Obstetric Anesthesia in High-Risk Situations
Women’s Health & Education Center’s Contribution. We wish to express our gratitude to the individual contributors whose hard work and dedication has helped to compile this article.
Pregnancy and delivery are considered “high-risk” when accompanied by conditions unfavorable to the well-being of the mother or unborn baby or both. Analgesia management in acute and chronic fetal distress and in maternal complications such as preeclampsia, eclampsia, hypertension, heart disease, renal disease, neurologic disorder, obesity, substance abuse and diabetes are affected by it. The analgesic management of obstetric complications such as placenta previa, cord prolapse, abruptio placentae, prematurity, multiple gestation, and breech presentation may increase the risk to the mother or the fetus. In general, the anesthetic management of the high-risk pregnant patient is based on the same maternal and fetal considerations as the management of healthy mothers and fetuses. These include maintenance of maternal cardiovascular function and oxygenation, maintenance and possibly improvement of uteroplacental blood flow, and creation of optimal conditions for a painless, atraumatic delivery of an infant without significant drug effect.
The purpose of this document is to understand the pain management of high-risk patient during labor and delivery. There is less room for error because many of these functions may be compromised before the induction of anesthesia. Significant acidosis is prone to develop in fetuses of diabetic mothers when delivered by cesarean section with spinal anesthesia complicated by even brief maternal hypotension. Because the high-risk pregnant patients may have received a variety of drugs, anesthesiologists must be familiar with potential interactions between these drugs and the anesthetic drugs they plan to administer.
Acute and Chronic Fetal Distress:
Acute fetal distress usually occurs intrapartum, without previously suspected fetal compromise. It may be heralded clinically by the sudden appearance of meconium, the development of bradycardia, or a deceleration pattern detected by fetal monitoring. With continuous heart rate monitoring equipment, a severe deceleration may be designated as variable or late. In either case, because uterine perfusion presently is correlated with blood pressure, it may be assumed that a maternal pressure fall greater than 20% of the baseline systolic figure will produce a substantial reduction in uterine perfusion. Because this will aggravate any acute intrapartum fetal distress, hypotension should be avoided. Although the incidence and degree of hypotension after sympathetic blockage can be minimized by thorough evaluation, fluid preloading, and patient positioning, it may occur even in the best circumstances. The anesthetic selection during labor with mild fetal distress is usually either a small dose of intravenous tranquilizer and narcotics or a segmental epidural block. If severe fetal distress requires immediate cesarean delivery, time must not be spent placing a regional block. General endotracheal anesthesia is required in this circumstance to enable the speediest delivery of the distressed fetus.
A serious problem in anesthetic management occurs when acute intrapartum distress is superimposed on chronic fetal distress. Underlying chronic distress may be preeclampsia, hypertension, postmaturity, or diabetes. These disorders all reduce fetal reserve. The anesthesiologist must meticulously avoid or manage even mild hypotension, since the primary aim should be maintenance of uterine blood flow (1). The probable choices are no analgesia, minimal systemic analgesia, or segmental epidural block.
Preeclampsia – Eclampsia:
This syndrome (often called toxemia of pregnancy) is composed of the triad of hypertension, generalized edema, and proteinuria. The primary pathologic characteristic of this disease process is generalized arterial spasm. As gestation lengthens, there is a tendency toward a fluid shift from the vascular to the extravascular compartment with resultant hypovolemia – in spite of an expanded extracellular fluid space. The most significant electrolyte disturbances are those of sodium and chloride. Although both sodium and water are retained in the extracellular space, sodium and chloride values in the intravascular compartment are subnormal. This is not reflected by serum electrolyte values because of hypovolemia. Arterial spasm takes its toll via the cardiovascular system. It is estimated that nearly 50% of eclamptic patients who die have myocardial hemorrhages or areas of focal necrosis.
Regional anesthesia is preferred for women with preeclampsia and eclampsia – both for labor and delivery. A secondary analysis of women with severe preeclampsia in the National Institute of Child Health and Human Development’s Maternal-Fetal Medicine Units Network trial of low-dose aspirin reported epidural anesthesia was not associated with an increased rate of cesarean delivery, pulmonary edema, or renal failure. Moreover, general anesthesia carries more risk to pregnant women than does regional anesthesia (2). Regional analgesia in women with preeclampsia is associated with an overall 15-25% reduction in systemic mean arterial pressure. Although the peripheral vasodilation seen with regional analgesia may be helpful in decreasing severe hypertension, and if severe hypotension occurs, that may require cautious treatment with ephedrine. In addition prehydration with crystalloid combined with intraoperative fluid boluses for hypotension results in an average additional fluid challenge of 600-800 mL in women with preeclampsia receiving regional anesthesia.
