It assist the fetal ability to cope with the contraction of high risk pregnancy and stress of labour. It helps physician in selecting the optimal time for delivery of high risk fetus. Contraction decreases the blood flow through intervillus space if the uteroplacental reserve is normal this decreases in blood flow by contraction, will not negatively affect the fetus.
If the uteroplacental reserve is diminished, there is decrease in blood blow by contraction, will affect he fetus, this alters the heart rate pattern by late deceleration.
CST is perform by the fetus response induced by nipple stimulation test and oxytocin using external fetal monitoring to record FHR and uterine contraction simultaneously.
PROCEDURE– The woman must empty her bladder and lie in semi fowler’s or left lateral position. Two method for inducing the contraction for CST are-
(A) Nipple stimulation test– In this there is release of endogenous oxytocin. This method of producing naturally occuring oxytocin avoid the risk of discomfort .At the begining of test warm cloth are applied to breast then the woman stimulate her nipple either by rolling or gentally pulling them. Initially it is unilateral if the contraction are inadequate the women then stimulate both nipple for another 10 min.If contraction are still inadequate (<3 contraction in 10 min.)IV oxytocin is used.
(B) Oxytocin challenge test– The administration of oxytocin to induced contraction, veni puncture is done and a very dilute solution of oxytocin control by an infusion pump is administered. Solution is prepared by dissolving 1 unit in 500 ml RL with the initial dose of 1-2 milliunit/min.(15 drops/ min.).The rate is increased at interval of 20 min. until the contraction are frequent of at least 3 in 10 min.or lasting for at least 30 sec. then the recording is done as-
1) A negative CST is one with no late deceleration occurs with contraction as frequent as 3 in 10 min.indicate fetal well being. Can predict that fetus will continue for another week without intervention of delivery.
2) A positive CST is one in which there is repeated late deceleration of FSH pattern after contraction has been passed. It indicate that fetus may not withstand with continuation of pregnancy.