Quantification of Fetal Heart Rate Variability in Relation to Oxygenation in the Sheep Fetus

Abstract

Beat-to-beat variability was suggested as a sensitive indication of fetal well-being. The suggestion originated from patient reports of observed decrease in variability preceding clinical signs of fetal asphyxia and death. The possible changes in heart rate variability in relation to changes in fetal oxygenation was studied in sheep. Chloralose-anesthetized sheep were used. Short-term hypoxia was produced without acidosis. The fetal ECG were recorded on magnetic tape and later analyzed by scrutinizing each QRS complex prior to the trigger of a rate meter. The differential index (DI) described by Yeh and co-workers was chosen because it was easily computed and reflected the variability of coefficient of variation for 2 successive R-R intervals. In contrast to the generally held view of a progressive diminution of variability during the development of asphyxia, the variability increased parallel with a decrease in fetal arterial PO2 [partial pressure of oxygen] determined on blood samples withdrawn at intervals and ranging between 26-10 mm Hg. The correlation between variability and PO2 in the range below 8-10 mm Hg was undetermined.