This study has shown that mean cervical tissue resistivity obtained with 5 mm and 9 mm EIS probes differs significantly in the frequency range 4–819 kHz. This difference was most marked at lower electrical frequencies, with the magnitude of tissue impedance values being twice as high with the small, compared to the larger, probe. However, at higher frequencies the resistivity values obtained were similar for both probes. Short-term intra-observer variability of cervical resistivity measurement did not differ between the two probes. We noted a close correlation between our in vivo observations and the predicted cervical resistivity using a FE cervical tissue computer model. Additionally, this model suggested that the fraction of injected current passing through the cervical stroma would be higher at all frequencies for the 9 mm compared to the 5 mm probe, and would be a maximum at about 100 kHz when approximately 90% stromal penetration was attained with the larger probe, compared to about 70% penetration with the 5 mm probe.
The underlying tissue characteristics summarised in the derived resistivity spectrum for a tissue are complex and not fully explained. Tissue resistivity as captured with different probes is likely to be influenced not only by the physical and electrical properties of the probes themselves (although this is minimised by calibration), but also by the intrinsic characteristics of the tissue studied. The resistivity spectrum obtained by EIS is influenced by such tissue properties as the surface mucus, structural changes in epithelium (such as stretching, nuclear-cytoplasmic ratio, cell orientation and the associated changes in extracellular volume), and the stromal tissue characteristics [3, 12–14]. Stromal contribution to tissue resistivity is influenced by such factors as extracellular hydration, matrix content and cellular density, as recently demonstrated using computational modelling . Larger probes are designed to have a wider distance between the injecting and sensing electrodes. The higher the inter-electrode distance associated with large diameter probes, the deeper the current penetration into stromal tissue and therefore the greater the relative contribution of stromal elements to the obtained resistivity spectrum. We believe that the difference in the depth of penetration of electrical current into cervical tissue accounted principally for the different cervical resistivity values obtained with the 5 mm and 9 mm probes in the same group of women.
However, the relationship between electrode spacing and measured resistivity is complex, due to the anisotropy in electrical properties that arises from the highly stratified nature of the tissue. Our computer modelling suggests that at low frequencies the tissue is still stratified and this draws the current into the higher conductivity stromal tissue. However, at high frequencies current penetrates the cell membranes and the tissue appears to be homogeneous so that the relative amount of current in the stroma is reduced. These two competing effects give rise to a frequency where current penetration is greatest. In principle it is possible to compute a full sensitivity analysis for the tissue strata but this would require a much larger and more realistic FE model of the tissues to be available.
The relative proportion of current flowing in the epithelial layers and the stroma will also vary according to the probe size. With the smaller electrode array, a greater proportion of current would flow through the shallower and more resistive epithelial layers. This could explain the higher resistivitymeasured for the 5 mm probe. As the frequency increases, cell membranes become progressively more 'invisible' to current, so the electrical properties (and hence measured impedance) converge towards the higher end of the frequency range, irrespective of the current distribution.
Our study is the first to compare uterine cervical tissue resistivity obtained in a single group of subjects using two different probes. Our observations are consistent with two previous reports, obtained from two separate groups of pregnant women, which show a lower cervical resistivitymeasurement with a 5 mm compared to an 8 mm probe at 4.8 kHz frequency [6, 7]. Using a 5 mm probe, O'Connell et al reported median resistivity values of 10.01 Ωm in a group of pregnant women , compared to mean resistivity values of 7.03 and 5.34 Ωm obtained with an 8 mm probe in two groups of pregnant women distinguished by an "unfavourable" or a "favourable" cervix for labour inducibility respectively . Although these two studies reported median and mean resistivity values respectively, previous reports on cervical epithelium employing the 5 mm probe described a difference between mean and median values of resistivity of only 5% , suggesting that the resistivity differences between 5 and 8 mm probes noted in the papers during human pregnancy are significant. Taken together, the larger diameter probes appear to pass current more deeply into cervical tissue and are likely to be more sensitive to the lower resistivity of cervical stroma.
It may be suggested that the differences in cervical resistivity observed with the two different probes were related to differences either in the voltage delivery to the tissues by the injecting electrodes, and/or in capture of resistivity data by the sensing electrodes. This is highly implausible for several reasons. Firstly, adjustments had been made to minimise these potential differences during the design process of the probes. Secondly, both probes were calibrated in the same saline solution and device-independent cervical impedance values then derived as absolute values of resistivity in Ωm. Variations in the applied pressure of the probe on the cervix as well as the shape of the tissue at the point of application can affect the measured CI. In our study, however, this variation is likely to be minimal as all the measurements were obtained by a single researcher using subjectively similar force on the probe, and applying the probe in the same area of the anterior lip of the cervix. However, because of the difference in area of the two probe tips the pressure applied with the larger probe may be less. It is possible that this might have affected the measurements, although the difference is likely to be small as at low pressures the affect on measurements is much less than at high pressures. The similarity in coefficient of variation for both probes suggests that any difference in short-term variability between both probes is likely to be small and insignificant.
Our observations are likely to prove relevant when choosing the appropriate probe for the investigation of cervical epithelial or stromal tissue. Several studies have shown that cervical epithelial assessment of preinvasive cancer is best undertaken using a small probe which enhances spectral separation of normal from premalignant cervical epithelium [4, 5]. Cervical prelabour changes are presumed to occur mainly in the stroma. If this were the case, our observations would suggest that the wider probe would prove more applicable for the study of the pregnant cervix. There is a paucity of histological evidence for this assumption, however, and there are emerging reports that changes in cervical stroma during pregnancy may be paralleled by changes in cervical epithelium. One study in mice has shown that the increase in the expression of the glycosaminoglycan hyaluron which is associated with cervical prelabour ripening not only occurs in the cervical stroma but also, to a lesser extent, in the epithelium. Studies of human and animal epithelial tissue have highlighted the physiological role of gap junctional proteins such as occludin and the claudins in modulating cell contact and permeability [16–19]. Little is known of the epithelial changes associated with human cervical preparation for birth and changes in these gap junctional proteins have not been described in the cervix during human pregnancy. Insight into these changes is likely to facilitate interpretation of cervical tissue resistivity data and the design of the appropriate probe for measuring and interpreting cervical resistivity changes associated with pregnancy and prelabour that may have clinical correlates.