Ray and colleagues (1972) used this concept in 66 complicated pregnancies and developed the oytocin challenge test, which was later called the contraction stress test. Intravenous oxytocin is used to stimulate contractions, and the criterion for a positive test result, that is, an abnormal result, is uniform repetitive late fetal heart rate decelerations. These reflected the uterine contraction waveform and had an onset at or beyond the contraction acme. Such late decelerations could be the result of uteroplacental insuiciency. In their study, the tests were generally repeated on a weekly basis, and the investigators concluded that negative contraction stress test results, that is, normal results, forecasted fetal health. A major disadvantage is that the average contraction stress test requires 90 minutes to complete.
To perform the test, the fetal heart rate and uterine contractions are recorded simultaneously with an external monitor. If at least three spontaneous contractions of 40 seconds or longer are present in 10 minutes, no uterine stimulation is necessary (American College of Obstetricians and Gynecologists, 2016). Contractions are induced with either oxytocin or nipple stimulation if there are fewer than three in 10 minutes. For oxytocin use, a dilute intravenous inusion is initiated at a rate of 0.5 mU/min and doubled every 20 minutes until a satisfactory contraction pattern is established (Freeman, 1975). The results of the contraction stress test are interpreted according to the criteria shown in Table
Nipple stimulation to induce uterine contractions is usually successful for contraction stress testing (Huddleston, 1984). One method involves a woman rubbing one nipple through her clothing for 2 minutes or until a contraction begins. This 2-minute nipple stimulation ideally will induce a pattern of three contractions per 10 minutes. If not, after a 5-minute interval, she is instructed to retry nipple stimulation to achieve the desired pattern. If this is unsuccessful, then dilute oxytocin may be used. Advantages include reduced cost and shortened testing times. Some have reported unpredictable uterine hyperstimulation and fetal distress, whereas others did not find excessive activity to be harmful (Frager, 1987; Schellpfefer, 1985).
Freeman (1975) and Lee and colleagues (1975) introduced the nonstress test to describe fetal heart rate acceleration in response to fetal movement as a sign of fetal health. This test involved the use of Doppler-detected fetal heart rate acceleration coincident with fetal movements perceived by the mother. By the end of the 1970s, the nons tress test had become the primary method of testing fetal
TABLE 17-1. Criteria for Interpretation of the Contraction Stress Test
Negative: no late or significant variable decelerations Positive: late decelerations following 50% or more of contractions (even if the contraction frequency is fewer than three in 10 minutes) Equivocal-suspicious: intermittent late decelerations or significant variable decelerations Equivocal-hyperstimulatory: fetal heart rate decelerations that occur in the presence of contractions more frequent than every 2 minutes or lasting longer than 90 seconds Unsatisfactory: fewer than three contractions in 10 minutes or an uninterpretable tracing health. he nonstress test was easier to perform, and normal results were used to urther discriminate false-positive contraction stress tests. Simplistically, the nonstress test is primarily a test ofetal condition, and it difers from the contraction stress test, which is considered a test of uteropacentalonction. Currently, nonstress testing is the most widely used primary testing method for assessment of fetal well-being. It has lso been incorporated into the biophysical profile testing system, subsequently discussed.
Autonomic inluences are mediated by sympathetic or para sympathetic impulses from brainstem centers to normally raise or slow the fetal heart rate. Beat-to-beat variability is also under the control ofthe autonomic nervous system (Matsuura, 1996).