Abstract

Objective: To examine the effect of hypoxemia on fetal heart rate (FHR) variability, using an instrumented pregnant 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 period, hypoxemia was induced by attaching the mother to a rebreathing circuit. Hypoxemia was sustained for 120 min, following which it was reversed until maternal and fetal partial pressure of oxygen (pO₂) returned to baseline. FHR recordings at baseline, after 30 and 120 min of hypoxemia and at recovery were analyzed to calculate short‐term variation (STV) in 16 epochs of 3.75 s, every minute. Phase‐rectified signal averaging (window length (L) = 10, time (T) = 2 and scale (S) = 2) was used to calculate FHR acceleration (AC) and deceleration (DC) capacities.

Results: At baseline, mean ± SD fetal pO₂ was 2.90 ± 0.38 kPa. Acute hypoxemia was associated with a significant reduction in mean pO₂ at 30 (1.62 ± 0.37 kPa) and 120 (1.51 ± 0.16 kPa) min. Mean ± SD fetal pO₂ at recovery was 2.86 ± 0.32 kPa. At baseline, median STV, AC and DC were 1.307 (interquartile range (IQR), 0.515–2.508) ms, 1.295 (IQR, 0.990–2.685) beats per minute (bpm) and 1.197 (IQR, 0.850–1.836) bpm, respectively. At 30 min of hypoxemia, the values were 1.323 (IQR, 0.753–2.744) ms, 1.696 (IQR, 1.310–3.013) bpm and 1.584 (IQR, 1.217–4.132) bpm, respectively. At 120 min of hypoxemia, they were 1.760 (IQR, 0.928–4.656) ms, 3.098 (IQR, 1.530–5.163) bpm and 3.054 (IQR, 1.508–4.522) bpm, respectively. At recovery, they changed to 0.962 (IQR, 0.703–1.154) ms, 1.228 (IQR, 1.071–2.234) bpm and 1.086 (IQR, 0.873–1.568) bpm, respectively. Hypoxemia for 30 and 120 min was associated with a significant increase in DC compared to baseline (P = 0.014 and 0.017, respectively). The changes in STV and AC were not significant.

Conclusion: Acute hypoxemia is associated with a significant increase in the DC of FHR in a fetal sheep model.