[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Clinical skills](https://durgical.com/en/clinical-skills/) › [The ECG](https://durgical.com/en/clinical-skills/areas/ecg/) › Paced rhythms

# Paced rhythms

- ECG tracing

A pacemaker sends small electrical pulses to the heart; each pulse shows as a sharp spike on the ECG followed by the heartbeat it triggers.

Updated Nov 13, 2026

## Key points

- Spike then P wave: atrial pacing; spike then broad QRS: ventricular pacing; two spikes: dual-chamber (atrial then ventricular).
- RV apical pacing gives a broad, LBBB-like QRS with left axis deviation and discordant ST-T; this is normal and not a new LBBB.
- Failure to capture: a spike with no P wave or QRS after it; failure to pace: no spike when one is due.
- Undersensing: spikes fall too early, inside native beats or on the T wave; oversensing: pauses because noise is read as beats.
- Code: chamber paced, chamber sensed, response (I inhibit, T trigger, D dual), R rate modulation; e.g. VVI, AAI, DDD, DDDR.

On this page

- The basics Key points
- What it is
- How to recognise it
- Why it happens
- Step by step
- Malfunctions on the ECG
- Next step
- Elsewhere in DurgiPedia

- Clinical Differential and pitfalls

- Study Test yourself

- Connections Connections

## What it is

A permanent pacemaker senses the heart's own activity and paces a chamber when that activity is too slow or absent; its main indications are symptomatic sinus node dysfunction and high-grade AV block, and cardiac resynchronisation adds a left ventricular lead in selected heart failure. The ECG sign of pacing is the pacing spike: a very short, sharp vertical line that may be tall with unipolar leads and tiny, sometimes invisible, with bipolar leads. An atrial spike is followed by a P wave; a ventricular spike is followed by a broad QRS. Pacing from the right ventricular apex activates the ventricles from the right and from below, so the QRS is wide with a left bundle branch block-like look and left axis deviation, and the ST-T is normally discordant (opposite to the main QRS direction). Biventricular pacing usually gives a narrower QRS, often with a dominant R in V1 and a right or north-west axis. The device's mode is written in a three-to-five-letter code: chamber paced, chamber sensed, response to sensing (inhibit, trigger or dual) and rate modulation, so VVI paces and senses the ventricle and is inhibited by a native beat, and DDD paces and senses both chambers. Reading a paced ECG means answering four questions: are there spikes, does each spike capture, is the device sensing native beats correctly, and is the rate what the device should deliver. Malfunctions are failure to pace, failure to capture, undersensing and oversensing, and pacemaker-mediated tachycardia.

## How to recognise it

- Pacing spike: a vertical deflection lasting a few ms; large with unipolar pacing, small with bipolar pacing; look in several leads and the rhythm strip.
- Atrial pacing: spike followed by a P wave, then the patient's own (narrow) QRS if AV conduction is intact.
- RV pacing: spike followed by a broad QRS (usually ≥120 ms, often much wider), LBBB-like in V1 (mainly negative), left axis with apical pacing; ST-T opposite to the QRS.
- Dual-chamber (DDD) pacing: atrial spike, P wave, a programmed AV delay, then a ventricular spike and broad QRS; or a sensed native P wave followed by a ventricular spike (atrial tracking).
- Biventricular pacing: QRS narrower than RV pacing, often a dominant R wave in V1 and right axis; loss of this pattern suggests loss of LV capture.
- Magnet over a pacemaker usually gives asynchronous pacing at a fixed magnet rate; over an implantable cardioverter-defibrillator it usually suspends shock therapy without changing pacing.

## Why it happens

- The pulse depolarises tissue next to the lead tip; the impulse then spreads cell to cell rather than through the fast conduction system, which is why ventricular paced beats are broad.
- Capture needs an output above the tissue's threshold; the threshold rises with lead dislodgement, fibrosis at the tip, hyperkalaemia, acidosis, ischaemia and some antiarrhythmics.
- Sensing depends on how large the native signal is at the lead: small signals are missed (undersensing), while muscle noise, far-field waves or lead fracture artefact may be counted (oversensing).

## Step by step

- Look for spikes in every lead and the rhythm strip — bipolar spikes can be almost invisible in some leads.
- Say what follows each spike: P wave, broad QRS or nothing — this shows which chamber is paced and whether it captures.
- Measure the paced rate and the intervals between spikes and native beats — compare them with the programmed lower rate on the device card.
- Check that native beats inhibit pacing — a spike within or just after a native QRS means undersensing.
- Look for pauses longer than the lower-rate interval with no spike — think oversensing, lead fracture or battery depletion.
- Review QRS shape and axis against the expected pacing site — a change in a biventricular device suggests loss of LV capture.
- Finish with the patient: symptoms, haemodynamics, potassium and recent procedures — and arrange device interrogation.

