Recording and reading a 12-lead ECG in a patient with a pacemaker follows the same electrode placement as any standard tracing, but interpreting what you see is fundamentally different. Pacing spikes, altered QRS shapes, and expected ST-segment changes all layer on top of the heart’s native electrical activity, masking some diagnoses and mimicking others. The key is learning to recognize what the pacemaker is supposed to be doing so you can spot when something has gone wrong or when a genuine cardiac emergency is hiding behind a paced rhythm.
Getting a Clean Recording
Standard 12-lead electrode placement does not change for pacemaker patients, but your ECG machine’s filter settings matter more than usual. Modern bipolar pacemakers produce tiny stimulus artifacts that can be nearly invisible on a standard recording. A study comparing different upper-frequency filter cutoffs found that a 300 Hz low-pass filter significantly improved detection of both atrial and ventricular pacing spikes compared to the more common 150 Hz setting, with the 300 Hz cutoff achieving an area under the curve of 0.93 for ventricular spike detection on bedside recordings.1PubMed Central. A low-pass filter of 300 Hz improved the detection of pacemaker spike on remote and bedside electrocardiogram If your machine has an “enhanced pacemaker detection” mode, switching it on can help. However, that feature occasionally introduces its own artifacts; one documented case showed a puzzling ECG pattern in an asymptomatic pacemaker patient that turned out to be caused by the machine picking up signals from the device’s built-in physiological sensor when enhanced detection was active.2PubMed Central. An uncommon ECG manifestation of normal pacemaker function The practical lesson: turn on pacemaker detection features when you need to see faint spikes, but be aware that the feature itself can produce confusing tracings.
If pacing spikes remain invisible on the surface ECG, that does not automatically mean the device is malfunctioning. Bipolar pacing configurations produce much smaller stimulus artifacts than older unipolar systems, and the spike can disappear into the baseline noise at standard filter settings. Before escalating to device interrogation, try the 300 Hz filter and check whether you have a bipolar device on your hands.
Reading the Pacing Spike and the QRS That Follows
A pacing spike is a thin vertical deflection on the ECG that appears just before the chamber it stimulates depolarizes. An atrial spike precedes a P wave; a ventricular spike precedes a QRS complex. In a dual-chamber device, you may see both. When you see a spike followed by an appropriately timed complex, the device has successfully “captured” the chamber, meaning its electrical pulse triggered a contraction.
The shape of the paced QRS depends heavily on where the lead tip sits in the heart. A lead at the right ventricular apex, the traditional location, produces a wide QRS that looks much like a left bundle branch block pattern because the impulse starts in the right ventricle and spreads leftward through muscle rather than the fast-conducting Purkinje fibers. The frontal-plane axis typically swings leftward and superiorly. A lead placed in the right ventricular outflow tract, a somewhat higher position, often produces a QRS that is still wide but shifts the axis differently, sometimes inferiorly.
Whether the pacing is unipolar or bipolar also changes the QRS shape. A comparison study found that bipolar ventricular pacing can produce QRS complexes noticeably different from unipolar pacing at the same catheter location, likely because the proximal electrode contributes to the initial depolarization wavefront. Increasing the distance between the two electrodes and the stimulus intensity amplified these differences.3PubMed. A comparison of QRS complexes resulting from unipolar and bipolar pacing: implications for pace-mapping For everyday clinical interpretation, this means you should not assume that a change in QRS shape necessarily signals lead displacement; it could reflect a reprogramming from unipolar to bipolar mode or a change in output settings.
Pacemaker Modes and How They Appear on the Tracing
Pacemaker modes are described using a standardized letter code. The first letter indicates which chamber is paced, the second which chamber is sensed, and the third what the device does in response to a sensed event. A fourth letter, “R,” indicates rate-responsive behavior. The modes you encounter most often include VVI (paces and senses only the ventricle, inhibits pacing when it detects a native beat), DDD (paces and senses both atrium and ventricle, can trigger or inhibit as needed), and AAI (paces and senses only the atrium).
