4.5 Permanent Pacemakers, CRT and Device Troubleshooting

Pacemaker NBG Coding System & Pacing Modes

Artificial cardiac pacemakers deliver electrical stimuli to the myocardium to initiate depolarization when intrinsic conduction fails. Device function is standardized using the NASPE/BPEG (NBG) 5-letter generic code.

The NBG Code Structure

Common Clinical Pacing Modes

• DDD Mode (Dual-Chamber Pacing & Sensing): The most sophisticated physiological mode.
  • Paces both atrium and ventricle when intrinsic signals are absent.
  • Senses intrinsic atrial and ventricular activity.
  • Response: Inhibits atrial/ventricular output if native beats are sensed; triggers ventricular pacing after a sensed intrinsic atrial beat (maintaining physiological AV synchrony).
• VVI Mode (Single-Chamber Ventricular): Paces and senses the ventricle; inhibited by native ventricular beats. Used commonly in patients with chronic permanent Atrial Fibrillation with slow ventricular response.
• AAI Mode (Single-Chamber Atrial): Paces and senses the atrium; used in sick sinus syndrome with intact AV nodal conduction.
• VOO / DOO (Asynchronous Pacing): Paces at a fixed rate regardless of underlying intrinsic cardiac activity. Senses nothing (Position II = O). Used during surgical procedures to protect against electromagnetic interference (EMI).

Troubleshooting Pacemaker Malfunctions

Recognizing pacemaker malfunction on telemetry is a core critical care nursing skill for the AACN CMC exam.

  Failure to Capture:
  | (Spike)               | (Spike)
  |                       |
--+-----------------------+-------------------------
  (No QRS follows spike)  (No QRS follows spike)

  Failure to Sense (Undersensing):
        (Native QRS)    (Pacing Spike on T wave! Risk of R-on-T VF)
             |                 |
      /\     |          /\     |
     /  \   /\         /  \   /\|
  --/----\-/--\-------/----\-/--|-------------------

1. Failure to Capture

• Definition: The pacemaker delivers an electrical impulse (a pacing spike is clearly visible on ECG), but no myocardial depolarization follows (no P wave after an atrial spike, or no QRS complex after a ventricular spike).
• Etiologies:
  • Lead dislodgement or micro-dislodgement from the endocardium.
  • Exit Block: Localized myocardial tissue edema, fibrosis, or ischemia at the lead tip raising the electrical threshold.
  • Electrolyte/Metabolic Derangements: Severe hyperkalemia, acidosis, or antiarrhythmic drug toxicity (e.g., flecainide).
  • Insufficient energy output setting (mA set below capture threshold).
• Nursing Interventions: Increase output (mA) on temporary generator; turn patient onto left lateral decubitus position (may re-establish lead contact); check serum electrolytes (STAT potassium/ABG); obtain STAT chest X-ray to evaluate lead placement.

2. Failure to Sense (Undersensing)

• Definition: The pacemaker fails to detect native cardiac electrical activity. As a result, it fires competitive pacing spikes randomly throughout the cardiac cycle, including inappropriately on top of native QRS complexes or T waves.
• Lethal Complication: A pacing spike falling on the vulnerable T wave phase can trigger the R-on-T phenomenon, precipitating polymorphic VT or VF!
• Etiologies: Sensitivity threshold set too HIGH (e.g., set to 5.0 mV instead of 2.0 mV, making the device less sensitive to low-amplitude native signals); lead displacement; low intrinsic signal amplitude (small EGM voltage).
• Nursing Interventions: Increase sensitivity by LOWERING the numerical mV threshold (e.g., dial down from 5.0 mV to 1.5 mV so smaller intrinsic waves are sensed).

3. Oversensing

• Definition: The pacemaker inappropriately senses non-cardiac signals (e.g., skeletal muscle artifact, pectoral muscle twitching, diaphragmatic movement, electromagnetic interference / EMI) or internal T waves as native depolarizations.
• Clinical Result: The pacemaker is inappropriately inhibited, leading to unexpected pauses, dizziness, or syncope in pacemaker-dependent patients.
• Etiologies: Sensitivity setting set too LOW (e.g., 0.5 mV, making the device overly sensitive to background noise); lead insulation breach causing wire noise.
• Nursing Interventions: Decrease sensitivity by RAISING the numerical mV threshold (e.g., adjust from 0.5 mV up to 2.5 mV); eliminate external sources of EMI.

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Temporary Pacing Modalities & Clinical Nursing Care

Temporary transcutaneous, transvenous and epicardial pacing — including threshold testing, the 2-3x safety margin, epicardial wire identification and the full capture/sensing troubleshooting matrix — is covered in depth in section 4.4. This section stays with permanently implanted devices.

Implantable Cardioverter-Defibrillators (ICD) & Magnet Protocols

ICD primary and secondary prevention thresholds, transvenous versus subcutaneous devices, tiered therapy, electrical storm, inappropriate shocks, wearable cardioverter defibrillators and the full magnet-effect comparison are covered in section 4.6. The one point to carry from here: a magnet over a pacemaker produces asynchronous pacing (VOO/DOO), whereas a magnet over an ICD suspends tachytherapy and does not change pacing behaviour — so a pacing-dependent ICD patient needs formal reprogramming, not a magnet.

Cardiac Resynchronization Therapy (CRT-P / CRT-D)

In patients with heart failure and Left Bundle Branch Block (LBBB), electrical propagation to the lateral wall of the left ventricle is markedly delayed relative to the right ventricle. This intraventricular dyssynchrony impairs ejection fraction, increases mitral regurgitation, and accelerates adverse LV remodeling.

CRT Indications (ACC/AHA/HFSA Guidelines)

1. Left Ventricular Ejection Fraction LVEF ≤ 35%.
2. Sinus Rhythm.
3. LBBB morphology with QRS duration ≥ 150 ms (Class I recommendation) or 120-149 ms (Class IIa).
4. NYHA Class II, III, or ambulatory Class IV symptoms despite maximal optimal medical therapy.

Lead Configuration & Biventricular Optimization

• Three Leads:
  1. Right Atrial (RA) lead.
  2. Right Ventricular (RV) lead.
  3. Left Ventricular (LV) lead: Threaded transvenously through the coronary sinus into a epicardial lateral or posterolateral vein branch of the left ventricle.
• Biventricular Pacing Percentage Target: Clinical response and functional recovery depend on achieving > 95% to 98% continuous biventricular pacing. Frequent PVCs, rapid Atrial Fibrillation, or loss of LV capture reduce pacing percentage and eliminate CRT benefit, requiring aggressive antiarrhythmic therapy, AV nodal ablation, or device reprogramming.