Fetal heart rate responses in chronic hypoxaemia with superimposed repeated hypoxaemia consistent with early labour: a controlled study in fetal sheep

• Published in: BJOG : an international journal of obstetrics and gynaecology Vol. 130; no. 8; pp. 881 - 890
• Main Authors: Lear, C. A., Georgieva, A., Beacom, M. J., Wassink, G., Dhillon, S. K., Lear, B. A., Mills, O. J., Westgate, J. A., Bennet, L., Gunn, A. J.
• Format: Journal Article
• Language: English
• Published: England Wiley Subscription Services, Inc 01.07.2023
• Subjects: Acidosis - etiology; Animals; asphyxia; Blood pressure; cardiotocography; computerised fetal heart rate monitoring; deceleration area; deceleration capacity; Female; Fetus; Fetuses; Heart rate; Heart Rate, Fetal - physiology; Hypotension; Hypotension - complications; Hypoxia - complications; hypoxia‐ischaemia; mortality; peripheral chemoreflex; phase rectified signal averaging; Pregnancy; Prospective Studies; Risk factors; Sheep; stillbirth; Umbilical cord; Umbilical Cord - blood supply; hypotension; hypoxia-ischaemia; phase rectified signal averaging; peripheral chemoreflex; mortality; stillbirth; deceleration capacity; computerised fetal heart rate monitoring; asphyxia; cardiotocography; deceleration area
• ISSN: 1470-0328; 1471-0528
• DOI: 10.1111/1471-0528.17425

Objective Deceleration area (DA) and capacity (DC) of the fetal heart rate can help predict risk of intrapartum fetal compromise. However, their predictive value in higher risk pregnancies is unclear. We investigated whether they can predict the onset of hypotension during brief hypoxaemia repeated at a rate consistent with early labour in fetal sheep with pre‐existing hypoxaemia. Design Prospective, controlled study. Setting Laboratory. Sample Chronically instrumented, unanaesthetised near‐term fetal sheep. Methods One‐minute complete umbilical cord occlusions (UCOs) were performed every 5 minutes in fetal sheep with baseline paO2 <17 mmHg (hypoxaemic, n = 8) and >17 mmHg (normoxic, n = 11) for 4 hours or until arterial pressure fell <20 mmHg. Main outcome measures DA, DC and arterial pressure. Results Normoxic fetuses showed effective cardiovascular adaptation without hypotension and mild acidaemia (lowest arterial pressure 40.7 ± 2.8 mmHg, pH 7.35 ± 0.03). Hypoxaemic fetuses developed hypotension (lowest arterial pressure 20.8 ± 1.9 mmHg, P < 0.001) and acidaemia (final pH 7.07 ± 0.05). In hypoxaemic fetuses, decelerations showed faster falls in FHR over the first 40 seconds of UCOs but the final deceleration depth was not different to normoxic fetuses. DC was modestly higher in hypoxaemic fetuses during the penultimate (P = 0.04) and final (P = 0.012) 20 minutes of UCOs. DA was not different between groups. Conclusion Chronically hypoxaemic fetuses had early onset of cardiovascular compromise during labour‐like brief repeated UCOs. DA was unable to identify developing hypotension in this setting, while DC only showed modest differences between groups. These findings highlight that DA and DC thresholds need to be adjusted for antenatal risk factors, potentially limiting their clinical utility.