1 What are normal arterial blood gas findings in pregnancy?

Pregnancy results in increased ventilation because of elevated carbon dioxide production, as well as an increase in respiratory drive mediated largely by progesterone. These changes cause a low arterial partial pressure of carbon dioxide, at about 30 mm Hg by term. Plasma bicarbonate is decreased to 18 to 21 mEq/L, maintaining the arterial pH in the range of 7.40 to 7.45. Alveolar to arterial oxygen tension difference is usually unchanged, and the mean arterial PO₂ is generally about 100 mm Hg.

2 How does pregnancy affect hemodynamics?

Cardiovascular physiology changes significantly during pregnancy, characterized by an increase in blood volume, an elevation in cardiac output, and a small decrease in blood pressure, resulting in a number of changes in the normal hemodynamic values in the third trimester (Table 83-1). In the supine position, the gravid uterus may produce significant mechanical obstruction of the inferior vena cava, reducing venous return and resulting in a decrease in cardiac output and hypotension. Maternal syncope or fetal distress may result. Supine hypotension syndrome may be avoided by positioning the patient on her left side, or at least with the right hip slightly elevated.

^{Table 83-1} Effect of Late Pregnancy on Pulmonary Artery Catheter Measurements

3 What factors affect oxygen delivery to the fetus?

Oxygen delivery to the fetus is determined by the maternal arterial oxygen content, uterine blood flow, and placental function. A number of factors may adversely affect blood flow to the uteroplacental vasculature, which is normally maximally vasodilated. A decrease in maternal cardiac output reduces fetal oxygenation. The maternal response to hypotension does not favor the uterus, and catecholamines (endogenous or exogenous) may aggravate fetal hypoxia by producing uterine vasoconstriction. Uterine blood flow may also be reduced by maternal alkalosis and during uterine contractions.

4 Are there any special concerns to be considered when inserting an endotracheal tube in a critically ill pregnant patient?

The upper airway in pregnancy may be edematous and friable because of the effects of estrogen and aggravated in the presence of preeclampsia because of excessive edema. The nasal route should be avoided, and a smaller endotracheal tube may be necessary. Because of the reduced functional residual capacity and increased oxygen consumption, hypoxemia will develop in a pregnant woman more rapidly than in nonpregnant, critically ill patients during intubation. Intubation should be carried out by the most skilled person available.

Note: The incidence of failed intubation is eight times higher in the obstetric population than in nonobstetric patients.

5 Describe the principles of management of severe preeclampsia

The most important aspect of management is the well-timed delivery of the fetus. Supportive treatment involves fluid management, control of hypertension, and prevention of seizures:

6 What are the clinical features of the HELLP syndrome?

The HELLP syndrome (i.e., hemolysis, elevated liver enzyme levels, and low platelet count) is a complication of preeclampsia characterized by multiorgan dysfunction. The diagnostic features are the presence of thrombocytopenia, elevated liver enzymes, and a microangiopathic hemolytic anemia. The patient may present with epigastric or right upper quadrant pain, nausea, and vomiting, with or without other features of preeclampsia. Significant hemorrhage may result from the thrombocytopenia. A rare but catastrophic consequence of HELLP syndrome is hepatic hemorrhage, manifesting with sudden shock or acute abdominal pain.

7 What is acute fatty liver of pregnancy?

This is an uncommon complication of pregnancy manifesting with acute fulminant hepatic failure during the third trimester. Increased awareness of this condition has resulted in earlier diagnosis, with milder liver disease and an improved outcome. The clinical presentation is with malaise, anorexia, and vomiting, followed by abdominal pain and jaundice. The patient deteriorates rapidly with acute liver failure manifested by coagulopathy, hemorrhage, renal failure, and encephalopathy. Management requires urgent delivery of the fetus and supportive therapy for fulminant hepatic failure.

8 How does amniotic fluid embolism present?

Amniotic fluid embolism is a rare but catastrophic obstetric complication usually associated with labor, delivery, or other uterine manipulations. The typical presentation is a sudden onset of severe dyspnea, hypoxemia, and cardiovascular collapse, which may be accompanied by seizures. The maternal presentation is accompanied or preceded by sudden fetal distress. A significant portion of patients die acutely within the first hour. Survivors commonly have a disseminated intravascular coagulopathy and acute respiratory distress syndrome. Management is supportive, and the prognosis for mother and fetus is poor.

9 What are the causes of acute respiratory failure in pregnancy?

The pregnancy-specific diseases (Box 83-1) include amniotic fluid embolism, pulmonary edema resulting from the use of tocolytic therapy or related to preeclampsia, or peripartum cardiomyopathy. Although pregnant patients may have diseases similar to those in nonpregnant patients, pregnancy may increase the risk for venous thromboembolism, acute asthmatic attacks, and gastric aspiration. Changes in immune function in pregnancy predispose to increased severity of influenza pneumonitis (particularly H1N1), varicella pneumonia, as well as coccidioidomycosis infections. Of interest is an association between the presence of pyelonephritis and the development of acute respiratory distress syndrome in pregnancy.

