It is estimated that 1 - 3% of pregnant women undergo a surgical procedure unrelated to their pregnancy. (1) Stated another way, more than 50,000 surgical procedures are carried out in the United States and 5,000 in Canada each year in parturients. A small percentage of elective procedures are carried out in the first trimester before the patient herself is aware of her pregnancy. The majority of surgery however, is needed for urgent and even life threatening conditions which require surgical intervention despite the concerns and risks this presents to the mother and her fetus.
Emergency surgery is indicated during pregnancy for the management of trauma, malignancy or acute medical illness. More recently procedures have been developed for the treatment of fetal abnormalities which produce progressive injury in-utero before the fetus has reached a viable gestational age. The anesthetic considerations for surgery during pregnancy include (1) the management of maternal risk factors, (2) exposure to teratogens, (3) the maintenance of uteroplacental sufficiency, and (4) the direct and indirect actions of maternally administered anesthetics on fetal well-being.
The traumatized pregnant patient requires special assessment and management, and specialized procedures associated with neurosurgery, cardiac surgery and minimal access surgery may require modification during pregnancy.
Finally, the advent of in-utero procedures to repair fetal abnormalities presents sophisticated management approaches for the anesthesiologist.
Women in the childbearing years are among the population at greatest risk for trauma. Trauma occurs in 5 - 10% of pregnancies and is responsible for 36 maternal deaths per 100,000 pregnancies, which is considerably higher than pregnancy-related mortality. (2) Today, many more women continue to work and travel throughout pregnancy, exposing themselves to the multiple trauma of motor vehicle accidents. Fatigue, anemia, unsteadiness of gait and ligamentous laxity associated with pregnancy predispose the parturient to falls and blunt abdominal trauma that jeopardize both herself and the fetus with direct injury and/or placental disruption. Penetrating abdominal injury from gunshot and knife wounds or associated with motor vehicle accidents results in 5% maternal mortality. A much higher perinatal death rate in the range of 41 - 71% is reported. (3) Fetal death can be the result of maternal instability, placental abruption or direct fetal injury and hemorrhage, or as a consequence of premature delivery. The fetal status must be assessed carefully for evidence of developing compromise. Evidence of ongoing fetal hemorrhage can be determined by serial acid elution evaluation of vaginal blood loss and will impact on the obstetrical management of an otherwise stable mother. Finally, disruption of the uteroplacental unit carries the potential of releasing tissue thromboplastins into the maternal circulation. Disseminated intravascular coagulation and potential adult respiratory distress syndrome may further complicate the perioperative management.
The pregnant patient and her husband will be concerned about the effects of the anesthetic on their baby, no matter how urgent the situation they are facing. The anesthesiologist must be prepared to discuss concerns about teratogenicity, primarily to allay fears and preconceived notions. A few moments of reassurance prior to induction will usually not delay the most urgent of proceedings, and will answer important unspoken concerns.
Animal evidence of teratogenicity cannot be extrapolated to the human situation, and likewise, data from chronic exposure surveys of dental assistants and nurse anesthetists do not relate to intraoperative exposure to modern anesthetics (4,5). Human data obtained from the medical registry banks in Sweden and Manitoba document that acute exposure to anesthesia for surgery during pregnancy is not associated with an increase in congenital malformation rates (6,7). An increase in abortion rates, premature labor, stillbirths, low birthweight neonates and perinatal death was reported in both of these studies. No specific anesthetic drug nor technique was identified as preventive or causal in the development of premature labor, which is the greatest risk issue for the fetus exposed the stresses of maternal surgery.
All drugs administered to the pregnant patient will rapidly cross the placenta and be distributed to the fetus. The exception to this general concept are muscle relaxants, which are charged quaternary ammonium molecules. Halothane will equilibrate in fetal tissues after 60 minutes of maternal exposure, and is associated with stable fetal hemodynamics and acid base response (8,9). Isoflurane will produce a reduction in fetal cardiac output and redistribution of fetal circulation away from the placenta (10). Within one hour of 1 MAC Isoflurane anesthesia to the mother, fetal sheep develop hypercarbic acidosis, making isoflurane potentially harmful to the fetus exposed to prolonged maternal administration.
Most of the commonly used induction agents, narcotics and muscle relaxants are devoid of harmful direct fetal effects. The indirect effects of any drug on uteroplacental circulation and oxygen delivery to the fetus must be considered in light of the maximally vasodilated uterine vasculature. The blood flow to the maternal side of the placenta is not capable of further dilatation and autoregulation in the face of maternal hypotension. The "pressure-passive" nature of uterine blood flow, particularly in the third trimester requires a more aggressive approach to the management of maternal hypotension than in the non- pregnant patient.
Increasing maternal P02 from 100mmHg to 400mmHg will result in an increase in fetal P02 from 30mmHg to 60mmHg, because an increases partial pressure gradient across the placental unit will increase oxygen flux to the fetus (11). This is in spite of the fact that the increased maternal P02 does little to increase maternal oxygen content. Increasing maternal Fi02 in the face of 100% S02 should be considered during surgery if the fetal status is in question.
