Response of Fetal Heart Rate to Scalp Stimulation Related to Fetal Acid-Base Status

ABSTRACT

The correlation of the response of the fetal heart rate (FHR) to scalp stimulation and the acid-base status of the fetal scalp blood (FSB pH) was studied in 104 term fetuses during labor. The mean FHR was determined for the 5-minute period before the preparation for fetal scalp blood pH determination and 1 minute following FSB pH. The analysis was performed for three pH groups: (a) Normal (pH more than 7.25, = 73); (2) preacidotic (pH 7.20 to 7.25, = 16); and (3) acidotic (pH less than 7.20, = 15). The pH was confirmed at delivery by determination of umbilical cord blood acid-base status. Regression analysis of the number of fetuses responding by changing their heart rate against time was significant only for the nonacidotic fetuses. There was no significant difference in the number of subjects whose mean FHR increased, decreased, or remained unchanged between the three pH groups. Furthermore, there was no difference between the three pH groups in the number of fetuses who demonstrated significant mean heart rate changes (p <0.05) following the stimulation compared to those who did not. Further analysis between fetuses in the three pH groups who increased FHR 15 beats/min or more following stimulation did not discriminate between groups.

We conclude that scalp stimulation results in a FHR response in all three pH categories. However, the net change in the mean heart rate following the stimulation may be negative or positive. The magnitude and the direction of this change in FHR is not significantly related to FSB pH.

Based on the time course of the FHR response, one needs only to look at the first 20 seconds following scalp stimulation to determine fetal response.