A paced rhythm on an ECG is identified by sharp, narrow vertical lines called pacing spikes, each immediately followed by a heart complex the pacemaker triggered. The spikes look like tiny needles or hash marks on the tracing, and the heart complexes that follow them are often wider and shaped differently than what you’d see in a normal, unpaced heartbeat. The exact appearance depends on where the pacemaker leads are placed and how many chambers they’re pacing.
The Pacing Spike
The most recognizable feature of any paced rhythm is the pacing spike: a thin, vertical deflection that appears just before the electrical activity it triggers. On paper or a monitor, it looks like a brief, sharp line, almost like a tiny antenna sticking up (or down) from the baseline. Every time the pacemaker fires, one of these spikes appears.
How tall that spike is depends on the type of lead. Modern pacemakers almost universally use bipolar leads, where the two electrodes sit close together on the same wire inside the heart. These produce very small spikes, averaging less than 1 mm tall on a standard ECG. On some monitors, bipolar spikes are so subtle they can be easy to miss. Older unipolar systems, where the electrical circuit runs between the lead tip and the pacemaker box itself, produce much larger, more obvious spikes. If you’re looking at an ECG and can barely see the spikes, that’s normal for a modern device.
Ventricular Pacing
Ventricular pacing is the most common and most visually striking paced pattern. The pacemaker lead sits in the right ventricle, so when it fires, it forces the ventricles to contract from that point outward rather than through the heart’s normal wiring. This produces a wide, broad QRS complex that looks very similar to a left bundle branch block pattern, because the right ventricle activates first and the left ventricle follows with a delay.
The QRS complex during ventricular pacing is noticeably wider than normal. In patients paced from the right ventricular apex (the most common position), the paced QRS typically measures around 140 to 150 milliseconds, compared to the normal range of under 120 milliseconds. You’ll see a pacing spike immediately before each of these wide complexes. The T wave often points in the opposite direction from the main QRS deflection, a feature called “appropriate discordance” that’s expected and normal in paced rhythms.
Because the electrical signal starts in the right ventricle and spreads leftward, the overall direction of the QRS shifts. In leads that look at the heart from the left side, the complex is predominantly upward. In leads looking from the right, it’s predominantly downward. This is one of the easiest ways to confirm the lead is positioned in the right ventricle.
Atrial Pacing
When only the atrium is paced, the ECG looks much closer to a normal rhythm. A pacing spike appears just before each P wave (the small bump representing atrial contraction). The P wave shape may look slightly different from a natural one depending on exactly where the lead sits in the atrium, but it can also appear essentially normal. The key giveaway is that tiny spike right before it.
Because the electrical signal still travels through the heart’s normal conduction system from the atria down to the ventricles, the QRS complex that follows is narrow and normal-looking. This is the main visual distinction: atrial pacing gives you a spike before the P wave with a normal QRS, while ventricular pacing gives you a spike before a wide, abnormal QRS.
Dual-Chamber Pacing
Dual-chamber pacemakers have one lead in the atrium and one in the ventricle, and they can pace either or both chambers as needed. When both are firing, you’ll see two pacing spikes per heartbeat. The first spike triggers the P wave (atrial contraction), and after a programmed delay that mimics the heart’s natural pause, the second spike triggers the wide QRS complex (ventricular contraction). That delay between the two spikes is typically programmed somewhere around 120 to 230 milliseconds, recreating the normal timing the heart uses to fill the ventricles before they squeeze.
The pattern isn’t always two spikes per beat, though. These devices are designed to stay out of the way when the heart is working on its own. If the heart generates a normal P wave, the pacemaker senses it and only paces the ventricle if needed. So you might see beats with two spikes, beats with one spike (before just the P wave or just the QRS), and beats with no spikes at all, all on the same tracing. This shifting pattern is normal and means the pacemaker is responding appropriately to the heart’s own activity.
Biventricular (CRT) Pacing
Biventricular pacing, used in cardiac resynchronization therapy, adds a third lead that paces the left ventricle through a vein on the heart’s outer surface. The goal is to make both ventricles contract simultaneously in patients whose ventricles have fallen out of sync. The ECG pattern is distinctive.
Instead of the typical left bundle branch block appearance you see with standard right ventricular pacing, biventricular pacing often produces a tall upward deflection in lead V1 (which normally shows a downward QRS during right ventricular pacing alone). You may also see a deep downward deflection in lead I. Research published in JACC found that adding the depth of the downward wave in lead I to the height of the upward wave in V1, a sum greater than 5 mm, identified biventricular pacing with 96% specificity. Some tracings also show two closely spaced ventricular pacing spikes, one for each ventricle, though these can be difficult to distinguish on a standard ECG.
What Normal Pacing Should Look Like
A properly functioning paced rhythm has a consistent, predictable relationship between spikes and the complexes they produce. Every pacing spike should be followed by its expected response: a P wave after an atrial spike, a wide QRS after a ventricular spike. The timing between beats should be regular at whatever rate the pacemaker is programmed to, commonly 60 to 70 beats per minute as a lower limit.
Some pacemakers have rate-responsive features that speed up the pacing rate when sensors detect physical activity, breathing changes, or other signs of exertion. On an ECG taken during exercise or movement, you might see the paced rate climb well above the programmed base rate. This is intentional and looks like a faster version of the same paced pattern.
Signs of Pacemaker Malfunction
Knowing what normal looks like makes it easier to spot problems. The two most important malfunctions to recognize are failure to capture and failure to sense.
Failure to capture shows up as a pacing spike that isn’t followed by the expected heart complex. You see the sharp vertical line, but no P wave or QRS comes after it. The spike just sits there on the baseline, doing nothing. This can happen if the lead has shifted out of position, if the wire is damaged, or if the heart tissue at the lead tip has become less responsive. It’s one of the most visually obvious pacemaker problems on an ECG.
Failure to sense (undersensing) looks different. The pacemaker doesn’t recognize the heart’s own beats, so it fires its spikes at inappropriate times, sometimes landing on top of or very close to the heart’s natural complexes. You’ll see pacing spikes scattered in places they shouldn’t be, including during the recovery phase of the heartbeat (the T wave), where stimulation can potentially trigger dangerous rhythms. The tracing looks disorganized, with paced and natural beats jumbled together without the clean, coordinated pattern of a properly sensing device.
The opposite problem, oversensing, produces a different appearance. The pacemaker mistakes other electrical signals (muscle movement, electrical noise, or even the T wave) for real heartbeats and holds back when it should be pacing. The result is unexpected pauses on the ECG where you’d expect to see pacing spikes but don’t. If the patient’s own heart rate is unreliable, these pauses can cause symptoms like dizziness or near-fainting.
Lead Damage on ECG
A damaged lead can sometimes reveal itself through changes in spike size. With bipolar leads, insulation damage causes current to leak, which makes the pacing spike suddenly appear much taller than expected. In one study, the average spike amplitude in leads with known insulation breaks measured about 7.9 mm, compared to under 1 mm for normal functioning leads. If a previously invisible bipolar spike suddenly becomes prominent, that’s a red flag for lead malfunction even before other signs appear.