Karsdon J, Shi S-Q2, Maner WL2, Saade G2, Garfield RE2

2 The University of Texas Medical Branch, Galveston, TX, USA


Objective: Uterine contractions are mechanical events resulting from the underlying electrical activity of the myometrium. Previous studies showed an electrical current applied to the uterus inhibits in vitro and in vivo contractility, in a dose dependant manner. The EI dose, at constant amperage, depends on the electrical pulse width (PW) and pulse frequency (Hz). Many authors use various PW and Hz with variable effects. Our aim was to determine, in vivo, the most important parameter of EI.

Methods: Timed-pregnant Sprague-Dawley rat on gestational day 19 was studied. 5 stainless steel electrodes were sutured along one of the bicornuate uterine horns and an intraperitoneal radiotelemetric transmitter measured intrauterine pressure (IUP) and electromyographic (EMG) activity. An on-line computer and data acquisition system collected the transmitter signals and calculated the intrauterine pressure maximum (IUPmax), area under the contraction curve (IUPauc) and mean EMG activity (EMGmean). After taking control values for spontaneous preterm labor contractions EI was started via the electrodes periodically for 5 seconds (T) with a constant electrical current of square wave pulses at 5 mA. Two or three recordings were made at each PW e.g.7.5, 15, 30, 60 and 120 ms/pulse and Hz e.g.120, 60, 30, 15 and 7.5 pulses/s. EI dose, using the root-mean-square (RMS) method, was calculated with the formula T x mA x [PW/((1/Hz) x 1000)]½ and was kept constant, i.e. as PW increased Hz decreased.

Results: 5 seconds of EI significantly decreased IUPmax and IUPauc, more than 50% of control values. However, there were no significant differences in IUPmax, IUPauc or EMGmean with variations in PW and Hz but RMS constant.

Conclusions: EI inhibits uterine activity in vivo and depends on the total current dose and not only the frequency. This is important for comparing studies with various parameters of PW and Hz.