Inverted Lead III (−III) at −60° as a Complementary Frontal Lead in Anterior STEMI: A Vector-Based Approach to Anterolateral and Basal Extension

BACKGROUND AND OBJECTIVES The standard 12-lead ECG leaves a 60° gap in the superior-left frontal quadrant between aVL (-30°) and -90°. Inverting lead III yields a derived lead (-III) at -60°, potentially improving the detection of superior-leftward injury vectors in anterior myocardial infarction (MI). The objective is to evaluate whether -III provides complementary diagnostic information to aVL and if its ST-segment amplitude reflects vector orientation. METHODS We analyzed 25 ECGs from patients with anterior MI and available cardiac catheterization data from PTB Diagnostic ECG Database. Leads III and aVR were mathematically inverted to obtain -III (-60°) and -aVR (+30°). ECGs were qualitatively assessed for ST-segment deviation, T-wave inversion, Q waves, fragmented QRS, and PR changes. RESULTS All patients had significant left anterior descending (LAD) artery obstruction. ST-segment elevation in -III occurred in 56% of cases and was frequently equal to or greater than in aVL, consistent with an injury vector oriented closer to -60° than -30°. Morphological concordance between -III and aVL 88%. T-wave inversion was present in 100% of -III and 88% of aVL leads. Discordant patterns corresponded to more inferiorly directed vectors in distal LAD occlusion. CONCLUSIONS Derived lead -III provides complementary frontal-plane information in anterior MI by directly sampling the -60° axis. When ST elevation in -III equals or exceeds aVL, it may indicate a more vertically oriented injury vector, potentially reflecting proximal LAD occlusion or anterolateral basal extension.