How to fix undersensing and overpacing in an epicardial pacemaker with no underlying rhythm without lowering heart rate?

Managing Undersensing and Overpacing in Epicardial Pacemakers

In a 100% pacemaker-dependent patient with an epicardial system experiencing undersensing and overpacing, increase the sensitivity setting (program to a lower millivolt value) to allow the device to detect smaller amplitude signals, while maintaining asynchronous pacing mode (VOO/DOO) to ensure continuous pacing despite the sensing malfunction. 1

Understanding the Core Problem

Undersensing in epicardial systems occurs when the pacemaker fails to detect intrinsic cardiac activity, leading to inappropriate pacing stimuli. Epicardial leads have inherently higher failure rates and sensing challenges compared to transvenous systems, making this a particularly critical issue in pacemaker-dependent patients. 2

The specific challenge here is that you cannot lower the pacing rate because the patient has no underlying rhythm—any reduction in pacing rate would leave the patient without cardiac output. This means you must fix the sensing problem through other programming adjustments.

Primary Solution: Optimize Sensitivity Settings

Program the sensitivity to a more sensitive setting (lower millivolt threshold) to detect smaller amplitude signals. 1, 3

• Start by checking current sensitivity settings during device interrogation
• Gradually increase sensitivity (decrease the mV value) in small increments (e.g., from 2.5 mV to 2.0 mV, then to 1.5 mV) 3
• Test at each setting to verify improved sensing without introducing oversensing of noise or myopotentials 3
• For ventricular sensing, most patients have adequate sensing thresholds at 4.0 mV or greater, so programming to 2.5 mV or less should provide adequate margin 3

Critical Caveat About Sensitivity

Avoid programming to unnecessarily high sensitivities (very low mV values like 0.4-1.0 mV) as this increases risk of myopotential oversensing, particularly in unipolar epicardial systems. 3 The goal is to find the optimal balance where intrinsic activity is detected without detecting noise.

Secondary Interventions for Epicardial Systems

Check and Adjust Output Settings

High ventricular pacing outputs can cause atrial undersensing through cross-chamber blanking effects, particularly in epicardial unipolar systems. 4

• Review ventricular pacing output—if elevated above 5.0 V, consider reduction if capture threshold allows 4
• Verify capture thresholds for both chambers and program output to 2-3 times threshold 1
• In pediatric/epicardial cases, ventricular outputs of 5.5-7.5 V have been associated with atrial undersensing due to amplifier saturation and quiet timer blanking 4

Verify Lead Integrity

Epicardial leads are prone to higher complication rates including loss of capture and sensing deterioration over time. 1

• Check lead impedance trends—a decline of 100 Ω or more over time suggests insulation failure 5
• Verify both bipolar and unipolar electrogram amplitudes 5
• Loss of capture occurs in up to 50% of epicardial leads by the fifth postoperative day in some series 1

Maintain Safe Pacing Mode

For a 100% pacemaker-dependent patient, program to asynchronous mode (VOO for single chamber, DOO for dual chamber) until sensing is corrected. 1

• Asynchronous pacing ensures continuous pacing regardless of sensing function 1
• This prevents life-threatening pauses from undersensing 1
• Once sensing is optimized and verified stable, you can return to demand mode (VVI/DDD) 1

Troubleshooting Algorithm

1. Interrogate device: Document current sensitivity, output, impedance, and electrogram amplitudes 1
2. Increase sensitivity (lower mV threshold) in stepwise fashion while monitoring for oversensing 3
3. If undersensing persists: Reduce ventricular output if elevated (>5.0 V) and capture threshold permits 4
4. If still problematic: Check impedance trends for lead failure 5
5. Consider lead revision if impedance abnormal or electrogram amplitude inadequate 5
6. Maintain asynchronous pacing throughout troubleshooting to ensure patient safety 1

Specific Pitfalls to Avoid

Do not attempt to lower the base pacing rate in a patient with no underlying rhythm—this will result in asystole and hemodynamic collapse. 2

Do not program to excessively sensitive settings (below 1.0 mV for ventricular, below 0.4 mV for atrial) as this dramatically increases myopotential oversensing risk, particularly with unipolar epicardial leads. 3

Do not ignore impedance trends—even "normal" impedance values can mask insulation failure if the trend shows significant decline over time. 5

Epicardial systems have unique challenges with amplifier saturation and cross-chamber blanking that are less common in transvenous systems, requiring specific attention to output programming. 4