The valve of the superior vena cava--the supernumerary structure of the precaval segment of the crista terminalis.

The primitive right sinuatrial valve persists in humans as the crista terminalis, the valve of the inferior vena cava and the valve of the coronary sinus, while according to the known data the primitive left sinuatrial valve is supposed to have no derivatives. Ten human right atria were opened with intercaval incisions and the precaval segment of each crista terminalis was studied macroscopically. Three specimens did not present any peculiarities at this level, but the other 7 had sagittal muscle bundles and supernumerary valves in individual arrangements. Supernumerary valves were present in 2 specimens, one complete and the second fenestrated; these valves were located immediately below the superior vena cava orifice and covered the medial end of the crista terminalis. The supernumerary valves at the superior vena cava orifice may be termed, mirroring that of the inferior vena cava, "valves of the superior vena cava". Their exact frequency of occurrence and their embryonic precursors must be further established. The presence of such valves in the right atrium may interfere with the flow to the right side of the heart, may represent conditions for thrombotic changes and may disturb a central venous catheter placement. If present, the valve of the superior vena cava will also interfere with the catheter ablation procedures used for supraventricular tachycardia.