Antenatal Nonstress Cardiotocography
Authors: Beischer Norman A. 
Published: February 2008

EDITORIAL COMMENT: There are problems with the interpretation of the value of antenatal cardiotocography in current obstetric practice. For example, a recent review by Thacker and Berkelman concluded that although nonstress and contraction stress cardiotocographs are used extensively in the United States of America, ‘neither have been demonstrated to be useful diagnostic tests’ (1). These authors stated that antepartum fetal surveillance techniques are potentially useful screening tests but still ‘require critical evaluation, with large randomized, controlled trials, to determine their efficiency and safety before their further diffusion into obstetrical practice’ (1).

This issue of the journal contains audits of the indications and results of antenatal cardiotocography (using nonstress testing) performed in the Fetal Monitoring Departments of 2 of the University Teaching Hospitals in Melbourne. It is evident that antepartum cardiotocography has become popular with clinicians; 42% of patients at the Queen Victoria Medical Centre were tested and 32% at the Mercy Maternity Hospital, although the tests were ordered on clinical indications and not as a routine. The results showed that when there was evidence of satisfactory fetal reserve the perinatal mortality rate was less than 0.3% after malformations incompatible with life were excluded. Both studies showed a highly significant increase in the incidence of fetal growth retardation and low Apgar scores in infants born from pregnancies with abnormal cardiotocography. The Mercy Maternity Hospital data also showed that the perinatal mortality rate was increased from less than 1% to nearly 4% when fetal reserve was reduced, and to almost 20% when there was evidence of critical reserve (presence of late decelerations plus loss of beat to beat variation). These papers do not consider the management of patients who have evidence of critical fetal reserve. Since the perinatal mortality rate in these cases is approximately 20%, such a finding warrants immediate delivery, usually by Caesarean section.

The clinician's problem is what to do with the patient who has evidence of reduced fetal reserve on cardiotocography. This is an important consideration since this group constitutes about 8% of the patients tested by antenatal cardiotocography — the critical reserve group comprising only 1–2% of those tested. Oats and colleagues from the Mercy Maternity Hospital give clinical details of all the perinatal deaths that occurred in their patients with normal antenatal cardiotocography or with evidence of reduced fetal reserve. These authors have designated those cases in which they regard, in retrospect, the perinatal death as being possibly avoidable, and the reader can also see the number of unexplained deaths that occurred within a week of the test being performed.

The great advantage of antenatal cardiotocography is that it allows the clinician to defer delivery when there is evidence of satisfactory fetal reserve and so avoid the problems of prematurity when there is no absolute maternal indication for delivery. The reader can also see from the cases quoted that the clinician should not defer delivery when there is evidence of reduced reserve when the pregnancy has reached practical viability — 37 weeks' gestation. It is possible that recognition of reduced reserve with appropriate clinical action accounts for the varying incidence of critical reserve reported from different centres.