Author: Nilo Pereira Luz, M.D., Head Professor Obstetrics
Institution: Faculdade de Medicina, Pontifícia Universidade Católica do RGS
Address: Rua Otávio Dutra 174 apto 602
Porto Alegre 90810-230, RS, Brazil
email (for contact): savaris@orion.ufrgs.br
Summary: The auditory evoked response test is a new method to evaluate intrauterine fetal asphyxia.
Background data
No adequate method, up to now has proven to be efficient to detect intrauterine asphyxia during pregnancy in time to avoid brain damage of the newborn (Low # 1).
It is known fetal electroencephalography has been obtained by external or implanted electrodes during animal or human pregnancies (Bernstine # 2).
Sakabe (# 3) was the first to describe fetal evoked potentials in electroencephalography elicited by intra-uterine sound stimulation. Barden (# 4) brought definitive evidence of the correlation between sound stimulation of human fetuses and characteristic evoked potentials thus demonstrating the fetus was able to perceive sound in its environment and to react to this special type of stimulation.
Scibetta (# 5) studied the fetal response to sound stimulation during labor and its variability in human fetuses. The same author, in the guinea-pig (# 6), described the negative effect of perinatal hypoxia in the diminution of fetal response to sound stimulation during hypoxic periods of fetal distress and the postnatal recovery when oxygen supply to newborn was adequate. On both studies tracings of fetal electroencephalograph were used to demonstrate their findings.
Granier-Deferre e Abrams (# 7) demonstrated increased depletion of fetal cerebral glucose in areas related to sound perception and their disappearance when fetal auditory structures were destroyed. This increased glucose utilization in auditory areas makes them very sensitive to hypoxia and, indeed, more able to have their behavior changed by hypoxia than any other cerebral area not actively consuming marked amounts of oxygen during prenatal life. Therefore it could be expected hearing structures and its behavior should reflect the stage of oxygenation of the fetal brain and, therefore, possibly useful to estimate the amount of oxygen available to the fetus during pregnancy.
Current data
Aware of the quoted publications he author began to study fetal reactions to sound stimulation as a new approach to evaluate fetal conditions during pregnancy (Luz # 8). First he developed a sound generator to produce a standardized stimulus to fetal environment. Frequency chosen was 1,500 cycles/second, where vibration was reduced to minimum values. Sound pressure levels were of 125-140 dB measured at 50 cm of sound driver, a midrange speaker. This equipment delivered 98 to 100 measured dB of sound pressure levels to fetal environment. The sound generator produced five pulses lasting 1 second each and were separated 2 seconds intervals. Modern versions of used equipment used ceramic capsules to deliver sound. Any used device was applied in direct contact with maternal skin over the area of fetal head.
A adequate method for measuring fetal response to auditory stimulation and correlating it with present or absent fetal risk factors was developed and the matured score ratings was able to separate normal response (scores 6 to 10) from abnormal responses (scores 0 to 5).
The initial experiments involved the measurements of fetal reactions to sound stimulation during labor (Luz # 9 # 10). The expectation of this study was to ascertain if the procedure would be sensitive enough to detect fetal hypoxia and distress during labor. This was obtained and in these papers he was able to detect early fetal distress, to discriminate, by the type of obtained response to sound stimulation early in labor, which the fetus was to be born vigorous or with low Apgar scores. Other major observations were also made, as the progressive decrease of fetal response to sound stimulation during labor and the protective effect of the membranes upon the intensity of fetal responses.
After this preliminary study we began to study Auditory Evoked Response as a fetal evaluation test during pregnancy. An extensive experience covering a period of more than 20 years was matured and several papers were published and available by request.
The established advantages of the Auditory Evoked Response Test are listed below: