Nursing care
Temporary pacemaker complications: capture, sensing, oversensing and wire safety
Written and reviewed by Dana Whitfield, RN, MSN · 4 min read · Updated October 2026
Short answer
With a temporary transvenous or epicardial pacemaker, failure to capture shows spikes without a following complex, failure to sense shows spikes that ignore the patient's own beats, and oversensing shows missing spikes when pacing is needed. Assess the patient first, check connections and battery, escalate, and handle exposed wires with gloves to prevent microshock.
How temporary systems work and where they fail
Temporary pacing delivers impulses from an external generator through a wire in the right ventricle via a central vein, or through epicardial wires placed on the heart surface during cardiac surgery. The generator settings include rate, output in milliamps and sensitivity. Problems arise from the patient, the wire, the connections or the box, so troubleshooting covers each part of the circuit.
Always start with the patient: level of consciousness, blood pressure, perfusion and the rhythm on the monitor. A malfunction that leaves an underlying rhythm adequate is less urgent than one that leaves severe bradycardia or asystole. Keep a transcutaneous pacing-capable defibrillator nearby, and confirm the ordered settings at the start of each shift.
Failure to capture
In failure to capture, pacing spikes appear at the expected rate, but no P wave or QRS follows them, so the heart is not depolarising. Causes include a dislodged or migrated wire, loose connections, an output set too low, and a rising capture threshold from ischaemia, electrolyte disturbance such as hyperkalaemia, or ageing epicardial wires, which often lose function after several days.
Check connections from the wire to the box and the battery, then report immediately if the patient is compromised. The provider may increase output, measure the threshold or reposition the wire. While waiting, monitor perfusion and be ready to start transcutaneous pacing or emergency treatment according to protocol. Document the rhythm strip showing the problem.
Failure to sense and oversensing
Undersensing, or failure to sense, occurs when the pacemaker does not detect the patient's own beats and fires anyway. Spikes appear in the middle of intrinsic complexes or on T waves. A spike landing on the T wave can trigger ventricular tachycardia or fibrillation, so undersensing is dangerous even when the heart rate looks adequate. Report it and expect the sensitivity setting to be adjusted.
Oversensing is the reverse: the device interprets non-cardiac signals, such as muscle activity, shivering, electrical interference or T waves, as heartbeats and withholds pacing. The monitor shows pauses without spikes when the rate falls below the set limit. The patient can become bradycardic or asystolic. Check for interference sources and escalate for sensitivity or mode changes.
Microshock and wire safety
Temporary wires provide a low-resistance path directly to the myocardium, so very small currents that would be harmless on the skin can cause ventricular fibrillation. Wear non-conductive gloves when handling exposed wire ends, cover unused terminals in an insulating container, keep wires dry, and avoid touching the wires and other electrical equipment at the same time.
Secure the generator so it cannot fall, protect connections from tension during repositioning, and keep the box away from fluids. After removal of epicardial wires, monitor for tamponade, including hypotension, tachycardia and rising venous pressure. For transvenous wires, observe the insertion site for bleeding or infection and report new chest pain, hiccups or dyspnoea.
Hypothetical troubleshooting item
In an invented question, a client after cardiac surgery is on epicardial pacing at a set rate. The monitor shows pacing spikes with no QRS complexes after them, and the client is dizzy with low blood pressure. The choices include documenting the rhythm, changing the sensitivity setting, checking connections and notifying the provider immediately, or removing the wires.
Checking connections and urgent notification is the best answer, because this is failure to capture in a symptomatic client. Changing the sensitivity addresses sensing, not capture, and removing wires is unsafe. Afterwards, document strips, settings, the patient's response and any changes made by the provider.
Sources and further reading
BJA Education (PMC): Temporary epicardial pacing after cardiac surgery. Definitions and causes of failure to capture, undersensing with R-on-T risk, oversensing from muscle activity and interference, threshold rise and tamponade after wire removal.
Life in the Fast Lane: Temporary epicardial cardiac pacing. Non-conductive gloves when handling wires, discharging static, insulating unused wires and microshock-induced dysrhythmia risk.
The next step on this is the same as on everything else here: answer questions and read the rationales. Our cardiovascular practice questions are the closest set to what this page covers.
Common questions
What does failure to capture look like on the monitor?
Pacing spikes appear at the set rate but are not followed by a P wave or QRS complex. The heart is not responding to the impulse, so check perfusion, connections and battery and escalate.
Why is undersensing dangerous?
The pacemaker fires without noticing intrinsic beats, so a spike can fall on the T wave and trigger ventricular tachycardia or fibrillation. Report it for sensitivity adjustment.
Why wear gloves when touching epicardial wire ends?
The wires conduct directly to the heart, so even tiny currents, including static, could trigger fibrillation. Gloves and insulated covers reduce microshock risk.
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