Analyzing uterine EMG: tracking instantaneous burst frequency

The uterine electromyogram (EMG) has been proven to be an efficient tool for pregnancy and parturition monitoring. Temporal and spectral characteristics of this signal, recorded by means of external electrodes, make it possible to discriminate between efficient and inefficient contractions in terms of electrical command capability. As it is possible to record the uterine electrical activity as early as 19 weeks gestational age, abdominal EMG can be of value for pregnancy monitoring especially if this noninvasive recording can be associated with ambulatory instrumentation. Much work has been achieved related to the characteristics of this signal as well as to its ability to explain, to a certain extent, uterine contractility. In previous studies on monkeys, the authors demonstrated the relationship between internal and external recordings, by simultaneously using internal wire electrodes on the uterine muscle itself and external abdominal bipolar Ag/AgCl electrodes. In the same preliminary studies, the authors defined the most relevant frequency bands in terms of significant modifications between pregnancy and parturition. Furthermore, they have proposed a criterion which is representative of contraction efficiency. However, the most important point has been to relate the observations made on the external recordings to internal cellular activity. In fact, uterine smooth muscle cells continuously exhibit resting potentials with small and slow spontaneous fluctuations. When these fluctuations reach a threshold, bursts of action potentials are induced. The frequency of the spikes within a burst appear to control the mechanical contraction. If it could be possible to extract such an information of within-burst frequency by means of external recordings, this should lead to a relevant characterization in terms of contraction efficiency. In addition, the EMG undergoes important modifications from pregnancy to parturition, primarily due to the appearance of gap junctions that facilitate electrical coupling between cells, and increase in the within-burst spike frequency