopathy
  • Life expectancy <12 months
  • Psychiatric patients that may be adversely affected by shocks (relative)

Procedural technique

Preoperative evaluation should include assessment of patency of venous system, history of radiation therapy to the upper extremities or to the chest, and handedness of the patient.

Temporary PM can be placed transcutaneously, transvenously, or epicardially. Permanent PM and AICD can be placed transvenously (usually via cephalic vein or subclavian vein) or epicardially via thoracotomy or subxiphoid approach. The ventricular lead is placed usually at the septum or inferior apical area of the RV, avoiding the free wall and the RVOT. The atrial lead is placed in the RA, ideally in the appendage. If needed for resynchronization therapy, an additional LV lead can be placed transvenously in the coronary sinus or epicardially.

Complications

Incidence of significant complications is <2% for transvenous devices.

Early complications are reported as injury to systemic veins and right-sided cardiac structures (including tamponade due to RA perforation or traumatic tricuspid regurgitation), pneumothorax, hemothorax, air embolus, venous thrombosis, and arrhythmias. Infection of the pocket or leads is the most important long-term complication (up to 3%), and usually requires removal of the device with placement of a temporary one if needed, until clearing of the infection.

Additional late complications include lead fracture, lead malfunction, delayed lead perforation, pocket erosion, and venous thrombosis. Lead fracture or malfunction (e.g., due to fibrous tissue reaction) should be suspected in the presence of an increased pacing threshold.

When indicated, lead extraction is a procedure not free from morbidity and mortality (up to 15.3% major complication rate with a 1.1% mortality rate at 6 months) and should be performed in a setting prepared and committed to the safe performance of the procedure, often with a cardiac surgeon and cardiopulmonary bypass on standby for backup.

Suggested Readings

  1. Ghzally Y, Mahajan K. Implantable Defibrillator. [Updated 2020 Oct 12]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2020 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK459196/
  2. Dalia T, Amr BS. Pacemaker Indications. [Updated 2020 Aug 31]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2020 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK507823/
  3. https://www.uptodate.com/contents/modes-of-cardiac-pacing-nomenclature-and-selection/print#subscribeMessage
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