Consumption coagulopathy may require corrective measures such as infusion of fresh whole blood, platelet concentrates, fresh frozen plasma, and cryoprecipitate. The administration of conduction anesthesia is contraindicated in patients with severe coagulopathy because of the increased risk of an epidural hematoma, leading to permanent neurologic damage. In volume-depleted patients positioned with left uterine displacement, epidural anesthesia does not cause an unacceptable reduction in blood pressure, and leads to a significant improvement in placental perfusion (3). For cesarean section, the sensory level of regional anesthesia must extend to T3-T4, making adequate fluid therapy and left uterine displacement even more vital. Epidural anesthesia has been preferred to spinal anesthesia in preeclamptic women because of its slower onset of action and controllability. The rapid onset of spinal anesthesia may be associated with hypotension, particularly in a volume-depleted patient.
General anesthesia in preeclamptic patients has its particular hazards. Rapid-sequence induction of anesthesia and intubation of the trachea necessary to avoid aspiration are occasionally difficult because a swollen tongue, epiglottis, or pharynx distorts the anatomy. In patients with impaired coagulation, laryngoscopy and intubation of the trachea may provoke profuse bleeding. Marked systemic and pulmonary hypertension occurring at intubation and extubation enhance the risk of cerebral hemorrhage and pulmonary edema. Magnesium sulfate may prolong the effects of all muscle relaxants through its actions on the myoneural junction; therefore relaxants should be administered with caution (using nerve stimulator) to avoid over dosage. General anesthesia may be necessary in acute emergencies, such as abruptio placentae, and in patients who do not meet the criteria for epidural anesthesia.
Hemorrhage and Shock:
Intrapartum obstetric emergencies demand immediate diagnosis and therapy for a favorable outcome for the mother and fetus. Placenta previa and abruptio placentae are accompanied by serious maternal hemorrhage. Aggressive obstetric management may be indicated. The primary threat to mother is that of blood loss, which reduces her effective circulating blood volume and her oxygenation potential. Similarly, the chief hazard to the fetus is diminished utero-placental perfusion secondary to maternal hypovolemia and hypotension. The perinatal mortality rate associated with placenta previa and abruptio placentae ranges from 15-20% in some studies. The overall morbidity and mortality rates for both the fetus and mother depend on the gestational age and health of fetus, the extent of hemorrhage, and the therapy given. Reliable intravenous lines should be established early. In addition, recommendations for the treatment and control of shock must be formulated.
The anesthesiologist may be involved in maternal resuscitation and provision of anesthesia. This may include the establishment of invasive monitoring (an arterial and a central venous catheter are usually adequate) and blood volume replacement, preferably through 14- or 16-gauge cannulae. To correct clotting abnormalities, blood components such as frozen plasma, cryoprecipitate, and platelet concentrates may be required. The anesthesiologist also performs the traditional role of providing appropriate anesthesia for a cesarean section or, occasionally, a hysterectomy. Epidural anesthesia should be considered, but general anesthesia is indicated in the presence of uncontrolled hemorrhage and coagulation abnormalities (4). As usual, it should be preceded by adequate denitrogenation. In hypovolemic patients, induction may be accomplished with ketamine, intravenous 0.5-1.0 mg/kg. In all obstetric patients, proper precautions should be taken to prevent aspiration of gastric contents. These include the administration of a clear antacid and rapid-sequence induction of anesthesia with application of cricoid pressure until tracheal intubation.
