Geometric distortion of the limb-lead system under Mason-Likar electrode placement.

BACKGROUND: The Mason-Likar (ML) modification is standard for exercise and ambulatory ECG but distorts the recording relative to standard distal placement. This was quantified against a proximal-limb (Lund) reference. METHODS: In 12 patient-specific torso-heart models, ML and Lund limb electrodes and precordials V1-V9 were positioned. We derived the Wilson-central-terminal (WCT) displacement, each limb lead's frontal-plane direction and antero-posterior (AP) fraction, and - in the one subject with a validated boundary-element-method (BEM) forward model - the signal-level limb amplitudes and the offset added to the chest leads across 32 beats. RESULTS: Across 12 models ML displaced the WCT 72.4 ± 15.9 mm anteriorly (range 51.9 to 96.3 mm). Limb‑lead directions departed from textbook (II +42°, aVF +65°), and the inferior lead axes tilted out of the frontal plane, acquiring anterior forces (AP-fraction roughly doubling). In the one subject with a forward model (32 paced beats), ML amplified II, III and aVF (×1.37, ×1.40, ×1.47) and attenuated I (×0.64) and aVR (×0.79), placing a + 45° QRS at about +67°, and added one offset to every chest lead (median 0.35 mV, range 0.10-0.59 mV), its relative weight reaching 90-140% in V7-V9. CONCLUSIONS: ML modestly tilts the measured frontal axis, reweights limb‑lead amplitudes with the inferior leads acquiring anterior forces, and adds a common offset to every chest lead. The geometry rests on 12 models, the signal-level magnitudes on one subject. Confirm important inferior, lateral or precordial findings on a standard 12‑lead.