Pacemaker Emergencies: Recognizing Malfunctions and Taking Action

Pacemaker patients present in emergencies more frequently than you might expect. Thousands of people live with implanted pacemakers or ICDs, and each of these devices can malfunction – often at the very moment when patients are already hemodynamically compromised. Correct ECG interpretation, recognition of typical malfunction patterns, and rapid initiation of life-saving measures are core competencies in ACLS care. This article gives you the tools to systematically recognize and manage pacemaker emergencies.

Fundamentals of Pacemaker Function

Before you can recognize malfunctions, you need to understand normal function. A cardiac pacemaker has two central tasks: Pacing (stimulation) and Sensing (detection). The internationally used NBG code describes the programming in five letters, with the first three being most relevant in emergencies:

• 1st letter: Pacing site (A = Atrium, V = Ventricle, D = Dual)
• 2nd letter: Sensing site (A, V, D)
• 3rd letter: Mode of response (I = Inhibition, T = Triggered, D = Dual)

The most common modes in clinical practice:

• VVI: Ventricular pacing, ventricular sensing, inhibition with intrinsic rhythm
• DDD: Dual pacing and sensing in atrium and ventricle
• AAI: Atrial pacing and sensing (rarely used as a standalone system)

On the ECG, you recognize pacemaker activity by pacing spikes – narrow, vertical deflections immediately preceding the paced complex. An atrial spike precedes a P-wave, a ventricular spike precedes a wide QRS complex. If these spikes are absent or have an unusual relationship to the subsequent depolarization, a malfunction may be present.

Pacing Failure: When the Pacemaker Doesn't Stimulate

Failure to Pace (Absent Stimulation)

In failure to pace, the pacing spike is absent where it would be expected based on the programmed rate. The most common causes are:

• Battery depletion (most common cause over the long term)
• Lead fracture or lead dislodgement
• Connector failure (loose connection between lead and generator)
• Oversensing (the pacemaker interprets artifacts as intrinsic rhythm and inhibits itself – strictly speaking a sensing problem with the consequence of absent stimulation)

ECG signs: Pauses without pacing spikes that are longer than the programmed escape interval. In pacemaker-dependent patients, this can lead to syncope, presyncope, or in the worst case, asystole.

Failure to Capture (Stimulation Without Myocardial Response)

Here you see the pacing spike, but it is not followed by an adequate myocardial complex. The spike stands isolated, or only a minimal, ineffective depolarization follows.

Causes:

• Increased pacing threshold (fibrosis at the lead tip, electrolyte disturbances, medications)
• Lead dislodgement (spike present, but lead no longer has myocardial contact)
• Myocardial infarction in the area of the lead tip
• Severe hyperkalemia (massively increases the pacing threshold)
• Antiarrhythmics (flecainide, propafenone, amiodarone can increase the pacing threshold)

ECG signs: Pacing spikes without a subsequent QRS complex (in ventricular pacing) or without a subsequent P-wave (in atrial pacing). The pattern may occur intermittently – a single non-captured spike between functioning paced beats is equally pathological.

Clinical tip: Pay particular attention to electrolytes. Hyperkalemia above 6.5 mmol/L can render a previously normally functioning pacemaker completely ineffective. Correcting the hyperkalemia often restores pacemaker function.

Sensing Errors: When Detection Fails

Undersensing (Failure to Sense)

In undersensing, the pacemaker fails to recognize intrinsic rhythm and paces despite the presence of intrinsic activity. The result is competition between intrinsic rhythm and pacemaker stimulation.

ECG signs: Pacing spikes fall into the existing intrinsic rhythm – they occur independently of the patient's own QRS complexes or P-waves. This phenomenon is particularly dangerous when a ventricular spike falls into the vulnerable phase of repolarization (T-wave) – this carries the risk of R-on-T phenomenon-induced ventricular tachycardia or ventricular fibrillation.

