Aust N Z J Obstet Gynaecol

1992; 32: 3: 222

Intrapartum Fetal Stimulation Testing Mark Umstad’, MBBS, MRACOG, Catherine Bailey’, MBBS, DipRACOG and Michael Permezel’, MD BS MRCP(UK), MRCOG, FRACOG Division of Obstetrics and The University of Melbourne Department of Obstetrics and Gynaecologj The Royal Women’s Hospital, Melbourne

EDITORIAL COMMENT: We accepted this paper for publication since it reports findings with a test which could rationalize selection of thefetus with cardiotocographic abnormalities in labour in whom capillary blood sampling is indicated. We agree with the authors that thefindings need confirmation in a larger series. Our reviewer commented that the vibroacoustic stimulation test (VAST) should only be used when the fetal heart abnormality is sufficient to cause clinical concern of f e t d well-being. We need more information in order to judge the importance of the findings. Table 2 shows that only 3 of the 60 tracingsshowed reduced variabilityprior to VAST;how then can provocation of accelerationsof 15 beats per minute or more reassure the obstetrician that fetal blood sampling is not indicated, when the trace was already reactive in for example, a case with late decelerations?Moreover further study should examine the response to VAST when cardiotocographic abnormalities are associated with fetal immaturity or maternal narcotic drug therapy. Thefact of the matter is that many major centres no longer perform fetal scalp blood sampling in labour; when there isfetal distress the patient is delivered, vaginally i f conditions are favourable, or by Caesarean section i f they are not. Most obstetricians and patients are happy with this philosophy of obstetric practice and the increased Caesarean section rate it causes. Is it really necessary to repeatedly, invasivelj samplefetal condition when things are not progressing favourably in labour? VAST would be marvellous if it could safelj from the fetal point of view, allow labour to continue when the obstetrician considers that vaginal delivery is imminent.

Summary: Intrapartum vibroacoustic stimulation testing (VAST) had a sensitivity of loo%, a specificity of 59.6% and a positive predictive value of 27.6% for the detection of fetal acidosis in this study of 60 cases. The use of VAST could significantly reduce the requirement for fetal capillary blood sampling. However, fetal scalp stimulation (FSS) was found to be an unreliable test to exclude fetal acidosis. Fetal capillary blood sampling (FCBS) is widely used to evaluate abnormal fetal heart rate (FHR) patterns. However, there are many reasons why FCBS may not be feasible (table 1). In recent years, 2 intrapartum fetal stimulation tests have been promoted either as an alternative to FCBS or as an adjunct to its use (1,2), Fetal scalp stimulation (FSS) involves stimulation of the fetal scalp with digital pressure, pinching with tissue forceps or puncture with a guarded scalpel blade. Vibroacoustic stimulation testing (VAST) utilizes an auditory stimulus applied over the fetal head to provoke the fetus. The FHR response to stimulation is assessed as the presence (Reactive) or absence (Nonreactive) of 1. Perinatology Research Fellow. 2. Registrar in Obstetrics and Gynaecology. 3. Senior Lecturer.

Address for correspondence: Dr. Mark Umstad, The Queen Mother’s Hospital, Glasgow G3 8SH, Scotland, United Kingdom.

accelerations. The healthy, nonacidotic fetus should respond with an acceleration of at least 15 beats per minute (bpm) for at least 15 seconds. The acidotic fetus should not respond. A reactive FHR response is reassuring; a nonreactive response requires either FCBS or delivery. The aim of this study was to evaluate the usefulness of intrapartum fetal stimulation tests in routine clinical practice.

PATIENTS AND METHODS All patients with a FHR tracing significantly abnormal such that FCBS was indicated were recruited to the study. Several minutes prior to FCBS, a 3-second vibroacoustic stimulus was applied over the fetal head via a Corometrics Fetal Acoustic Stimulator (model 146). This particular acoustic stimulator generates a sound level of 82 db at 1 metre in air. FCBS was performed in the usual manner in either lithotomy (with appropriate tilt) or left lateral positions. A Corometrics Model 220 pH Analyzer was used to assess the pH of both fetal capillary and umbilical artery blood samples.


Table 3. Relationship Between VAST Response and Fetal Capillary oH above or below 7.25

Table 1. Reasons For Not Performing FCBS 1. Inaccessible presenting part


3. 4.


Response to VAST

Undilated cervix Intact membranes High presenting part Contraindication to percutaneous puncture infection Chorioamnionitis Hepatitis B Human immunodeficiency virus Active genital herpes simplex Fetal presentation Face Unknown Hereditary bleeding disorders Untrained operator Lack of equipment

Nonreactive Reactive

Response to VAST

20 15 12 6 2 2 2 1

L 7.25

23 0

6 31

Nonreactive Reactive

Fetal capillary p H

Intrapartum fetal stimulation testing.

Intrapartum vibroacoustic stimulation testing (VAST) had a sensitivity of 100%, a specificity of 59.6% and a positive predictive value of 27.6% for th...
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