Uterine first pass effect in postmenopausal women

Vaginally administered drugs distribute preferentially to the uterus; counter-current transfer from the vaginal veins to the uterine artery probably plays a pivotal role. In each side, the ovarian and uterine arteries form arterial anastomoses and controversy exists regarding the origin of the arterial supply to the Fallopian tube and tubal part of the uterus, and consequently whether these tissues can be reached through vaginal administration.

A thermocatheter with four measurement points, each separated by 5 mm, was inserted under endoscopic control into the tubal corner of uterus in 10 conscious, menopausal women and the temperatures registered every 2 s. The vagina was then flushed for 15 min with 1.5 l of saline at room temperature, after which the probe position was re-assessed by the endoscope.

The lowest measurement point (15 mm from the tip) cooled significantly more than the other points (P < 0.0001). At 15 min, mean temperature reduction at point 4 was significantly greater than at all other measurement points (P < 0.05) due to local transfer of cold from vaginal vein blood to the uterine arterial blood (but not the ovarian artery).

The results support the theory that, at least in postmenopausal women, the uterine artery supplies most of the uterus while the corneal part of cavity (up to 5-10 mm from the ostium) receives the blood supply from the ovarian artery. This finding represents a rationale for vaginal administration of drugs when a local effect on the uterus (e.g. progestational or relaxation) in postmenopausal women is requested.