c. Methods of Contraction Production.
(1) Oxytocin challenge test (OCT). A dilute of IV solution of oxytocin is
administered to the mother until a contraction pattern is developed. When sufficient
information is obtained to evaluate the test, the medication is turned off.
(a) The Oxytocin challenge test evaluates the ability of the placenta to
supply fetal needs in a stressed environment. Contractions, such as those of labor, are
a stress on the pregnancy. During a contraction, the flow of oxygen from the uterus to
the placenta is diminished. The healthy placenta stores an oxygen reserve so that the
fetus does not suffer a diminished supply of oxygen during the contraction.
(b) The OCT involves application of the fetal monitor to record fetal
heart rate and contraction activity. Acceptable results include acceleration of fetal heart
rate or no change in fetal heart rate baseline during a contraction. Unacceptable results
include deceleration of fetal heart rate during a contraction.
(c) The OCT is used to evaluate the high-risk pregnancy where the
placental compromise is suspected. It is often applied to the same high-risk patients
listed under NST. In addition, it is used to evaluate the patient when a suspicious result
is obtained on an NST. The OCT is more invasive than the NST; it provides more
conclusive results than the NST.
(2) Breast stimulation test (BST). This test involves stimulation of the
nipples (by rubbing), which causes the posterior pituitary to release the hormone
oxytocin, which in turn, causes contractions.
(3) Contraction stress test (CST). Evaluation is done in the presence of
naturally occurring contractions. It is a means of evaluating the respiratory function
(oxygen and carbon dioxide exchange) in the placenta.
2-11. COMPONENTS OF FETAL CIRCULATION
As the placenta acts as the intermediary organ of transfer between the mother
and fetus, fetal circulation differs from that required for extrauterine existence. The
fetus receives oxygen through the placenta because the lungs do not function as organs
of respiration in the uterus. To meet this situation, the fetal circulation contains certain
special vessels that shunt the blood around the lungs, with only a small amount
circulating through them for nutrition. See figure 2-9. The following functions occurs: