Читать книгу Protocols for High-Risk Pregnancies - Группа авторов - Страница 87

The CST was historically the first method of fetal assessment using the noninvasive technique of fetal heart rate monitoring during the antepartum period. The test is based upon the fact that normal uterine contractions will restrict fetal oxygen delivery in a transient manner resulting from stasis of blood flow secondary to compression of maternal blood vessels in the uterine wall. Alterations in respiratory exchange in the maternal–fetal interface at the level of the placenta will result in differing responses of the fetus to interruption of maternal blood flow secondary to uterine contractions. If such contractions result in episodic fetal hypoxia, this will be demonstrated by the appearance of late decelerations of the fetal heart rate.

The CST is performed over a period of 30–40 minutes with the patient in the lateral recumbent position while both the fetal heart rate and uterine contractions are simultaneously recorded utilizing an external fetal monitor. A frequency of at least three 40‐second or longer contractions in a 10‐minute period of monitoring is required and these contractions can be either spontaneous or induced with nipple stimulation or resulting from the intravenous infusion of oxytocin.

The results of the CST are negative (no late or significant variable decelerations), positive (late decelerations following 50% or more of contractions), equivocal (intermittent late or significant variable decelerations or late decelerations following prolonged contractions of 90 seconds or more or with a contraction frequency of more than every two minutes), or unsatisfactory. A major problem associated with the CST is the high frequency of equivocal test results.

Relative contraindications to the CST are those conditions associated with a significant increased risk of preterm labor, preterm rupture of membranes, placenta previa with bleeding or history of classic cesarean delivery or extensive uterine surgery.

The CST has a remarkably low false‐negative rate of 0.04% (antepartum stillbirth within one week of a negative test) but up to 30% of positive tests when followed by induction of labor do not require intrapartum interventions for continued abnormalities of the fetal heart rate or adverse neonatal outcome. Because of the fact that the CST is more labor intensive, takes more time and has a high rate of equivocal test results, this test has generally been abandoned as the primary means of antepartum fetal surveillance. In some centers, the CST remains the primary test for women with type 1 diabetes. However, the CST remains a very reliable means of either primary or back‐up fetal surveillance for any number of high‐risk pregnancy conditions.