Comparison of short term variability indexes in cardiotocographic foetal monitoring

Concise indexes related to variability of foetal heart rate (FHR) are usually utilised for foetal monitoring; they enrich information provided by cardiotocography (CTG). Most attention is paid to the short term variability (STV), which relates to activity and reaction of autonomic nervous control of foetal heart. There is not a unique method to compute short term variability of the FHR but different formulas have been proposed and are employed in clinical and scientific environments: this leads to different evaluations and makes difficult comparative studies. Nine short term variability indexes: Arduini, Dalton, Organ, Sonicaid 8000, Van Geijn, Yeh, Zugaib a modified version of Arduini index and Standard Deviation were considered and compared to test their robustness in CTG applications. A large set of synthetic foetal heart rate series with known features were used to compare indexes performances. Different amounts of variability, mean foetal heart rate, storage rates, baseline variations were considered. The different indexes were in particular tested for their capability to recognise short term heart rate variability variation, their dependence on heart rate signal storage rate (as those provided by commercial cardiotocographic devices), on mean value of the foetal heart rate and on modifications of the floatingline, such in case of accelerations or decelerations. Concise statistical parameters relative to indexes scores were presented in comparative tables. Results indicate that although the indexes are able to recognise STV variation, they show substantial differences in magnitude and some in sensibility. Results depend on the frequency used to acquire and store FHR data (depending on devices); in general, the lower is data rate the more degraded are the results. Furthermore, results differently depend on FHR mean, some for their intrinsic definition; differences arise also in correspondences of accelerations and decelerations. Our results demonstrate that only indexes which refer directly to differences in FHR values, such as Organ and SD indexes, not show dependence on FHR mean. The use of the Standard Deviation index may provide efficient information while showing independence from the considered variables. Indexes performance in case of real cardiotocographic signals were also presented as examples.