What pacemaker problem caused the patient's initial symptoms of profound weakness, hypotension, and bradycardia?

Oversensing

The patient's initial symptoms were caused by oversensing (Option C). The pacemaker inappropriately detected extraneous electrical signals as intrinsic cardiac activity, leading it to inhibit pacing output, resulting in profound bradycardia (heart rate 38), hypotension, and weakness. 1

Diagnostic Reasoning

Why Oversensing is the Correct Answer

• Magnet application confirms the diagnosis: When a magnet is placed over a pacemaker, it converts the device to asynchronous (non-sensing) mode, forcing it to pace at a fixed rate regardless of any sensed activity. 1 The immediate improvement in heart rate (38→70 bpm) and blood pressure when the magnet was applied proves the pacemaker hardware was functional but was being inappropriately inhibited. 2

• Clinical presentation matches oversensing: The patient had profound weakness, hypotension (BP 92/48), and severe bradycardia (HR 38) despite having a functioning pacemaker—classic signs that the device was failing to deliver necessary pacing stimuli because it was "fooled" into thinking the heart was beating on its own. 1

• Pacemaker-dependent patient: In a patient with sick sinus syndrome requiring a pacemaker, a heart rate of 38 indicates complete failure of the device to pace appropriately, which is life-threatening in pacemaker-dependent individuals. 1, 3

Why Other Options Are Incorrect

Battery failure (Option A) is excluded because:

• The magnet application successfully triggered pacing at the programmed rate of 70 bpm, demonstrating that the pulse generator had adequate power to deliver electrical impulses. 1
• Battery depletion typically causes gradual onset of symptoms over weeks to months, not acute presentation. 1
• A depleted battery would not respond to magnet application with normal pacing function. 4

Failure to capture (Option B) is excluded because:

• When the magnet was applied and the pacemaker paced at 70 bpm, there was hemodynamic improvement (increased blood pressure), proving that the pacing stimuli were successfully capturing the myocardium and producing effective ventricular contractions. 2
• Failure to capture would show pacing spikes on ECG without subsequent QRS complexes, and magnet application would not improve the clinical situation. 2

Undersensing (Option D) is excluded because:

• Undersensing causes the pacemaker to fail to detect intrinsic cardiac activity, leading to inappropriate pacing stimuli that can fall during the vulnerable period of the cardiac cycle. 2
• This typically presents with pacing spikes occurring too frequently or at inappropriate times (potentially causing pacemaker-induced arrhythmias), not with bradycardia and failure to pace. 2
• The patient's bradycardia indicates the pacemaker was not delivering stimuli when needed, which is the opposite of undersensing. 1

Clinical Pearls

Common Sources of Oversensing

• Electromagnetic interference: External electrical signals from medical equipment, muscle potentials (myopotentials), or other sources can be misinterpreted as cardiac activity. 2
• Lead malfunction: Insulation breaks or lead fractures can cause the pacemaker to sense electrical noise. 1, 2
• T-wave oversensing: The pacemaker may inappropriately sense the T-wave as a QRS complex, particularly problematic in unipolar systems. 2
• Inappropriate sensitivity settings: If the device is programmed too sensitively, it may detect non-cardiac signals. 2

Critical Management Steps

• Immediate magnet application converts the pacemaker to asynchronous mode (VOO or DOO), providing temporary stabilization while the underlying problem is addressed. 1, 2
• Urgent device interrogation is required to identify the specific cause of oversensing and reprogram sensitivity settings. 1, 2
• Lead evaluation with chest X-ray and impedance measurements should be performed to exclude lead fracture or displacement. 1, 2
• Consider external pacing availability in case magnet application fails or the patient becomes pacemaker-dependent during evaluation. 1

Pitfalls to Avoid

• Do not assume symptoms are non-cardiac in pacemaker patients presenting with confusion, weakness, or syncope—always suspect device malfunction first. 1
• Do not delay magnet application in symptomatic bradycardia with suspected pacemaker malfunction; this simple intervention can be life-saving. 1, 2
• Do not miss intermittent oversensing: Symptoms may be episodic if the interference source is intermittent, requiring prolonged monitoring or event recording. 1