Causes:

• Lead dislodgement (the sensing signal at the new location is too small)
• Low signal amplitude due to myocardial infarction, fibrosis, or electrolyte disturbances
• Lead fracture (intermittent contact)
• Incorrect programming (sensing threshold set too high)

Oversensing (Excessive Detection)

The opposite: the pacemaker falsely interprets interference signals as intrinsic rhythm and inhibits itself. The patient needs the pacemaker but receives no stimulus.

Common sources of interference:

• Myopotentials (especially with unipolar leads; diaphragm, pectoralis muscle)
• Electromagnetic interference (diathermy, MRI with non-MRI-conditional systems, electrocautery)
• T-wave oversensing (tall T-waves are misinterpreted as R-waves)
• Lead fracture with high-frequency fracture-site artifacts

ECG signs: Unexpectedly long pauses without pacing spikes, despite the absence of an adequate intrinsic rhythm. The ECG shows bradycardia or asystole, even though the pacemaker should be pacing.

Clinical tip: Distinguishing between failure to pace and oversensing can be difficult – in both cases the pacing spike is absent. The difference lies in the cause: in failure to pace, the pacemaker is defective or without power; in oversensing, it is technically functioning correctly but is being deceived by erroneous signals.

Pacemaker-Mediated Tachycardias (PMT)

Pacemaker-mediated tachycardia is a special case that occurs primarily with DDD pacemakers. The mechanism is a reentry loop:

1. A retrograde impulse conducted from the ventricle to the atrium is sensed by the atrial channel.
2. The pacemaker interprets this as intrinsic atrial activity and triggers a ventricular stimulus after the programmed AV interval.
3. This ventricular stimulus is again conducted retrogradely to the atrium – the circuit is complete.

ECG signs: Regular wide-complex tachycardia (because of ventricular pacing) at a rate that typically corresponds to the programmed upper tracking rate of the pacemaker – commonly between 110 and 130 bpm, but may be higher depending on programming.

Differential diagnosis: Differentiation from VT, SVT with aberrant conduction, and atrial flutter with 1:1 conduction. The key clue is the regular rate exactly at the upper tracking limit and the presence of ventricular pacing spikes.

Other Tachycardic Malfunctions

• Runaway Pacemaker: An extremely rare but life-threatening malfunction in which the pacemaker stimulates at an uncontrollably high rate (sometimes over 200 bpm). The cause is component failure. This is an absolute emergency.
• Sensor-driven tachycardia: Rate-responsive pacemakers (VVIR, DDDR) can generate an inappropriately high pacing rate due to vibration, fever, or other nonspecific stimuli.

Magnet Application: Your Most Important Emergency Tool

Placing a magnet over the pacemaker generator is the only way to influence device behavior in an emergency without a programmer. The response differs fundamentally between pacemakers and ICDs:

Magnet on Pacemaker

The pacemaker switches to asynchronous mode (VOO, DOO, or AOO depending on the base programming). This means:

• Sensing is deactivated – the pacemaker stimulates at a fixed rate, regardless of intrinsic rhythm
• The rate is manufacturer-dependent, typically around 65–85 bpm
• With battery depletion, the magnet rate is often reduced (e.g., below 65 bpm) – this is a diagnostic clue

When magnet application on a pacemaker is useful:

• Oversensing with inhibition: Asynchronous mode bridges the problem since sensing is no longer active
• PMT termination: Switching to asynchronous mode interrupts the reentry loop
• Diagnostics: Checking capture capability and battery capacity

Caution: In undersensing, magnet application does not significantly worsen the situation (the pacemaker is already stimulating asynchronously anyway), but in patients with a relevant intrinsic rhythm, asynchronous mode carries the risk of competitive pacing.

Magnet on ICD

With an ICD, the magnet has a different effect: it exclusively deactivates the tachycardia therapies (shock delivery and antitachycardia pacing). The antibradycardia pacing function remains unchanged.

Most important indication: Inappropriate ICD shocks (e.g., atrial fibrillation that the ICD misinterprets as ventricular fibrillation, or lead fracture with artifacts). The magnet immediately stops the repeated shock deliveries.

Important: Once you remove the magnet, the shock function is reactivated in most devices. There are individual manufacturers where the shock function remains permanently deactivated until actively reprogrammed – in an emergency, continuous magnet application until the arrival of the electrophysiologist or the availability of a programmer is the safe strategy.