On the ECG, a VVI device in a patient whose native rate drops below the programmed lower limit shows ventricular spikes at regular intervals, each followed by a wide QRS. When the patient’s own rhythm is fast enough, the device stays quiet and you see native complexes with no spikes at all. A DDD device can produce several patterns on the same tracing: atrial spike followed by P wave, then ventricular spike followed by QRS; or atrial spike and P wave followed by a native QRS (meaning the device sensed intrinsic conduction and inhibited its ventricular output); or no atrial spike at all if the patient’s sinus node is firing on its own, followed by a ventricular spike after a programmed delay. A case report of a dual-chamber pacemaker nicely illustrated how a single tracing can show multiple QRS morphologies when the degree of fusion between the paced beat and the patient’s intrinsic conduction varies from beat to beat.4PubMed Central. Different QRS morphologies in a dual-chamber pacemaker: what is the mechanism? This is normal behavior, not a malfunction.
Rate-responsive devices add another layer. These adjust the pacing rate upward when their internal sensors detect that the patient is exercising or under physiological stress. The sensors can be accelerometers responding to chest motion, algorithms tracking metabolic oxygen demand, intrathoracic impedance measurements, or QT-interval analysis.5PubMed Central. Rate-Responsive Cardiac Pacing: Technological Solutions and Their Applications On the ECG, rate-responsive pacing simply looks like a faster paced rate than you might expect for a resting patient, and it can be mistaken for a pacemaker-mediated tachycardia if you are not aware the feature is turned on. The “R” in the mode designation (DDDR, VVIR) is your clue.
His Bundle Pacing and Biventricular Pacing
Newer pacing strategies aim to produce a more natural activation pattern than traditional right ventricular apical pacing, and each has a distinct ECG fingerprint.
His bundle pacing places the lead at the bundle of His, the narrow bridge of conduction tissue that sits between the atria and the ventricles. Because the impulse enters the normal conduction system, the paced QRS can look nearly identical to the patient’s native QRS rather than the wide, bundle-branch-block-like morphology of conventional pacing. The hallmark is a narrow QRS with a characteristic left ventricular lateral wall activation time seen in lead V6.6PubMed Central. A New Era of Physiologic Cardiac Pacing In some patients, the capture is “non-selective,” meaning the impulse also activates a small amount of septal muscle tissue near the lead tip, adding a subtle initial slur (sometimes called a pseudo-delta wave) before the QRS transitions into a clean, narrow complex. Conduction system pacing at the His bundle typically shows a very short time from pacing spike to ventricular activation, roughly 35 to 55 milliseconds, compared with 120 to 200 milliseconds when pacing occurs inadvertently at the atrioventricular node itself.7PubMed Central. Current Advance, Challenges and Future Perspectives of Conduction System Pacing If you see what appears to be a narrow paced QRS and you know the patient has a pacemaker, His bundle pacing or left bundle branch area pacing is the most likely explanation.
Biventricular pacing, used in cardiac resynchronization therapy, places one lead in the right ventricle and a second lead in a coronary sinus branch vein to pace the left ventricular free wall simultaneously. The expected ECG pattern during biventricular stimulation with a right ventricular apical lead is a positive (tall R wave) QRS in lead V1, because left ventricular pacing sends the wavefront toward V1 rather than away from it. One study found a dominant R wave in V1 in about two-thirds of biventricular pacing patients, with right axis deviation in the frontal plane in over half.8Indian Pacing and Electrophysiology Journal. Electrocardiographic patterns in biventricular pacing delivered by second-generation cardiac resynchronization devices A negative QRS in V1 during biventricular pacing can still represent normal left ventricular capture in certain substrates, but it should prompt you to rule out loss of left ventricular capture, lead displacement, or accidental placement of two leads in the right ventricle.9PubMed Central. Electrocardiographic follow-up of biventricular pacemakers
Fusion beats complicate the picture. In patients with a relatively short PR interval and intact native conduction, the patient’s own impulse can collide with the paced impulse, producing a hybrid QRS that is narrower than the fully paced complex. This narrowing can look like effective biventricular capture when it is actually just fusion with native conduction.9PubMed Central. Electrocardiographic follow-up of biventricular pacemakers Device interrogation or manipulating the AV delay can help distinguish the two.
Spotting Malfunctions on the ECG
Pacemaker malfunctions fall into a handful of recognizable categories, and most can be picked up on a surface ECG if you know what to look for.
