Changes in Fetal TCPo2 Values Occurring during Labour in Association with Lumbar Extradural Analgesia

Abstract

Continuous lumbar extradural analgesia has been reported to both improve and impair fetal acid-base status in the first stage of labor. Changes in fetal heart rate (FHR) also have occurred with the use of local analgesic agents. It was the authors' intention to determine whether the latter changes were associated with alterations in the degree of fetal oxygenation.

Sixty-four patients in spontaneous labor were studied, 46 of whom received continuous lumbar extradural analgesia. One hundred and sixty-six hours of fetal tcPo2 measurements were obtained from the 64 labors. Fifty-three hours were from labors in which extradural analgesia was not used (18 patients). Nonrecurring late decelerations with exaggerated FHR variability occurred five times in four fetuses (9 per cent) during the 113 hours of extradural analgesia, but these were not seen in the 53 hours without extradural blockade. Two of the four fetuses were in the group that did not demonstrate any FHR abnormality before blockade. The third fetus had diminished FHR variability, and the fourth had had occasional mild variable decelerations before induction of the extradural block. Following the sudden severe decelerations that were temporarily related to the onset of the extradural block, each fetus demonstrated a return to the previous FHR pattern.

Although the supine position generally decreased fetal tcPo2 by 0.27 kPa, the decrease was as great as 0.8 kPa in three fetuses. With the exclusion of the four instances reported above, however, these changes were not associated with fetal distress. All other tcPo2 values were essentially unaltered by extradural analgesia. The two lateral positions were compared before and after the block, and were found to give compatible fetal tcPo2 values. In 18 of the 46 mothers, the right lateral position gave greater tcPo2 values than the left before the block, with changes of 0.27 to 0.4 kPa after blockade. The left lateral position was associated with greater fetal tcPo2 values than the right in 28 patients before and after the extradural block (Table 1). The better of the two lateral positions is referred to as the “preferred” lateral position.

In the first patient with abnormal changes, a decrease in the fetal tcPo2 from 2.13 to 1.46 kPa occurred after the mother had been placed in the supine position. A further decrease occurred 8 minutes after the administration of the extradural analgesia. The uterine contractions, as measured by an internal catheter, became more frequent and, although less intense than before the blockade, caused the uterine pressure to remain between 2.7 and 4 kPa for 5 minutes (normal, 0.67 to 2.7 kPa). The FHR deceleration was accompanied by a decrease in tcPo2, which began to recover after the uterine contractions had ceased. A sustained pattern of contraction recurred, however, and decreases in FHR and tcPo2 were noted. The maternal arterial pressure decreased from 122/80 to 90/55 mm Hg during the first deceleration. Oxygen was administered to the mother; she was placed in the right lateral position, and the rate of fluid infusion increased, whereupon improvement was seen in FHR and tcPo2. The fetus was delivered 1 hour later with an umbilical artery pH of 7.24 units and Apgar scores of 8 at 1 minute and 9 at 5 minutes. Gestational age was assessed as being 39 weeks.

The course of the other three labors was similar to that described above. In all instances, the mothers were in the supine position when the FHR abnormalities occurred.