What this simulator does
It recreates a left bundle branch area pacing implant: choosing the entry site on fluoroscopy, screwing the lead through the septum, and proving conduction-system capture with the EHRA 2023 criteria. ECGs and electrograms are generated from a heart-vector model, so every lead responds consistently to where the tip is, what is captured and at what output.
Quick start: your first case
- Pick a case. Use the Case menu at the top. Start with "Sinus node disease · narrow QRS".
- Choose the entry site (RAO 30°). Drag the lead tip on the fluoroscopy image to about 15–20 mm below the yellow His / TV summit tag, along or just inferior to the red His–apex line. Turn on "Target sector" to see the commonly used zone.
- Pace map the right septum. Pacing starts at 2.5 V. Look for a W-shaped notch at the bottom of V1, lead II positive or equiphasic and lead III negative or equiphasic. The coaching panel under the analyzer comments on this.
- Orient the sheath (LAO, septum on edge). Each case starts with the sheath pointing anteriorly, as it usually does after crossing the tricuspid valve. In RAO this looks almost the same as a septal position; in LAO the tip sits to the left of the septal line, on the RV free wall. Torque the sheath counter-clockwise (or drag it on the LAO image) until it is perpendicular to the septal line, usually 10–40° above horizontal (about 2 o'clock). Steeper slides the tip anterior onto the free wall; much flatter slides it posterior-inferior. Off-axis angles on the septum cause a drill effect.
- Rotate the lead. Use "Rotate lead ×1" and watch after every turn: QRS narrowing, a terminal R appearing in V1, V6RWPT shortening, fixation beats, unipolar impedance (rises then falls) and the COI (rises to 20–35 mV, then falls). The analyzer trends plot each turn.
- Stop when V1 shows a terminal R. Then prove capture, ideally with a transition at decrementing unipolar output (the ESC 2025 gold standard); timing criteria alone give only probable capture. Run a threshold test, deliver S1–S2, turn pacing off to look for an LBB potential, and use the calipers. Click any criterion for an explanation and an example.
- Finish the implant. Withdraw the sheath, measure unipolar and bipolar thresholds, turn pacing off briefly for the R wave, set slack, slit the sheath, check slack with a deep breath, then press "Finish case".
The screen
FluoroscopySimulated x-rays from real CT scans in RAO 30° and LAO 40°. Pulsed at 7.5 frames/s with last-image hold. Hold Space or the Fluoro button to screen; auto-fluoro screens while you move the lead. Contrast puff outlines the RV and septum.
EP recording system12-lead ECG plus the tip EGM, filtered (30–500 Hz) and unfiltered (0.5–500 Hz, which shows the COI). V1 and V6 are yellow. Sweep mode by default; change speed and gain at the top.
Analyzer screenLive unipolar EGM with the COI bracket, and trends of impedance, COI and Stim–V6RWPT after every turn. Coaching messages sit underneath.
Control bar (bottom)Always on screen. Lead row: Rotate ×1, ×3, Unscrew, forward support, the Lead menu (lead type, retract & reposition), the LAO sheath angle and the Sheath & slack menu (withdraw, slit, deep breath, slack). Pacing row: pacing on/off, output, the settings menu (pulse width, polarity, rate, backup RV lead), threshold test, the S1–S2 menu (auto sweep or single S2) and "Wait 5 min" (COI decay). Pace 10–20 bpm above sinus to avoid fusion.
Phase bar and hintsThe header shows where you are: Target, Pace map, Penetrate, Confirm capture, Accept & secure. The hint line underneath says what to do in that phase. Press H to hide or show it.
Follow-up clinicFrom the case summary: day 1 and 6-week checks with matured thresholds, possible micro- or macro-dislodgement (more likely with a shallow tip, too little slack or a drilled tunnel), implant-versus-today ECG, a clinic threshold test and a decision to make.
AnalysisOpens in place of the live recording when you freeze a beat, finish a threshold test or S1–S2, or press "Analysis". Calipers (drag across the beat), transitions side by side or overlaid, a 25 mm/s 12-lead and, in instructor view, the septal cross-section and refractory periods. "Back to live" or Esc returns.
Capture assessmentThe verdict follows the phase: grey while pace mapping, blue while penetrating, amber for likely or unproven capture, green for LBB area capture, red for hazards (perforation, loss of capture). Each EHRA criterion has a tick or cross; click one to see why, how to measure it, and an example. The event log under the recording records everything.
Proving capture: suggested order
1. Terminal R in V1? Yes: stop and test. No: usually not yet deep enough, so check impedance, COI and fixation beats before each further turn; if V6RWPT is already short, test capture first (a terminal R is occasionally absent). With baseline RBBB this step does not discriminate.
2. Stim–V6RWPT. < 75 ms (narrow QRS or isolated RBBB) or < 80 ms (LBBB, IVCD, RBBB + fascicular block, escape, asystole) = LBBP; < 85 / < 100 ms = likely LBBP (EHRA). Higher values do not exclude it.
3. V6–V1 interpeak. > 44 ms is highly specific; > 33 ms supports non-selective capture.
4. Transition on decremental output. ns-LBBP → s-LBBP (local EGM splits), or ns-LBBP → LVSP with V6RWPT prolonging ≥ 15 ms. The most convincing single test. Start from a high output (5 V or more) and test unipolar.