Failure to Capture
You see a pacing spike but no QRS (or no P wave, for an atrial lead) follows it. The device fired, but the heart muscle did not respond. Causes include lead dislodgement, a rise in the stimulation threshold at the lead-tissue interface, lead fracture, or an output setting that is too low. On the tracing, isolated spikes sitting on the baseline without a subsequent complex are the giveaway. When the ECG amplitude of a bipolar pacing stimulus is unusually large, that can itself signal lead trouble: one study found that leads with confirmed malfunction had an average ECG bipolar stimulus amplitude of about 8 mm/V, compared to about 0.9 mm/V for normal leads, and a cutoff of 5 mm/V (normalized for programmed output) distinguished malfunctioning leads with over 90% sensitivity and specificity.10PubMed. Accurate detection of lead malfunction from ECG-derived bipolar pacing stimulus amplitude
Oversensing
The device detects a signal it interprets as a native heartbeat when no real beat occurred, so it withholds pacing when it should have fired. The result on the ECG is unexpected pauses, longer than the programmed pacing interval, with no visible spike. Oversensing is considered the most common cause of pacemaker-related pauses and a frequent clinical problem during follow-up.11PubMed. Oversensing by single-chamber pacemakers: mechanisms, diagnosis, and treatment The culprit signals can range from T waves or P waves bleeding into the sensing window, to skeletal muscle electrical activity, to electromagnetic interference from external sources. In rare cases, far-field sensing of high-amplitude atrial signals by a ventricular lead can suppress ventricular pacing altogether, particularly when the lead tip is placed high on the septum near the right atrium. One case report documented near-fatal ventricular pacing inhibition from a combination of myopotential oversensing and far-field atrial wave sensing, noting that the far-field component persisted even after the patient lost consciousness and stopped moving.12Heart Rhythm Case Reports. Near-fatal ventricular pacing inhibition caused by inappropriate safety mode in an Ingenio pacemaker: A case report
Undersensing
The opposite problem: the device fails to see a native heartbeat and delivers a pacing spike when it should have stayed quiet. On the ECG, you see pacing spikes landing at inappropriate times relative to the native rhythm, often falling on or near native QRS complexes at intervals shorter than the programmed pacing rate would dictate. The pattern can look like asynchronous pacing even though the device is programmed to sense. Causes include lead malfunction, low-amplitude native signals, or programming the sensitivity threshold too high.
Pacemaker-Mediated Tachycardia
Dual-chamber pacemakers can create a re-entrant loop: the ventricle contracts, the impulse travels backward to the atrium, the atrial channel senses that retrograde P wave, and the device dutifully paces the ventricle again after the programmed AV delay, perpetuating the cycle. The ECG shows a regular, paced tachycardia with a 1:1 relationship between atrial events and ventricular paced beats. One documented case presented as a tachycardia at 130 beats per minute with pacing spikes preceding every QRS.13PubMed Central. Reversible left ventricular dysfunction due to endless loop tachycardia in patient with dual chamber pacemaker- A case report The tachycardia was perpetuated by a short post-ventricular atrial refractory period (the window after a ventricular event during which the device ignores atrial signals) combined with intact retrograde conduction. Prolonging that refractory period is usually the fix.14PubMed. Spontaneous endless loop tachycardia
Using a Magnet
Placing a magnet over a pacemaker switches it to an asynchronous pacing mode, meaning it fires at a fixed rate regardless of what the heart is doing. A DDD device becomes DOO (paces both chambers, senses nothing), VVI becomes VOO, and so on. The rate-response feature turns off.15EP Europace. Clinical applications of magnets on cardiac rhythm management devices This maneuver is useful for two things: confirming capture (if the device fires and you see a QRS after every spike, capture is intact) and checking battery status (the magnet rate varies by manufacturer and drops as the battery approaches end of life).
The expected magnet rate depends on the device. Older Medtronic models, for instance, deliver three initial beats at 100 bpm for threshold testing and then pace at 85 bpm, while newer-generation Medtronic devices may skip the threshold test and simply pace at 85 bpm.16PubMed Central. Unusual response to magnet application: What is the Mechanism? Different manufacturers use different rates. Knowing the expected magnet rate for the device brand and model matters because a rate lower than expected is your signal that the battery is running low. Removing the magnet should restore the device to its original programming in virtually all pacemaker brands.15EP Europace. Clinical applications of magnets on cardiac rhythm management devices
Diagnosing a Heart Attack Through a Paced Rhythm
This is arguably the hardest diagnostic challenge in pacemaker ECG interpretation. Right ventricular pacing produces a wide QRS with ST-segment changes that are “discordant,” meaning the ST segment deflects in the opposite direction from the main QRS deflection. That discordance is expected and normal for a paced rhythm, which means the usual ST-elevation criteria for detecting a heart attack do not apply. Applying standard STEMI criteria to a paced ECG will produce a flood of false positives.
