Linear and Nonlinear Analysis of Fetal Heart Rate Variability

Cardiotocography (CTG) is widely used in pregnant women for fetal monitoring and has provided extensive knowledge on the pathophysiology of fetal hypoxia. Computer analyses of CTG tracings were developed to reduce intraobserver and interobserver disagreement inherent to visual analyses, but commonly rely on the use of simple linear time-domain indices, providing a limited understanding of fetal heart rate (FHR) variability. Additional information can be obtained from linear analysis of FHR in the frequency-domain and from nonlinear methods such as measures of entropy. Such indices are able to characterize fetal behavioral states, different gestational ages, and to identify situations of fetal acidemia. However, the technical specifications of fetal monitoring equipment and fetal presentation should be considered in order to ensure an adequate application of these indices. In addition to these, the inclusion of fetal gender may improve the identification of fetal acidemia and intrauterine growth-restriction. In conclusion, linear and nonlinear analysis of FHR variability is likely to improve the performance of existing computer systems for CTG analysis, justifying more extensive clinical trials with these methods.