Each of the ~3,000 dots samples the epicardial surface of the left (LV) and right (RV) ventricle. Every point's activation time is found with an eikonal-style approximation used in real electro-anatomic mapping systems (CARTO, EnSite):
t_arrival(p) = min over active sources s of
[ t0(s) + dist(p, s) / CV(p) ]
Conduction velocity CV(p) is a real physiological range: specialized His-Purkinje fibers conduct at ≈0.2–0.4 cm/ms (2–4 m/s); working ventricular myocardium conducts ≈0.03–0.05 cm/ms (0.3–0.5 m/s), about 8–10× slower; a fibrotic scar patch (toggle) drops local CV by a further 80%.
- Intrinsic LBBB — the left bundle branch is blocked, so the impulse leaves the AV node/His bundle, races down the still-intact right bundle to activate the RV quickly, then must cross the septum myocyte-to-myocyte at the slow rate to reach the LV free wall last — the hallmark of left bundle branch block, and why untreated LBBB widens the QRS to ~150–180 ms.
- RV-only pacing — a right-ventricular apical lead captures local myocardium directly (bypassing the fast conduction system entirely), so the wavefront must still cross to the LV free wall at the slow myocardial rate — this iatrogenic dyssynchrony is why isolated RV pacing can worsen heart failure.
- BiV pacing (CRT) — a second lead placed in a coronary-sinus branch over the LV free wall fires as well. Two wavefronts now start close to their respective free walls and fuse near the septum, sharply cutting the distance either one must travel alone — this fusion is the entire mechanism of cardiac resynchronization therapy.
- VV offset — the programmed delay between the two pacing sources; a small negative offset (LV slightly first) often compensates for the naturally longer RV-to-LV path and further shortens total activation time.
- LV lead position — swept around the free wall via a simulated coronary-sinus branch; placing it over scar (toggle) forces conduction through the slow scar CV and can worsen resynchronization even in BiV mode — the real-world reason implanters use imaging to steer the LV lead away from scar.
The colored sweep is an isochronal (equal-activation-time) map: color encodes arrival time from early (blue) to late (red), replaying once per simulated heartbeat, exactly how a clinical electro-anatomic mapping system displays a chamber's activation sequence.