The original Sgarbossa criteria, developed for left bundle branch block and later extended to paced rhythms, identify three patterns that suggest acute infarction despite the expected ST changes: concordant ST elevation of 5 mm or more in leads with a positive QRS, concordant ST depression of 1 mm or more in V1 to V3, and excessively discordant ST elevation. The modified Sgarbossa criteria refined the third rule, replacing the fixed 5 mm cutoff for discordant ST elevation with a proportional rule: ST-segment deviation that exceeds 25% of the preceding S-wave depth. A study specifically validating the modified criteria in ventricular-paced patients found they were substantially more sensitive than the originals, detecting about 81% of acute coronary occlusions compared with 56% for the original criteria, while maintaining specificity above 96%.17PubMed. Electrocardiographic Diagnosis of Acute Coronary Occlusion Myocardial Infarction in Ventricular Paced Rhythm Using the Modified Sgarbossa Criteria Adding concordant ST depression in leads V4 through V6 pushed sensitivity up to about 86%.
In practice, if you are evaluating chest pain in a patient with a ventricular paced rhythm, applying the modified Sgarbossa criteria gives you the best available surface ECG tool. A case report illustrated how the original weighted scoring system missed an infarct that the unweighted modified criteria caught, because any single positive criterion in the modified system is enough to raise the alarm rather than requiring a point total.18PubMed. Identifying acute myocardial infarction in ventricular-paced patients: The effectiveness of modified Sgarbossa criteria The sensitivity is still not perfect, so clinical suspicion, troponin trends, and echocardiography remain essential when the ECG is equivocal.
Cardiac Memory and Why It Mimics Ischemia
After a pacemaker has been pacing the ventricles for a period and then stops (either because the device is turned off, the battery runs out, or the patient’s native rhythm resumes), the heart’s repolarization pattern does not immediately snap back to normal. Instead, the T waves can remain altered for hours to days, a phenomenon called cardiac memory. These T-wave inversions can look disturbingly similar to the changes seen in myocardial ischemia, leading to unnecessary catheterizations if the clinician is not aware of the history.
Distinguishing the two is possible with surface ECG criteria. A study comparing post-pacing cardiac memory T waves with ischemic T-wave inversions found that the T-wave axis in cardiac memory pointed in a significantly different direction from that seen in most ischemic territories. A simple two-step criterion combining a positive T wave in lead aVF with either a specific amplitude relationship or a positive T wave in V5 and non-negative T wave in lead I identified cardiac memory with 91% sensitivity and 92% specificity.19PubMed. Surface ECG criteria can discriminate post-septal pacing cardiac memory from ischemic T wave inversions The takeaway for anyone interpreting a post-pacing ECG: before assuming those deep T-wave inversions represent acute ischemia, check the T-wave axis and ask when the pacemaker last paced.
Automated ECG Interpretation and Its Limits
Most modern ECG machines generate an automated interpretation, and those algorithms struggle with paced rhythms. One major challenge is simply detecting the pacing artifact itself. An automated detection algorithm tested on a dataset of paced ECGs correctly classified whether a given data point belonged to a pacing artifact with a specificity of about 99.6% but a sensitivity of only about 74.5%.20medRxiv. Detection and removal of pacing artifacts prior to automated analysis of 12-lead ECG Missing a quarter of pacing artifacts means the algorithm may try to interpret a paced QRS as if it were native, producing misleading automated reads like “left bundle branch block” or “ST elevation” that are really just normal paced morphology.
The broader implication is that automated ECG reports for pacemaker patients should be treated as unreliable until reviewed by a human who understands the device. This is especially true in emergency settings where a machine read of “acute MI” could trigger an inappropriate catheterization lab activation. When reviewing a machine interpretation on a paced ECG, mentally set the computer’s diagnosis aside and apply your own systematic approach: identify the spikes, check for capture after each spike, assess the pacing mode, and only then consider whether the ST changes meet modified Sgarbossa criteria or fall within the range of expected discordance.