Assessing Velocity and Directionality of Uterine Electrical Activity for Preterm Birth Prediction Using EHG Surface Records

Abstract

The aim of the present study was to assess the capability of conduction velocity amplitudes and directions of propagation of electrohysterogram (EHG) waves to better distinguish between preterm and term EHG surface records. Using short-time cross-correlation between pairs of bipolar EHG signals (upper and lower, left and right), the conduction velocities and their directions were estimated using preterm and term EHG records of the publicly available Term-Preterm EHG DataSet with Tocogram (TPEHGT DS) and for different frequency bands below and above 1.0 Hz, where contractions and the influence of the maternal heart rate on the uterus, respectively, are expected. No significant or preferred continuous direction of propagation was found in any of the non-contraction (dummy) or contraction intervals; however, on average, a significantly lower percentage of velocity vectors was found in the vertical direction, and significantly higher in the horizontal direction, for preterm dummy intervals above 1.0 Hz. The newly defined features-the percentages of velocities in the vertical and horizontal directions, in combination with the sample entropy of the EHG signal recorded in the vertical direction, obtained from dummy intervals above 1.0 Hz-showed the highest classification accuracy of 86.8% (AUC=90.3%) in distinguishing between preterm and term EHG records of the TPEHGT DS.

Keywords: conduction velocity; conduction velocity directionality; contraction intervals; dummy intervals; electrohysterogram; preterm birth prediction; propagation of EHG waves; short-time cross-correlation.

Figure A1

A five-minute excerpt of an originally recorded EHG signal $S{4}_{\mathrm{M}}$ (upper trace), calculated EHG signal $S{4}_{\mathrm{C}}$ (middle trace), and the difference between samples of signals, $\Delta S4$ (lower trace), of a selected term EHG record. The standard deviation (Equation (A1)) for this entire record equals 0.78 ($\mathsf{\mu }$V)${}^2$.

Figure A2

A detailed example of passing an EHG wave with an amplitude of, e.g., 10 $\mathsf{\mu }$V, over the electrodes placed on the abdominal wall, the values of the corresponding EHG signals, the cross-correlation similarity shifts, $s_{\mathrm{V}}=C_{S1,S3}$ and $s_{\mathrm{H}}=C_{S2,S4}$, the transition times, $t_{\mathrm{V}}$ and $t_{\mathrm{H}}$, and the calculated incidence angle $\varphi$.

Figure A3

Conduction velocity amplitudes, $CV$, and conduction velocity amplitudes in the vertical, $V_{\mathrm{V}}$, and horizontal, $V_{\mathrm{H}}$, directions of the record tpehgt_p006 in the frequency band B1. $\overline{CV}$—the average conduction velocity. ${\overline{V}}_{\mathrm{V}}$, ${\overline{V}}_{\mathrm{H}}$—the average conduction velocities ($CV\ne 0$) in the vertical and horizontal directions. $P_{\mathrm{V}}$, $P_{\mathrm{H}}$—the percentages of conduction velocities in the vertical and horizontal directions. Blue: dummy intervals, red: contraction intervals.

Figure A4

Conduction velocity amplitudes, $CV$, and conduction velocity amplitudes in the vertical, $V_{\mathrm{V}}$, and horizontal, $V_{\mathrm{H}}$, directions of the record tpehgt_t009 in the frequency band B1. $\overline{CV}$—the average conduction velocity. ${\overline{V}}_{\mathrm{V}}$, ${\overline{V}}_{\mathrm{H}}$—the average conduction velocities ($CV\ne 0$) in the vertical and horizontal directions. $P_{\mathrm{V}}$, $P_{\mathrm{H}}$—the percentages of conduction velocities in the vertical and horizontal directions. Blue: dummy intervals, red: contraction intervals.

Figure 1

Positions of electrodes. The electrodes were placed symmetrically at a distance of 7 cm in two horizontal rows, above and under the navel [5].

Figure 2

The electrohysterogram (EHG) and external tocogram (TOCO) signals of the record tpehgt_p008 (preterm, delivery in the 32nd week, recorded in the 26th week of pregnancy). Blue: human-annotated dummy intervals, red: human-annotated contraction intervals.

Figure 3

(a) Positions of the electrodes measuring the EHG potentials on the abdominal surface; (b) measured EHG signals of the records of the Term–Preterm EHG DataSet with Tocogram (TPEHGT DS) and their orientations. (c) Calculated and consistently oriented EHG signals.

Figure 4

(a) Schematic description of the method used to estimate conduction velocity amplitudes and directions of EHG waves (adopted from [36]). (b) Distribution of sectors (South, East, North, and West) and propagation directions (Vertical and Horizontal) on the abdominal wall according to the selected coordinate system.

Figure 5

Box plots of the percentages of correlation intervals with velocity vectors in the horizontal direction, $P_{\mathrm{H}}$. P—preterm intervals, T—term intervals.

Figure 6

Box plots of the ratio between the percentages of velocity vectors in the vertical and horizontal directions, $R{P}_{\mathrm{V}\mathrm{H}}$. P—preterm intervals, T—term intervals.

Figure 7

Box plots of the average conduction velocity amplitudes in the vertical direction, ${\overline{V}}_{\mathrm{V}}$. P—preterm intervals, T—term intervals.

Figure 8

Box plots of the average conduction velocity amplitudes in the horizontal direction, ${\overline{V}}_{\mathrm{H}}$. P—preterm intervals, T—term intervals.

Figure 9

Box plots of sample entropies of the EHG signal S2, $S{E}_{\mathrm{S}2}$. P—preterm intervals, T—term intervals.