Heart disease during pregnancy occurs in 0.4-4.1% of patients and is the leading non-obstetric cause of maternal mortality, with a mortality rate ranging from 0.4% among patients in Class I or II of the New York Heart Association’s functional classification to 6.8% among those in Classes III and IV. The following lesions pose the greatest risk for the mother; pulmonary hypertension, particularly with Eisenmenger’s syndrome, mitral stenosis with atrial fibrillation, tetralogy of Fallot, Marfan’s syndrome and coarctation of the aorta. Cardiac decompensation and death occur most commonly at the time of maximum hemodynamic stress, that is, in the third trimester of pregnancy, during labor and delivery, and particularly during the immediate postpartum period. During labor, cardiac output increases above antepartum levels. Between contractions this increase is approximately 15% in the early first stage, approximately 30% during the late first stage, approximately 45% during the second stage, and after delivery 30-50%. With each contraction, approximately 200 ml of blood is squeezed out of the uterus into general circulation. The greatest change occurs immediately after delivery of the placenta, when cardiac output increases to an average of 80% above antepartum values, and in some patients it may increase by as much as 150%. These changes in cardiac output can be reduced by administration of regional anesthesia. In patients managed with continuous caudal anesthesia, cardiac output increases only 24% above antepartum control values during the second stage and 59% in immediate postpartum.
Until 1960, rheumatic fever was responsible for almost 90% of the heart disease encountered in pregnant women. Mitral stenosis is the sole or predominant valvular lesion in most parturients with rheumatic heart disease. The primary defect is obstruction to diastolic blood flow from the left atrium to the left ventricle. This becomes hemodynamically significant when the valve orifice is diminished or the rate of blood flow through the constricted orifice is increased sufficiently to raise left atrial pressure and, consequently, pressure in the pulmonary veins and capillaries. Continuous epidural block offers particular advantages to the pregnant cardiac patient. It not only eliminates pain and tachycardia throughout labor and delivery but prevents the progressive increase in cardiac output and stroke volume that normally occurs in parturition. Should general anesthesia be deemed necessary for cesarean section, the standard thiopental-nitrous oxide-halogenated anesthetic-muscle relaxant technique is recommended. In cases of severe mitral stenosis, etomidate 0.2-0.3 mg/kg, or a slower induction with halothane or intravenous fentanyl is preferred. The benefit of high-dose narcotic induction should be weighed against the risk of transient neonatal depression. In patients with severe aortic stenosis and evidence of left ventricular compromise, halogenated agents should be avoided (5).
Patent ductus arteriosus, atrial septal defect, and ventricular septal defect are the more common congenital cardiovascular abnormalities. In all these conditions, anomalous communicating channels exist between the cardiac chambers or the great vessels. Late in the natural history of these diseases, pulmonary hypertension may develop, causing a reversal of the shunt (Eisenmenger’s syndrome). Pregnancy is poorly tolerated because the gestational decrease in systemic vascular resistance results in a significant increase in the right-to-left shunt. Tetralogy of Fallot is the most common cyanotic congenital heart defect. Anesthesia for parturients with cyanotic heart disease should provide effective pain relief while avoiding hypotension, struggling, or coughing, and eliminating bearing-down efforts, all of which could increase the right-to-left shunt. For labor, intrathecal opioids rather than conventional epidural anesthesia can be administered. Light planes of general anesthesia are usually well tolerated for cesarean section (6).
Preterm labor and delivery present a significant challenge to the anesthesiologist because both the mother and the infant may be at risk. Obstetricians frequently try to inhibit preterm labor to enhance fetal lung maturity. Delaying delivery by even 24-48 hours may be beneficial if glucocorticoids are administered to the mother. Premature newborns are more vulnerable than the term newborns to the effect of drugs used in obstetric analgesia and anesthesia. For labor and vaginal delivery, well conducted epidural anesthesia is advantageous in providing good perineal relaxation. The anesthesiologist should ascertain that the fetus is neither hypoxic nor acidotic before induction of epidural blockade. Asphyxia results in a redistribution of fetal cardiac output, which increases oxygen delivery to vital organs such as the brain, heart, and adrenals. These changes in the preterm fetus may be better preserved with bupivacaine or chloroprocaine than with lidocaine (7). Preterm newborns with breech presentation are usually delivered by cesarean section. General anesthesia with uterine relaxation provided by a halogenated drug, facilitates delivery of the after-coming head. If regional anesthesia is used, nitroglycerin should be available for uterine relaxation.
The adequacy of the uteroplacental circulation, so vital to the well-being of the fetus, is easily affected by drugs and anesthetic procedures. Well conducted obstetric analgesia, in addition to relieving pain and anxiety, may benefit the mother. The most frequently chosen methods for relieving the pain of parturition are psychoprophylaxis, systemic medication, and regional analgesia. Inhalational analgesia, conventional spinal analgesia, and paracervical blockade are less commonly used. General anesthesia is rarely necessary but may be indicated for uterine relaxation in some complicated deliveries.