THE INFLUENCE OF MATERNAL OXYGEN (O₂) ADMINISTRATION ON FETAL HEART RATE (FHR) AND LONGTERM VARIABILITY AMPLITUDE (LVA)

Abstract

breathing in healthy adults causes a small decrease in heart rate (HR) (2). In healthy fulHerrn infants 0 2 breathing also leads to a small decrease in HR (1). When the mother is breathing pure 0 2 , a decrease of FHR and LVA in the fetus can be observed (4). These observations have been confirmed previously (5). Using an algorithm for Computer determination (CD) of FHR level and LVA (3), i t was the aim of this paper to study the behaviour of FHR and LVA before and during maternal 0 2 breathing with Computer assistance. MATERIAL AND METHODSIn 14 cases the mothers were breathing 0 2 via a face mask for about 10 min., having been instructed not.to hyperventilate and were kept in the lateral position. Recordings were made of FHR and uterine activity s well s fetal tcPo 2 . FHR and LVA were estimated visually and by using an algorithm, which makes a histogram of all FHR values. From this distribution the 10th and 90th percentile i s estimated and it's distance multiplicated with a constant factor of 1.2. The result represents the actual LVA (3). When comparing the CD FHR and LVA with the visually estimated one, a very good agreement was found (2*«0.001). RESULTSIn 11 of the 14 cases the tcPo 2 increased and FHR and LVA decreased when the mother was breathing pure 0 2 . When plotting LVA against fetal tcPo 2 before and during the period of 0 2 breathing i t became obvious that LVA decreased with an increase in fetal tcPo 2 (Fig. 1). There was a significant increase in fetal tcPo 2 by χ = 5 mmHg (2·«43.001), a significant decrease in FHR by χ = 4 bpm (2*0.05) and a significant decrease in LVA by χ = 6,7 bpm (2rf<0.001). In the other three cases the FHR remained constant whilst the LVA slightly increased during fetal tcPo 2 increase. The fetal tcPo 2 level prior to maternal 0 2 breathing was not significantly different in both groups. There was however a significant difference between the initial basal FHR level in the two groups before the onset of maternal 0 2 breathing. The mean basal FHR in the group of FHR and LVA decrease was 138 bpm compared with 126 bpm in those cases showing only an increase in" LVA (2«0.01). To draw any conclusions from the difference, the number .of cases in the second group i s too small. It might be possible that the change in activity-status was masked in some way. DISCUSSION 0 2 breathing in normal subjects causes a small decrease in HR, which is abolished by atropine and therefore probably vagal in origin, and results in a comparable rate-dependent decrease in cardiac out-0300-5577/82/0102-0037 g 2.00