Why would a pacemaker (permanent artificial cardiac pacemaker) cause intermittent chest pain that feels like being shocked in an asymptomatic patient?

Pacemaker-Related Intermittent Chest Pain with Shock-Like Sensation

The most likely cause of intermittent chest pain that feels like being shocked in a patient with a pacemaker is phrenic nerve or diaphragmatic stimulation, though lead malposition, myocardial perforation, or inappropriate high-voltage output should also be considered and require urgent device interrogation.

Primary Mechanisms of Shock-Like Sensations

Extracardiac Muscle Stimulation

• Phrenic nerve stimulation occurs when the pacing lead is positioned too close to the phrenic nerve or diaphragm, causing involuntary diaphragmatic contractions that patients describe as sharp, shocking chest pain 1, 2
• This typically presents as intermittent discomfort synchronized with pacing impulses and may be accompanied by hiccups or visible diaphragmatic twitching 2
• Pectoral muscle stimulation can occur if the pacing output is excessively high or if there is insulation failure in the lead, allowing current to stimulate chest wall muscles 1, 2

Lead-Related Complications

• Lead perforation through the myocardium can cause pericardial stimulation or direct phrenic nerve contact, producing sharp, shocking sensations that may be positional 2
• Lead dislodgement or microdislodgement can result in intermittent loss of capture with high-output pacing attempts, causing uncomfortable sensations 1, 2
• Insulation breach in the lead allows electrical current to leak and stimulate surrounding tissues, creating shock-like pain 1, 2

Critical Evaluation Steps

Immediate Assessment Required

• Device interrogation is mandatory to evaluate pacing thresholds, lead impedances, and sensing parameters 1, 2
• Check for elevated pacing thresholds (suggesting poor lead contact) or decreased impedances (suggesting insulation failure or perforation) 2
• Review stored electrograms to correlate symptoms with pacing events 3, 2

Imaging Considerations

• Chest X-ray in multiple views (PA, lateral, and oblique) to assess lead position and detect perforation or dislodgement 2
• Echocardiography if perforation is suspected to evaluate for pericardial effusion 2

Common Pitfalls to Avoid

Misattribution of Symptoms

• Do not assume the patient is simply "feeling" normal pacing—true shock-like sensations indicate a device problem requiring correction 1, 2
• Avoid dismissing symptoms in "asymptomatic" patients; the shock sensation itself is a symptom requiring evaluation 2

Programming Issues

• Excessive output voltage may be programmed unnecessarily high, causing extracardiac stimulation without clinical benefit 1, 2
• Safety margins should be adequate (typically 2:1) but not excessive 2

Management Algorithm

Step 1: Urgent Device Interrogation

• Evaluate all electrical parameters (capture thresholds, sensing, impedances) 1, 2
• Review pacing mode and output settings 2

Step 2: Attempt Reprogramming

• Reduce output voltage to lowest safe level (maintaining 2:1 safety margin) to eliminate extracardiac stimulation 1, 2
• Adjust pulse width if threshold elevation is modest 2
• Consider bipolar pacing configuration if unipolar is being used, as this reduces extracardiac stimulation 1

Step 3: Imaging if Reprogramming Fails

• Obtain chest X-ray to assess lead position 2
• Consider echocardiography if perforation suspected 2

Step 4: Surgical Revision if Necessary

• Lead repositioning required for persistent phrenic nerve stimulation unresponsive to programming changes 2
• Lead replacement needed for insulation failure or perforation 2

Important Caveats

This is NOT an ICD shock: The sensation described is from pacing output (low energy, continuous), not defibrillation therapy (high energy, discrete events). ICD shocks are unmistakable, extremely painful events that patients clearly remember 4.

Electromagnetic interference from endoscopy or other sources can cause inappropriate pacing or sensing, but this would be episodic and related to specific exposures 4.

The patient requires evaluation even if "asymptomatic" in other respects—the shock sensation itself warrants device assessment to prevent potential complications and improve quality of life 1, 2.