DYSTOCIA & ABNORMAL FETAL PRESENTATIONS & POSITIONS
A prolonged latent phase by definition exceeds 20 h in a nulliparous parturient and 14 h in a multiparous patient. The cervix usually remains at 4 cm or less but is completely effaced. The etiology is likely ineffective contractions without a dominant myometrial pace-maker. Arrest of dilation is present when the cervix undergoes no further change after 2 h in the active phase of labor. A protracted active phase refers to slower than normal cervical dilation, defined as less than 1.2 cm/h in a nulliparous patient and less than 1.5 cm/h in a multiparous parturient. A prolonged deceleration phase occurs when cervical dilation slows markedly after 8 cm. The cervix becomes very edematous and appears to lose effacement. A pro-longed second stage (disorder of descent) is defined as a descent of less than 1 cm/h and 2 cm/h in nul-liparous and multiparous parturients, respectively. Failure of the head to descend 1 cm in station after adequate pushing is referred to as arrest of descent.
Oxytocin is usually the treatment of choice for uterine contractile abnormalities. The drug is given intravenously at 1–6 mU/min and increased in incre-ments of 1–6 mU/min every 15–40 min, depending on the protocol. Use of amniotomy is controversial. Treatment is usually expectant management, as long as the fetus and mother are tolerating the prolonged labor. When a trial of oxytocin is unsuccessful or when malpresentation or cephalopelvic dispropor-tion is also present, operative vaginal delivery or cesarean section is indicated.
Breech presentations complicate 3–4% of deliveries and significantly increase both maternal and fetal morbidity and mortality rates. Breech presentations increase neonatal mortality and the incidence of cord prolapse more than 10-fold. External cephalic version may be attempted after 34 weeks of gestation and prior to the onset of labor; however, the fetus may spontaneously return to the breech presenta-tion before the onset of labor. Some obstetricians
may administer a tocolytic agent at the same time. External version can be facilitated, and its success rate improved, by providing epidural analgesia with 2% lidocaine and fentanyl. Although an external version is successful in 75% of patients, it can cause placental abruption and umbilical cord compression necessitating immediate cesarean section.
Because the shoulders or head can become trapped after vaginal delivery of the body, some obstetricians employ cesarean section for all breech presentations. Manual or forceps-assisted partial breech extraction is usually necessary during these vaginal deliveries. The need for breech extraction does not appear to be increased when epidural anes-thesia is used for labor—if labor is well established prior to epidural activation. Moreover, epidural anesthesia may decrease the likelihood of a trapped head, because the former relaxes the perineum. Nonetheless, the fetal head can become trapped in the uterus even during cesarean section under regional anesthesia; rapid induction of general endo-tracheal anesthesia and administration of a volatile agent may be attempted in such instances to relax the uterus. Alternatively, nitroglycerin, 50–100 mcg intravenously, can be administered.
When the fetal occiput fails to spontaneously rotate anteriorly, a persistent occiput posterior presenta-tion results in a more prolonged and painful labor. Manual, vacuum, or forceps rotation is usually necessary but increases the likelihood of maternal and fetal injuries. Regional anesthesia can be used to provide perineal analgesia and pelvic relaxation, allowing manual or forceps rotation followed by for-ceps delivery.
A face presentation occurs when the fetal head is hyperextended and generally requires cesarean section. A compound presentation occurs when an extremity enters the pelvis along with either the head or the buttocks. Vaginal delivery is usually still possible because the extremity often withdraws as the labor progresses.
Shoulder dystocia, or impaction of a shoulder against the pubic symphysis, complicates 0.2–2% of deliveries and is one of the major causes of birth injuries. The most important risk factor is fetal macrosomia. Shoulder dystocias are often difficult to predict. Several obstetric maneuvers can be used to relieve it, but a prolonged delay in the delivery could result in fetal asphyxia. Induction of general anesthesia may be necessary if an epidural catheter is not already in place.