Analysis of abdominal electrical activity of uterus-approximate entropy approach

Spontaneous depolarization and repolarization due to ionic currents are the main properties of smooth muscle cells in the uterus. Their functioning as contractile tissue depends on intercellular electrical and metabolic communication. Due to the lack of a depolarization conducting system, the relationship between the electrical and mechanical activity of the uterus is determined by the amount of contractile cells being activated at a time. Changes in the distribution of electrical charges create the electric fields manifested on an electrohysterogram (EHG). Since the waveforms observed in EHGs are subject-dependent, they cannot be compared between patients. However, for reliable use of EHGs in obstetrics there is a strong need to establish methods of tracking selected parameters of the EHG signal. In this paper we describe a procedure which was designed to be applied to short and noisy time series data. This statistical instrument named "approximate entropy" (ApEn), was applied to EHG data analysis. In this study we evaluated the effectiveness of this method for tracking the level of uterine activity in the prelabor period.