The smallest cardiac veins (or Thebesian veins) are minute valveless veins in the walls of all four heart chambers. The veins are frequently confused with arterial fistula and a distinct set of artery connections eponymously referred to as the "vessels of Wearn". The Thebesian veins are reportedly most abundant in the right atrium and least in the left ventricle. They drain the myocardium and pass through the endocardial layer to empty mostly into the right atrium, but a few empty into the ventricles. This is different from most cardiac veins, which normally empty into the coronary sinus, which then empties into the right atrium. The Thebesian venous network is considered an alternative venous drainage of the myocardium.
The openings of smallest cardiac veins are located in the endocardium. Here the smallest cardiac veins return blood into the heart chambers from the capillary bed in the muscular cardiac wall, enabling a form of collateral circulation unique to the heart. Not every endocardial opening connects to the Thebesian veins as some connect to the vessels of Wearn, which are arteries. Therefore, the endocardial opening must be traced to a vein before it is definitely called an opening of the smallest cardiac veins.
^Snodgrass, Brett Thomas (1 July 2012). "Vessels Described by Thebesius and Pratt Are Distinct From Those Described by Vieussens and Wearn". The American Journal of Cardiology110 (1): 160. doi:10.1016/j.amjcard.2012.04.005. PMID22704295.