Monitoring fetal heart rate is an important aspect of these procedures, and is technically feasible after the 16th week for non-abdominal surgery. It may be necessary to reinforce to the surgeon and obstetrician that FHR monitoring will help guide the management of maternal cardiorespiratory parameters, and is useful even if it will not influence a decision to deliver the fetus (12). Intermittent abdominal ultrasound assessment of FHR can be used for abdominal procedures that do not permit the use of standard external Doppler probes. Anesthetic drugs create loss of heart rate variability, presumably by anesthetizing the brainstem center that modulates intrinsic cardiac automaticity. Vasoactive agents will cross the placenta and produce predictable changes to FHR, and will influence further the interpretation of the fetal tracing. Abrupt changes in heart rate and baseline rates outside the acceptable range of 120 to 160 beats per minutes should prompt the anesthesiologist to look for obvious causes of uteroplacental insufficiency and measures to increase blood pressure and oxygen delivery should be taken, even if these are within normal limits at the time a non-reassuring tracing is identified.
Pregnant patients who require urgent neurosurgical or cardiopulmonary bypass procedures present conflicting issues for perioperative management, and creative compromise is required to meet the best interests of mother and fetus. The first rule of management is that what is best for the mother's condition is usually best for the fetus. Controlled hypotension is however extremely hazardous for the fetus, and will likely produce demise. Fortunately, there are usually alternatives to hypotension, and neurosurgeon now routinely preclamp vessels feeding aneurysms and AV malformations. As a result, requests for controlled hypotension are less frequent and can usually be avoided during pregnancy.
High-pressure, high-flow cardiopulmonary bypass is recommended for the management of pregnant cardiac surgical candidates (13). Successful fetal outcome has been reported using mean pump pressures above 60 mmHg and flows above 2.0L/min/m2. Systemic cooling to temperatures below 28 C carries a high risk of fetal cardiac arrest and demise. With the advent of continuous retrograde cardioplegia via the coronary sinus, it is now possible to use systemic normothermia on bypass (14). Experience is reported of good fetal outcome following normothermic cardiopulmonary bypass.
Advances in prenatal diagnosis and the development of percutaneous and open fetal surgical procedures now enables treatment of fetal abnormalities that would otherwise produce progressive organ damage and neonatal demise if untreated prior to delivery. Congenital bilateral hydronephrosis can be prevented by the percutaneous placement of vesicoamniotic shunts or bladder marsupialization. The drainage of hydro- and chylothorax with thoracoamniotic catheter shunts prevent the development of pulmonary hypoplasia (15). The in-utero repair of congenital diaphragmatic herniation has met with limited success (16). Techniques for the management of obstructive hydocephalus, sacrococcygeal teratoma and selected congenital cardiac lesions are being developed and offer promise for improved neonatal survival for these lethal conditions (17).
The diagnosis of severe twin-twin transfusion in monozygotic twin pregnancy has prompted the development of fetoscopic procedures to protect the viable fetus from a 15-20% risk of developing severe neurologic injury as a consequence of the in-utero demise of the compromized twin (18). We have successfully carried out fetoscopic umbilical cord ligation at 24 weeks' gestation, and others have developed techniques for laser separation of the placental beds. These procedures require open laparotomy and fetoscopic visualization, which challenge the anesthesiologist to provide safe maternal conditions, uterine relaxation as well as fetal immobility and anesthesia (19). The high risk of premature labor associated with these procedures has led to the development of aggressive prophylactic protocols. Perioperative indomethacin, magnesium sulfate, beta agonists are added to inhalational anesthesia, and postoperatively, refractory uterine activity has been managed with intravenous nitroglycerin using intra- arterial and central venous monitoring (20).
Many medical and ethical questions are raised by fetal surgery. Is it adequate to immobilize the fetus with intramuscular muscle relaxants for percutaneous procedures which otherwise do not require maternal anesthesia? What are the psychologic and neurodevelopmental consequences of fetal surgery if fetal anesthesia is not addressed? What ethical and legal questions are raised when the late trimester fetus is sacrificed to improve the outcome of the in-utero sibling? Parents and medical caregivers are being presented with difficult options as a result of the development of these new technologies.
Semi-elective and urgent surgery is not contraindicated by pregnancy, although anesthetic and surgical approaches must be modified to address the safety of the mother and her fetus. When possible, surgery should be postponed to the second trimester, although there is no evidence to suggest significant teratogenic effects exist with any of the anesthetic agents in use today. Premature labor presents the greatest risk to the fetus in the perioperative period, but cannot be modified by the anesthetic approach. Neonatal mortality is 90% at 25 weeks' gestation and drops to less than 15% by week 30 (21). The postponement of surgery during this period of rapid fetal maturation should weigh the advantage to the fetus against the hazards that delay poses to the mother.
The combined expertise of obstetrician, anesthesiologist, neonatologist and surgeon are needed to manage the pregnant patient who requires emergency surgery.
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