Abstract

OBJECTIVE: We examined the effect of hypoxemia on fetal heart rate variability using the instrumented fetal sheep model. METHODS: In this prospective study, 19 pregnant sheep were instrumented under general anesthesia at a mean gestational age of 127 days. After a 5-day recovery, hypoxaemia was induced by attaching the mother to a re-breathing circuit. Hypoxemia was further extended till 120 minutes, following which it was reversed till matenal and fetal pO2 returned back to baseline. The heart rate recordings at baseline, hypoxemia of 30 and 120 minutes, and recovery were analysed to calculate short term variation (STV) in 16 epochs of 3.75sec each, every minute. Phase rectified signal averaging (window length L= 10, time T= 2 and Scale S=T) was used to calculate acceleration capacity (AC) and deceleration capacity (DC). RESULTS: At baseline, mean (SD) fetal pO2 was 2.90±0.38 kPa. Acute hypoxaemia was associated with a significant reduction in mean pO2 at 30 (1.60±0.37 kPa) and 120 (1.50±0.16 kPa) minutes. Mean (SD) fetal pO2 at recovery was 2.80±0.32 kPa. The median STV, AC and DC were 1.307 msec (IQR: 0.515 to 2.508), 1.295 (IQR: 0.990 to 2.685) BPM and 1.197 (IQR: 0.850 to 1.836) BPM respectively, at baseline. With 30-minute hypoxaemia, the values were 1.323 (IQR 0.753 to 2.744) msecs, 1.696 (IQR: 1.310 to 3.013) BPM & 1.584 (IQR 1.217 to 4.132) BPM. With 120-minute hypoxaemia, the values were 1.760 (IQR: 0.928 - 4.656) msecs, 3.098 (IQR: 1.530 - 5.163) BPM & 3.054 (IQR: 1.508 - 4.522) BPM. At recovery they changed to 0.962 (IQR: 0.703 - 1.154) msecs, 1.228 (IQR: 1.071 - 2.234) BPM & 1.086 (IQR: 0.873 - 1.568) BPM respectively. Hypoxemia for 30 and 120 minutes were associated with a significant increase in the DC compared to baseline (p = 0.014 & 0.017 respectively). The changes in STV and AC were not significant. CONCLUSION: Acute hypoxaemia is associated with a significant increase in the deceleration capacity of the fetal heart rate. This article is protected by copyright. All rights reserved.