Subspecialty Management |
|Chapter 57:||Anesthesia for Obstetrics|
Obstetric hemorrhage is classified according to timing: antepartum or postpartum. Although most patients tolerate the normal blood loss associated with either vaginal or cesarean delivery, occasionally, blood loss is excessive. Obstetric hemorrhage is an important cause of maternal mortality. 396
Antepartum hemorrhage complicates about 4 percent of pregnancies. Important causes include placenta previa, abruptio placentae, and uterine rupture.
Placenta previa exists when the placenta lies over the uterine cervix in front of the fetal presenting part (Fig. 5726). The diagnosis is suspected from the clinical picture of painless vaginal bleeding and is confirmed by ultrasonography. Vaginal examinations are avoided, and delivery is via cesarean section. Tocolytic therapy may be instituted when premature uterine contractions trigger bleeding. If the gestation is term, delivery may need to proceed urgently. Anesthetic management depends on the degree of urgency and the maternal and fetal status. Regional anesthesia is appropriate for elective surgery, but general anesthesia is appropriate when surgery is prompted by maternal hemorrhage. Excessive bleeding can also occur during surgery because delivery may require cutting through the placenta, and the placental implantation site in the lower segment of the uterus may not contract well after delivery.
|FIGURE 5726 Types of placenta previa. (A) Low implantation of the placenta. (B) Partial placenta previa. (C) Total placenta previa. (From Bonica and Johnson436 )|
Abruptio placentae is bleeding behind the placenta, causing partial separation (Fig. 5727). Both mother and fetus can be adversely affected: the mother from acute blood loss and coagulopathy from disseminated intravascular coagulation, and the fetus from reduced uterine blood flow and loss of functional placenta. A large abruption can result in fetal demise. Risk factors for abruptio placentae include chronic hypertension, abdominal trauma, cocaine use, advanced maternal age, multiparity, and history of prior abruption. 397 Abruptio placentae typically presents with painful, frequent uterine contractions and vaginal bleeding. The amount of maternal blood loss can be significant and may not be reflected by the amount of vaginal bleeding because blood can be sequestered behind the placenta. About 10 percent of cases of abruptio placentae are complicated by disseminated intravascular coagulation. Management of patients with abruptio placentae depends on the severity of the abruption and the maternal and fetal condition. A small abruption may trigger labor, which may continue to vaginal delivery if the status of the mother and the fetus is stable. A more significant abruption may lead to emergent cesarean delivery because of maternal and/or fetal instability. The anesthesiologist should be aware that uterine atony can occur because blood extravasated into the myometrium may preclude normal uterine contraction after delivery.
|FIGURE 5727 Abruptio placentae. (A) Internal or concealed hemorrhage. (B) External hemorrhage. (C) Complete placental separation and prolapse. (From Bonica and Johnson436 )|
Uterine rupture is a less common (0.4%) but potentially devastating cause of obstetric hemorrhage. It may occur in a previously scarred uterus (particularly after a classical [vertical] uterine incision) or may result from uterine manipulation (forceps, uterine curettage), trauma, or overaggressive use of IV oxytocin. Other risk factors are uterine anomalies, tumors, and placenta percreta (invasion of the placenta through the uterine wall). Uterine rupture is different than uterine scar dehiscence. Uterine scar dehiscence is a defect in the uterine wall at the site of a previous incision (usually low transverse) that does not result in obstetric hemorrhage. It is reported to occur in about 0.7 percent of patients undergoing vaginal birth after prior cesarean. 398 Uterine rupture is reported to occur in 0.6 to 0.8 percent of vaginal birth-after-cesarean patients. 398, 399 Rupture of an unscarred uterus can result in massive maternal hemorrhage. Symptoms can be nonspecific and include maternal hemodynamic instability, fetal bradycardia, vaginal bleeding, and loss of function of uterine pressure monitors. Treatment requires emergent laparotomy and may require obstetric hysterectomy. 400
Postpartum hemorrhage complicates about 10 percent of deliveries and is defined as a blood loss of greater than 500 mL. Important causes include uterine atony, retained placenta, placenta accreta, birth trauma, and uterine inversion.
