Effect of probe location on changes in vaginal electrical impedance during the porcine estrous cycle.

The impedance technique is one of many methods that can be used for noninvasive monitoring of reproductive events occurring in cyclic animals. The influence of the depth of probe insertion on changes in vaginal impedance in sows during the estrous cycle was examined. Sows were checked twice a day for estrus via exposure to a sexually mature boar. The criterion for confirmation of ovulation was an increase in plasma progesterone levels above 4.0 ng/ml 8 and 12 days after the beginning of estrus. The impedance measurements were carried out using a four-terminal method at a distance of 8, 10, 12, 14, 16 and 18 cm from the vulva. In all six locations of the vagina the mean impedance values decreased gradually after weaning (P<0.01), achieved a nadir 1-2 days before estrus and increased during estrus (P<0.01). It was found that the probe location within the vagina and the parity of sows significantly affected the impedance measured by means of a four-terminal method. The study suggests that the causes of impedance fluctuation are not only technical but also include a number of poorly understood biological causes.