Spectral analysis of fetal sheep EEG during hypoxia.
Abstract Power spectra and coherence functions were obtained on samples of EEG recorded from fetal sheep in utero . Hypoxia was produced by respiring the ewe on gas mixtures containing reduced oxygen pressure. With both halothane and local anesthesia, the EEG changes associated with hypoxia consisted of an early power reduction in higher frequencies followed by a reduction of over-all amplitude leading to the isoelectric stage. The times at which statistically significant changes occurred were not earlier during the progressive fall of fetal arterial pO 2 than visual estiamtes of abnormality made on line from the paper record and were in all cases later than significant cardiovascular changes.
EEG and spectral edge frequency: analysis in posthypoxic newborn piglets.
OBJECTIVE
To evaluate the frequency content of the electroencephalogram (EEG) during recovery after a severe hypoxic insult in newborn piglets.


METHODS
EEG was continuously monitored in nine newborn piglets exposed to a severe hypoxic period. Power spectra in five frequency bands were calculated using Fourier transformation. Spectral edge frequency 90 (SEF90) was defined as the frequency below which 90% of the power in the EEG was located. The piglets were divided into two groups; Group 1 represented piglets with some EEG recovery and Group 2 represented piglets without any EEG recovery.


RESULTS
The recovery of the EEG in Group 1 had the same time course in all frequency bands. SEF90 indicates recovery earlier than the value of total power. But SEF90 also signals activity in the EEGs that were almost completely suppressed. When SEF90 was calculated during periods of periodic EEG activity during the very early phase of recovery, the values fell within the same range as during the control period.


CONCLUSION
Spectral analysis of continuous EEG in newborn piglets exposed to very severe hypoxia showed that no specific frequency band of the EEG preceded the other ones during recovery. The results of the SEF90 measure, demonstrates the need for critical analysis of the raw EEG before any reliable estimation of cerebral function can be made.
The diffusibility of oxygen across the sheep placenta.
Estimates have been made of: (1) the oxygen tension in blood samples drawn from unanesthetized ewes and their fetuses via chronic catheters in the uterine vein, umbilical artery and umbilical vein between 90 days and full term; (2) the rate of oxygen uptake by the umbilical circulation between 80 and 134 days in a series of anesthetized ewes, and (3) the ‘diffusing capacity’ of the placenta for oxygen in eleven anesthetized ewes in stages of gestation from 81 to 134 days. The results demonstrate: (1) there is no regular change with advancing gestation in the oxygen tension of the blood in the vessels sampled, and (2) the quantity of oxygen taken up by the umbilical blood and the ‘diffusing eapacity’ of the placenta for oxygen both increase as the fetal weight.