Fetal heart rate patterns associated with acidemia at fetal blood sampling during labor-An observational study.

INTRODUCTION: Cardiotocography (CTG) is used worldwide for fetal surveillance with the aim to detect fetal hypoxia and aid timely intervention. The method is associated with shortcomings; for example, low interobserver reliability and low specificity for fetal hypoxia. Fetal scalp blood sampling (FBS) is an adjunct method to improve the specificity for fetal acidemia. Given the updated Swedish CTG classification introduced in 2017, the aim of this study was to evaluate which fetal heart rate (FHR) patterns are most strongly associated with acidemia as determined by FBS. MATERIAL AND METHODS: An observational cohort study on all fetuses that during the year 2019 underwent indicated FBS during labor. The data were collected from the Stockholm-Gotland Perinatal cohort. CTG traces were retrospectively interpreted, focusing on the last hour prior to FBS. All analyses were made with the bedside meter Lactate Pro 2™, and a lactate concentration > 7.3 mmol/L was used as a proxy for fetal acidemia. RESULTS: During the study period, 1116 fetuses underwent FBS (8%-9% of the laboring population). Of the 1116, 148 (13%) had acidemia at the first FBS, and 38% had more than one FBS. The corresponding number of fetuses with acidemia at the last FBS was 239/1116 (21%). The FHR patterns with the strongest association with acidemia at FBS were fetal tachycardia and/or reduced variability with repetitive late decelerations (20% at 1st FBS, 34% at last FBS) or severe variable decelerations (22% at first FBS, 33% at last FBS). At the last FBS, 6/23 (26%) fetuses with a normal baseline frequency, normal variability, and non-repetitive decelerations since >60 min of the types severe variable or late had acidemia. These FHR patterns would be classified as normal with the current classification. CONCLUSIONS: Among high-risk fetuses with FHR patterns giving rise to FBS, the FHR patterns most often associated with fetal acidemia during labor are tachycardia and/or reduced variability combined with repetitive late or severe variable decelerations. All FHR patterns with the highest risk of acidemia are classified as abnormal in the current Swedish CTG classification system. However, longstanding milder FHR alterations warrant increased attention.