## Malfunctions on the ECG

| Problem | What you see | Common causes |
| --- | --- | --- |
| Failure to capture | Spike with no P wave or QRS after it | Lead dislodgement, raised threshold (hyperkalaemia, ischaemia, drugs), low output |
| Failure to pace | No spike where one is due; pause | Oversensing, lead fracture, battery depletion, loose connection |
| Undersensing | Spikes that ignore native beats, fall inside the QRS or on the T wave | Low native signal, lead problem, sensitivity set too low |
| Oversensing | Pauses or slow pacing with no spikes | Muscle activity, far-field signals, lead fracture artefact |
| Pacemaker-mediated tachycardia | Regular paced tachycardia at the upper rate limit | Dual-chamber device tracking retrograde P waves |

## Differential and pitfalls

- Normal behaviours that mimic malfunction: hysteresis (pacing starts below the programmed rate), rate response during exercise, a lower sleep rate, mode switching in atrial fibrillation and pacing at the upper tracking rate.
- Paced rhythm hides ischaemia and conduction diagnoses; in chest pain, look for concordant ST change, which strongly suggests infarction, and treat on the clinical picture.
- Pacemaker-mediated tachycardia: a dual-chamber device tracking retrograde P waves gives a regular paced tachycardia at or near the upper rate limit.
- Failure to capture with a slow rhythm and symptoms is an emergency; check potassium and recent drug changes as well as the lead.
- Tiny bipolar spikes can be missed, making a paced rhythm look like LBBB or ventricular tachycardia; ask about a device and look for a scar under the clavicle.
- Digital filters and artefact can create false spikes; confirm with another recording or the device interrogation.

## Next step

- Failure to capture or to pace with bradycardia and symptoms: call a senior now, follow the bradycardia algorithm (atropine, transcutaneous pacing) and get cardiology to interrogate the device.
- Stable patient with a suspected problem: arrange device interrogation, check potassium, magnesium and the drug list, and get a chest X-ray to look at lead position and integrity.
- Suspected pacemaker-mediated tachycardia: a magnet over the pacemaker usually stops it; then ask for reprogramming.
- Before surgery with electrosurgery or an MRI, tell the team and the device clinic so the device can be reprogrammed and checked afterwards.
- Record the device type and mode in the notes and keep a copy of the paced ECG for comparison.

## Test yourself

Think of the answer, then tap the question to see it.

## A rhythm strip shows sharp spikes at 60 per minute; every second spike is followed by nothing, and the pulse is 30 per minute. What is happening?

Intermittent failure to capture: the device paces but some pulses do not depolarise the ventricle. Check potassium, consider lead dislodgement, and get urgent device interrogation; treat symptomatic bradycardia.

## A patient with a pacemaker has a broad QRS after every spike, negative in V1, with left axis and ST-T opposite to the QRS. Is this left bundle branch block?

No: this is the expected pattern of right ventricular apical pacing. Paced beats activate the ventricles slowly from the right apex, so they look LBBB-like with discordant ST-T.

## What does a pacing spike landing on the T wave of a native beat suggest?

Undersensing: the device did not see the native beat and paced anyway. It can trigger ventricular arrhythmia and needs device review.

## What do the letters in a DDD pacemaker mode mean?

The first D: both chambers paced; the second D: both chambers sensed; the third D: dual response, inhibiting on sensed beats and tracking sensed atrial beats to pace the ventricle.

[Add to my cards (4)](https://durgical.app/estudiar/?add=/en/clinical-skills/paced-rhythms/)

## Elsewhere in DurgiPedia

- [ECG basics: waves, intervals and leads](https://durgical.com/en/physiology/ecg-basics-waves-intervals-and-leads/)
- [The cardiac electrical axis](https://durgical.com/en/physiology/the-cardiac-electrical-axis/)
- [The SA node pacemaker potential](https://durgical.com/en/physiology/the-sa-node-pacemaker-potential/)

## Connections

## Related 8

- [Transcutaneous pacing](https://durgical.com/en/clinical-skills/transcutaneous-pacing/)
- [AV blocks: first, second (Mobitz I and II) and third degree](https://durgical.com/en/clinical-skills/av-blocks-first-second-and-third-degree/)
- [Right and left bundle branch block and fascicular blocks](https://durgical.com/en/clinical-skills/right-and-left-bundle-branch-block-and-fascicular-blocks/)
- [Sinus rhythm, sinus tachycardia, bradycardia and arrhythmia](https://durgical.com/en/clinical-skills/sinus-rhythm-sinus-tachycardia-bradycardia-and-arrhythmia/)
- [STEMI and its territories, and STEMI mimics](https://durgical.com/en/clinical-skills/stemi-and-its-territories-and-stemi-mimics/)
- [Hyperkalaemia and other electrolyte changes](https://durgical.com/en/clinical-skills/hyperkalaemia-and-other-electrolyte-changes/)
- [Defibrillation and synchronised cardioversion](https://durgical.com/en/clinical-skills/defibrillation-and-synchronised-cardioversion/)
- [Reading an ECG step by step](https://durgical.com/en/clinical-skills/reading-an-ecg-step-by-step/)

## Related conditions 5

- [Atrioventricular (heart) block](https://durgical.com/en/conditions/atrioventricular-block/)
- [Sinus node dysfunction (sick sinus syndrome) and symptomatic bradycardia](https://durgical.com/en/conditions/sinus-node-dysfunction-and-symptomatic-bradycardia/)
- [Atrial fibrillation](https://durgical.com/en/conditions/atrial-fibrillation/)
- [Heart failure](https://durgical.com/en/conditions/heart-failure/)
- [Syncope (vasovagal, orthostatic hypotension, cardiac)](https://durgical.com/en/conditions/syncope/)

## In Pharmacology 1

- [Atropine](https://durgical.com/en/pharmacology/atropine/)

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Educational content; it does not replace supervised hands-on training, your institution's protocols or a prescriber's order. A procedure is learnt with a teacher and done only once you are trained to do it.

Sources: [Pacemaker Types and Selection](https://www.ncbi.nlm.nih.gov/books/NBK556011/) (StatPearls (NCBI Bookshelf), 2022); [Pacemaker Malfunction](https://www.ncbi.nlm.nih.gov/books/NBK553149/) (StatPearls (NCBI Bookshelf), 2023).

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Representative model, for educational purposes only.

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License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/clinical-skills/paced-rhythms/