5. No transition? Try S1–S2 (LBB and myocardium recover at different times), a shorter pulse width (may separate the thresholds), or pacing off to compare the LBB potential–V6RWPT with the paced Stim–V6RWPT.
Off the septum. A pace map from the RV free wall is wide (about 170–190 ms) with a long V6RWPT: anterior and outflow sites give II and III positive, posterior-inferior sites II and III negative. Screwing in there gives no terminal r′, little impedance rise and a small COI, and the 4 mm wall perforates within a few turns (impedance falls, COI and capture are lost). Retract, torque back onto the septum and start again.
Pitfalls. If perforation is suspected, reposition the lead rather than withdrawing it a few turns. Anodal capture in bipolar mode hides the R′ in V1. Perforation: unipolar impedance < 450 Ω or a fall > 200 Ω, COI < 3–5 mV or absent, rising threshold, loss of the LBB potential, contrast reaching the LV. Fixation beats with QR/rSR′ in V1 mean you are close.
Controls reference
- Drag the tip (RAO, lead not fixed): entry site. In LAO, dragging swings the sheath angle instead. The HUD shows distance from the TV summit.
- Rotate ×1 / ×3, Unscrew ×1: advance or back out. Lumenless leads pause pacing while you rotate; stylet-driven leads pace continuously but need forward support.
- Retract & reposition: start again at a new site (counts as an extra site).
- Output ±, pulse width, polarity: working output for pacing and capture. Bipolar output above the ring threshold causes anodal capture when the ring touches the septum.
- Run threshold test: steps down every 3 beats and records each morphology in Transitions and Overlay.
- S1–S2 auto sweep / Deliver S1×8 + S2: 8-beat drive at 600 ms then an S2, decremented automatically in 10 ms steps from 400 ms (about a minute), or one S2 at the coupling you type.
- Pacing ON/OFF: off shows the intrinsic rhythm and LBB potential, and records the R wave.
- Freeze & review beat: sends the latest beat to the review panel for calipers. Turn off auto markers to measure blind.
- Sheath orientation in LAO: drag on the LAO image to swing the sheath, use the CCW / CW torque buttons (5° per click, keys [ and ]), or the slider. Counter-clockwise torque brings the tip onto the septum, flatter towards 2–3 o'clock; clockwise turns it steeper towards 12 o'clock (anterior). Locked once the lead is screwed in.
- LAO angle readout: in LAO the badge on the image compares the sheath angle with the angle perpendicular to this heart's septal line: green within 10°, amber 11–20° (drill risk), red beyond 20°. Hidden in blind assessment.
- Keyboard: Space fluoro, R rotate (Shift+R ×3), U unscrew, ↑/↓ output, P pacing, T threshold, S S1–S2, F freeze, Esc back to live, V RAO/LAO, C contrast, ? list.
- Guided first case: the link in the hint bar walks through one easy case a control at a time (LAO torque, entry site, pace map, rotation, calipers, threshold test). It is offered once on first visit.
- Instructor view: reveals depth, thresholds, conduction anatomy on fluoro and the septal cross-section.
Teaching traps, blind assessment and the case log
Teaching traps (Case menu) each set up one situation where a single criterion misleads you, with a teaching point at the end. Blind assessment (header) hides the case, coaching and verdict; you perform the tests and declare the capture type and level, then see the answer. Finish case scores the procedure; the Case log tab keeps your scores and a learning curve in this browser. Add #case-iso or #blind-rbbb (any case id) to the page address to share a specific case.
About and limits
Educational use only. This simulator is a teaching tool for trainees learning conduction system pacing. It is not a medical device, has not been clinically validated, and must not be used to plan, guide or document the care of any patient. Always follow current guidelines, local protocols and supervision by an experienced implanter.
Created by Dr Joseph Assad, Cardiologist. Feedback, corrections and case ideas are welcome at feedback@pacingsimulator.com, or via LinkedIn or X.
Build 1.5 (19 October 2026). It is a teaching model, not a validated clinical tool. The procedure score is illustrative and does not measure implant competence. Thresholds, refractory periods and the scoring are illustrative; the ECG model reproduces published patterns and intervals but not every patient's morphology.
Copyright and terms of use
© 2026 Joseph Assad. All rights reserved. The simulator, including its source code, ECG and electrogram models, case designs, scoring, text and graphics, is protected by copyright.
You may use the simulator online, free of charge, for personal learning and for teaching within a training programme. Without prior written permission you may not:
- copy, download, mirror, host or redistribute the simulator or any part of its code;
- modify it, create derivative works from it, or reverse engineer its models or case parameters;
- use it, or any part of it, for any commercial purpose, including paid courses, sponsored training or inclusion in a commercial product;
- remove or alter this notice, the educational-use notice or the author credit.
Screenshots may be shown in teaching sessions and academic presentations with the credit "PacingSim, Dr Joseph Assad, pacingsimulator.com". For licensing, course use or any other permission, contact feedback@pacingsimulator.com.
Third-party material keeps its own licence: the CT data are from the TotalSegmentator dataset (CC BY 4.0), and published criteria are cited in the references. The site collects anonymous usage statistics (visits, time spent and which features are used) to measure its reach and improve it; it uses no cookies, stores nothing on your device, keeps no IP addresses and honours Global Privacy Control and Do Not Track. The simulator is provided "as is", without warranty of any kind, and the author accepts no liability for any use of it.