Case Report: A collateral-supplying major septal artery occlusion: electromechanical consequences leading to low left ventricular ejection fraction and late-onset complete atrioventricular block

Background Septal arteries (SAs) are often considered minor branches, yet certain anatomies—particularly when a large septal artery (SA) provides collateral perfusion to a chronically occluded coronary vessel—carry substantial electromechanical risk. Abrupt loss of such a large SA during percutaneous coronary intervention (PCI) may compromise both conduction system perfusion and myocardial territories in selected anatomical settings. Although SA occlusion is typically benign, delayed complete atrioventricular (AV) block and severe left ventricular dysfunction are rarely reported complications. Case A 62-year-old man presented with typical chest pain and was diagnosed with non–ST-elevation myocardial infarction. The baseline electrocardiogram showed normal sinus rhythm without conduction abnormalities. Coronary angiography revealed culprit proximal-mid left anterior descending artery (LAD) lesions and a chronic total occlusion of the right coronary artery, supplied by a large collateral-providing first SA. During intravascular ultrasound-guided PCI of the LAD, this major SA became unintentionally occluded despite protection with a jailed wire. Immediately afterward, serial electrocardiograms demonstrated a new bifascicular block pattern—complete right bundle branch block with right axis deviation—which persisted over the next two days. At 62 hours post-PCI, the patient developed late-onset complete AV block. Echocardiography showed a marked decline in left ventricular ejection fraction (LVEF) from 61% to 32%, consistent with new septal akinesia and inferior/inferolateral wall-motion abnormalities, suggesting a clinically significant ischemic insult following SA occlusion. Given the persistence of complete AV block requiring pacing support and the severely reduced LVEF, early cardiac resynchronization therapy with defibrillator (CRT-D) implantation was performed. The patient stabilized thereafter and was discharged in good condition. Conclusion This case highlights that occlusion of a collateral-supplying major SA may result not only in conduction disturbances but also in significant left ventricular systolic dysfunction, producing an uncommon combined electromechanical presentation. Recognition of this high-risk anatomy during PCI planning—including an understanding that even anatomically appropriate protection strategies may not fully prevent septal branch loss—underscores the need for individualized, anatomy-guided side-branch protection, vigilant rhythm monitoring, and timely CRT-D implantation in selected patients.