ARTERIAL OXYGEN-SATURATION IN RELATION TO METABOLIC-ACIDOSIS IN FETALLAMBS

OBJECTIVE: We studied the relationship between preductal arterial oxyg en saturation and metabolic acidosis in 18 chronically instrumented fe tal lambs (gestational age 119 to 133 days) in two experimental design s. In the first group the onset of metabolic acidosis was determined. In the second group the progression of metabolic acidosis was studied as was the cardiovascular and hormonal changes resulting from hypoxemi a. STUDY DESIGN: In nine fetal lambs maternal fraction of inspired oxy gen was lowered stepwise by increasing flows of nitrogen delivered int o the trachea through a small indwelling catheter (group 1), and in ni ne fetal lambs maternal bloodflow was reduced stepwise by means of a v ascular occluder (group 2). RESULTS: Baseline arterial oxygen saturati on Values ranged from 26% to 67% with normal pH and extracellular flui d base excess values in both groups 1 and 2. In both groups pH and ext racellular fluid base excess started to decrease below 30% arterial ox ygen saturation, with a progressive decrease below 20% arterial oxygen saturation to an end value for pH of 7.14. In some fetal lambs pH and extracellular fluid base excess decreased initially at 20% to 30% art erial oxygen saturation and then stabilized at the lower level. Fetal heart rate in group 1 increased during hypoxemia from 155 to 179 beats /min. In group 2 baseline fetal heart rate was 153 beats/min and fell with every step change in arterial oxygen saturation but subsequently increased to 172 beats/min by the end of the period of hypoxemia. Base line values for epinephrine, norepinephrine, dopamine, cortisol, and m ean arterial pressure were not related to baseline arterial oxygen sat uration levels, and each of these variables was increased at the end o f hypoxemia in group 2. CONCLUSION: Preductal arterial oxygen saturati on can reach values between 20% and 30% before anaerobic metabolism st arts. During the progressive acidosis blood pressure was increased, wh ich can be attributed to a strong rise in catecholamines.