Emergency Management: A Systematic Approach

Acute Bradycardia in Pacemaker-Dependent Patients

If a pacemaker-dependent patient is bradycardic or asystolic, follow this algorithm:

1. Secure ABC – if unconscious, begin CPR per ACLS algorithm
2. Apply magnet over the generator – forces asynchronous pacing
3. Atropine 0.5–1 mg IV – effectiveness is limited in pacemaker failure, as there is often no functioning intrinsic rhythm
4. Transcutaneous pacing (TCP) – apply and start immediately:
   • Position electrodes anterior-posterior
   • Maintain at least 10 cm distance from the generator
   • Rate 60–80 bpm, gradually increase output until capture (typically 40–120 mA)
5. Epinephrine infusion (2–10 µg/min) or isoproterenol (1–4 µg/min) as bridging if TCP is not immediately available
6. Identify reversible causes: Electrolytes (K⁺, Ca²⁺, Mg²⁺), medications (antiarrhythmics?), chest X-ray (lead position?)

Tachycardic Pacemaker Malfunction

1. Hemodynamically unstable? → Electrical cardioversion per ACLS standards, with paddles/pads placed at least 10 cm from the generator and preferably in anterior-posterior position
2. PMT suspected? → Magnet application usually terminates the tachycardia immediately
3. Inappropriate ICD shocks? → Apply magnet over the ICD to deactivate shock function
4. Runaway pacemaker? → Magnet application (may help reduce rate to magnet rate), otherwise emergency reprogramming, or in extreme cases, surgical generator disconnection

Special Considerations During Resuscitation

Resuscitation of pacemaker and ICD patients follows the standard ACLS algorithms with the following modifications:

• Do not delay CPR – chest compressions are safe, even with an implanted device
• Defibrillation: Place pads in anterior-posterior position, minimum 10 cm distance from the generator. If the pacemaker no longer functions after defibrillation: TCP as backup
• ICD patients: The ICD may deliver shocks during resuscitation that are perceptible but not dangerous to rescuers. Wear gloves nevertheless. If the ICD delivers inappropriate and repeated shocks: apply magnet
• After successful resuscitation: Every implanted device must be interrogated and checked promptly by the manufacturer's service or electrophysiology – defibrillation energy can alter programming or damage the lead

Diagnostic Checklist in Emergencies

For every patient with a pacemaker or ICD and cardiac symptoms, you should proceed systematically:

• 12-lead ECG: Pacing spikes present? Every spike with capture? Relationship of spikes to intrinsic rhythm?
• Pacemaker ID card: Manufacturer, model, implantation date, programmed mode, base rate
• Chest X-ray: Lead position (dislodgement? fracture?), generator position
• Laboratory: Potassium, sodium, calcium, magnesium, creatinine, troponin, thyroid values
• Medication history: Antiarrhythmics, beta-blockers, calcium channel blockers?
• Magnet test: Asynchronous stimulation present? Rate as indicator of battery status?

Summary of Key Points

Malfunction	ECG Signs	Acute Intervention
Failure to Pace	Absent spikes, pauses	Magnet, TCP, epinephrine
Failure to Capture	Spikes without QRS	Correct electrolytes, TCP
Undersensing	Competitive pacing	Reprogramming, electrolytes
Oversensing	Inappropriate pauses/inhibition	Magnet (asynchronous mode)
PMT	Regular wide-complex tachy at upper tracking rate	Magnet, reprogramming
Inappropriate ICD shocks	Shock deliveries without VT/VF	Magnet on ICD

Practical Training

Pacemaker emergencies require rapid pattern recognition on the ECG and confident handling of magnet application, transcutaneous pacing, and ACLS algorithms – skills that can only be reliably recalled under time pressure if they have been practiced beforehand. In the ACLS Refresher course by Simulation Tirol, you practice these scenarios in realistic simulations, interpret device ECGs under expert guidance, and solidify your emergency management for this challenging patient population. All details and dates can be found at Simulation Tirol – ACLS Refresher.