Non-stress test (NST)
This test is based on the observation that in a healthy foetus with an intact CNS and responsive autonomic system (sympathetic and parasympathetic), gross foetal movements are associated with foetal cardiac acceleration. This response is blunted by hypoxia, acidosis, drugs (sedatives, narcotics, beta-blockers), foetal sleep, congenital foetal anomalies and foetal compromise from any cause.
The test is best performed in a quiet environment, the patient in semi-Fowler or left lateral recumbent position. External foetal heart rate and tocodyna-mometer monitoring leads are applied. A continuous foetal heart rate tracing is obtained. Whenever the patient perceives any foetal movement, she presses on a button which prints a mark on the tracing at that moment in time. The test is continued until the criteria for reactivity are satisfied or the time limit of 40 minutes has been reached.
The NST is interpreted as 'reactive' if at least 2 accelerations of 15 b.p.m. are observed, lasting for at least 15 seconds in any 20 minute period. It is indicative of a healthy foetus.
A 'non-reactive test' calls for further evaluation.
All `NST' tests should be reviewed for the following information:
Baseline heart rate.
Beat to beat variability.
Number, duration and extent of FHR accelerations.
Correlation of FHR accelerations with foetal movements.
Number, duration and extent of FHR accelerations in relation to uterine contractions.
The advantages of the NST test are its simplicity, suitability to an OPD setting, rapidity, low cost and free from contraindications.
A 'non-reactive test has a high positivity but a lower specificity for foetal compromise as compared to a contraction stress test (CST), oxytocin challenge test or the nipple stimulation test or biophysical profile.
Uterine contractions diminish placental perfusion. Hence transient FHR decelerations occur in consonance with the duration of uterine hypertonus induced by the uterine contraction. In situations of uteroplacental insufficiency, the uterine contractions may decrease the placental perfusion sufficiently to cause foetal hypoxia which outlasts the duration of the contraction. The FHR tracing shows decelerations which begin with the peak of the uterine contractions and persist even after the conclusion of the contraction (late deceleration). Late decelerations are indicative of foetal compromise