Uterine atony is the most common cause of postpartum hemorrhage. Contraction of the uterine muscle is necessary to stop uterine bleeding after delivery of the placenta. Ineffective uterine muscle contraction leads to hemorrhage. Risk factors for uterine atony include prolonged labor, overdistended uterus (e.g., with twin gestation), grand multiparity, and use of drugs known to decrease uterine muscle contraction (halogenated inhaled anesthetics, b-adrenergic agonists, and magnesium sulfate). Treatment includes volume resuscitation and administering uterotonic medications. Such medications include oxytocin, carboprost tromethamine, and ergot preparations. Oxytocin is often routinely given as a dilute infusion (20 U/L of IV fluid) after delivery to promote uterine contraction. Bolus doses are avoided because hypotension can result. 391 Carboprost-tromethamine (Hemabate) is the 15-methyl analogue of prostaglandin F2a and is a potent uterotonic medication given IM (250 mg). Side effects include nausea, vomiting, diarrhea, and bronchoconstriction. 401 The ergot derivatives ergonovine maleate and methylergonovine maleate are effective uterotonic medications. The usual dose is 0.2 mg IM. Hypertension is an important side effect that results from the drugs a-adrenergic activity. These medications should be avoided in patients with hypertension or intracranial vascular disease and in those who have recently received vasopressors.
Retained placenta exists when all or part of the placenta fails to deliver spontaneously within 1 hour of birth. Hemorrhage results from failure of the uterus to contract where the placenta is adherent. Treatment involves placental removal either manually or via curettage. Anesthetic management involves volume resuscitation, provision of adequate analgesia (with epidural or spinal anesthesia if maternal volume status allows, IV or inhalational analgesia, or general anesthesia), and if necessary, assisting with uterine muscle relaxation to allow uterine manipulation and delivery of the placenta (with halogenated anesthetics administered via an endotracheal tube, or IV nitroglycerin, 50- to 100-mg bolus doses. 402 )
Placenta accreta, increta, or percreta (Fig. 5728) involves invasion of the placenta through the endometrium, into the myometrium, and through the myometrium, respectively. Risk factors include previous uterine surgery or trauma. 403 Normal placental delivery is often precluded and results in hemorrhage perhaps necessitating obstetric hysterectomy. 404 Placenta accreta occurs in about 0.04 percent of pregnancies, but the risk increases markedly in patients with placenta previa (5%) and increases even more dramatically in patients with placenta previa with prior cesarean delivery (24% after one prior cesarean, 4060% after three to four prior cesareans). 403 The anesthesiologist should be aware of this potential problem when caring for a patient with placenta previa undergoing repeat cesarean delivery and should be prepared to manage massive hemorrhage.
|FIGURE 5728 Classification of placenta accreta based on degree of pen-etration of the myometrium. Placenta accreta: adherence of the placenta to the myometrium; placenta increta: invasion of the myometrium; placenta percreta: the placenta erodes through the myometrium to involve the serosa of the uterus and even the surrounding structures. (From Kamani et al437 )|
Cervical and vaginal lacerations can result in persistent vaginal bleeding postpartum. The anesthesiologist is needed to provide the necessary analgesia or anesthesia to allow the obstetrician to visualize and repair the injury. The anesthesiologist should be aware that the amount of blood loss from such lacerations is difficult to assess and is often underestimated.
Uterine inversion is a rare cause of postpartum hemorrhage in which the uterine fundus actually inverts through the cervix into the vagina, precluding uterine contraction. 405 Predisposing factors include retained placenta, prolonged labor, and precipitous labor. Management involves correction of the inversion and may require uterine muscle relaxation. 275, 406
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