CST (Contraction stress test) or OCT (Oxytocin challenged test)

It is an invasive method to assess the fetal well being alteration in fetal
heart rate in response to uterine contraction.

Principal -
The test is based on determination of the respiratory function of the
fetoplacental unit during induced contraction when the blood flow
through the unit is curtained. The objective is to detect the degree of
fetal compromise, so that a suitable time can be selected to terminate
the pregnancy.

Indication - 
i) IUGR (Intrauterine growth retardation).
ii) Post maturity.
iii) Hypertensive disorder of pregnancy.
iv) Diabetes mellitus.
v) Woman with non-reactive NST.

Contraindication -
- Third trimester bleeding.
- Incompetent cervix.
- Multiple gestation.
- Previous classical uterine incision.
- Hydramnios.
- History of preterm labour.
- Premature rupture of membrane

Procedure - 
The oxytocin infusion is started. The initial rate of infusion is 1 mu/min which
is stepped up at intervals of 20 min until the effective uterine contraction are
established. The alteration of the fetal heart rate during concentration is recorded
by electronic monitoring. Alternatively clinical monitoring can effectively be
performed using hand to palpate the hardening of the uterus during contraction
and auscultation of fetal heart rate during contraction and for 1 min. There after
it takes at least 1-2 hours to perform the test.

Interpretation - 
i) Positive - Persistent late declaration of fetal heart rate following 50% or more of uterine contraction.
ii) Negative - No late declaration or significant variable declaration.
iii) Suspicious - Inconsistent but definite declaration do not persist with most uterine contraction.
iv) Unsatisfactory (Poor quality of records) - Poor or adequate uterine contraction is not achieved.