A technique for chronic, extraluminal measurement of uterine activity

The construction, calibration, and surgical placement of a device for monitoring uterine motility are described. The device, a linear stretch gauge, consisted of a length of flexible tubing filled with mercury and connected at both ends to copper wire leads. An increase in the length of the mercury-filled tubing caused a change in resistance. This change was quantitated, using a modified Wheatstone bridge circuit. In in vitro test, the stretch gauges demonstrated linear response in millivolt output to elongation over a range that was comparable to physiologic responses observed with the gauges placed in vivo. If surgically placed around 1 uterine horn, stretch gauges responded to uterine contractile events with specificity and sensitivity. Calibration of the device at time of placement permitted monitoring for possible increases in uterine circumference.