Objectives: The coronary sinus (CS) plays a key role in cardiac electrophysiology, including cardiac resynchronization therapy (CRT) and ablation of arrhythmias such as left-sided accessory pathways, premature ventricular complexes (PVCs), and atrial fibrillation (AF). Some procedures cannot be performed without CS access, requiring alternative approaches when cannulation fails. CS cannulation can be difficult and, rarely, unsuccessful. Known causes include anatomical abnormalities such as atresia, diverticulum, anomalous ostial location, and fixed stenosis at the ostium or along the vessel course. Here, we present three cases of difficult CS cannulation due to dynamic obstruction — a novel anatomical variant.


Methods: We describe three consecutive patients referred for CRT-D implantation in whom CS cannulation or lead advancement was unexpectedly difficult. In each case, standard CS sheath and catheter techniques were used via subclavian venous access. When resistance to catheter or lead advancement was encountered, contrast venography (with balloon occlusion in select cases) was performed to delineate CS anatomy across the cardiac cycle, and catheter/lead manipulation timed to the non-systolic phase was used to overcome the obstruction.


Results: In all three patients, venography revealed dynamic narrowing of the CS main trunk or a branch during ventricular systole, with resolution of the compression during diastole — a pattern consistent with an underlying muscle bridge rather than a fixed structural lesion. Case 1 showed severe systolic compression of the CS main trunk at the posterolateral branch takeoff; Case 2 showed a similar constriction in the mid-CS body; Case 3 showed compression confined to the distal lateral branch. Despite these dynamic obstructions, successful CS lead placement was achieved in all three patients using gentle catheter manipulation and advancement timed to diastole, without need for alternative pacing approaches.

Conclusions: Dynamic, systole-phase compression of the coronary sinus or its branches — attributable to an epicardial-to-intramyocardial muscle bridge — represents a previously underrecognized and reversible cause of CS cannulation difficulty, distinct from fixed anatomical anomalies. Recognition of this pattern on cine-venography, combined with balloon-occlusion imaging and diastole-timed catheter advancement, allows most cases to be managed successfully without abandoning the CS approach.