10 Does the management of pulmonary embolism differ in pregnant patients?

Investigation of suspected pulmonary embolism is similar to that in nonpregnant patients, beginning with duplex ultrasound. False-positive results may occur because of venous occlusion by the enlarged uterus. Ventilation-perfusion scanning and chest computed tomography angiogram can be carried out with a low risk for fetal radiation exposure. Unfractionated heparin and low-molecular-weight heparin are safe and effective in pregnancy. Warfarin is usually avoided because of the risk for embryopathy with first-trimester use and central nervous system abnormalities and bleeding risk with second- and third-trimester use. Thrombolysis has been used successfully during pregnancy and the postpartum period but should be limited to life-threatening situations.

11 What are the risks of radiologic procedures in pregnancy?

Estimated fetal radiation exposure varies from <0.01 rad (0.1 mGy) for a chest radiograph to about 2 to 5 rad (20-50 mGy) for pelvic computed tomography (Table 83-2). Abdominal shielding with lead and use of a well-collimated x-ray beam can effectively reduce exposure. The potential adverse effects of fetal exposure to radiation are oncogenicity, teratogenicity, and neurologic compromise. A twofold increased risk for childhood leukemia may occur with relatively low-dose radiation (2-5 rad). Teratogenicity is thought to require greater than 10 rad exposure; microcephaly and hydrocephaly have been described after exposure of 10 to 150 rad. Although radiation exposure in pregnancy carries definite risks, the likelihood of any adverse effect is about 0.1% per rad. The perception of risk by patients, family members, and physicians is often vastly higher than the actual risk.

^{Table 83-2} Estimated Fetal Radiation Exposure During Radiographic Studies with Appropriate Shielding

CT, Computed tomography.

12 How do the manifestations of severe trauma differ in pregnant patients?

Trauma is a common cause of morbidity and mortality in pregnancy. The increased blood volume allows the mother to tolerate moderate blood loss, but this higher volume of hemorrhage necessitates more rapid IV fluid replacement. Fetal or amniotic injury may cause maternal coagulopathy, which can exacerbate hemorrhage. Occult uterine or retroperitoneal hemorrhage always should be considered. Deceleration injuries may precipitate placental abruption, after 20 weeks gestation. In the third trimester, abdominal trauma usually involves the uterus. Other intraabdominal organs may be compressed in the upper abdomen, and injury in this area may cause significant organ damage. Physical signs of peritonism may be reduced because of stretching of the peritoneum. The bladder is at increased risk for injury as it extends above the pubis, and it should be remembered that some degree of ureteric dilation is normal in pregnancy. The fetus is at risk for morbidity resulting from maternal hypotension, direct injury, fetomaternal hemorrhage, or placental abruption.

13 Is management of cardiac arrest different for pregnant patients?

Management of cardiac arrest in pregnancy follows usual protocols with some modifications. Because cardiopulmonary resuscitation in the supine position may cause impaired venous return, the uterus should be manually displaced to the left, or alternatively a left lateral tilt position should be achieved with a firm wedge. IV access should be established above the diaphragm, and a difficult airway should be anticipated. No change in pharmacologic therapy is necessary, and drugs should not be withheld when clinically indicated. Consider calcium administration if the mother was receiving a magnesium sulfate infusion. Electrical defibrillation may be performed in pregnancy after removal of any fetal monitoring device. When initial attempts at resuscitation have failed, perimortem cesarean section should be considered if the fetus is at a viable gestation and no return of spontaneous circulation has occurred within 4 minutes. Ideally, delivery should occur within 5 minutes of cardiac arrest for optimal fetal outcome. Cesarean section has been reported to reverse aortocaval compression and allow successful resuscitation of both the mother and infant.

14 How is massive obstetric hemorrhage managed?

Supportive measures include adequate venous access, rapid volume replacement, and blood product support. A dilutional coagulopathy should be anticipated. Ultrasound allows assessment of the uterine cavity for retained placental fragments that necessitate uterine curettage. Uterine massage and intramuscular methylergonovine (which should be avoided in the presence of hypertension) are used for uterine atony. Oxytocin infusion is administered in a dose higher than that used for augmentation of labor (e.g., 20-40 units in 1000 mL normal saline solution at a rate up to 0.1 units/minute). Prostaglandin analogs are effective; carboprost tromethamine is given by intramuscular or intramyometrial injection in a dose of 0.25 mg, which may be repeated. Intrauterine balloon tamponade is increasingly being used as an early adjunctive intervention. Prohemostatic drugs such as tranexamic acid and recombinant factor VIIa may be used, but good evidence is lacking. If bleeding remains uncontrolled, more invasive approaches may be required. These include radiologic arterial embolization and surgical exploration to repair lacerations, to reduce blood flow by arterial ligation, or, if necessary, to remove the uterus.

15 Which cardiac lesions present problems in pregnancy?

The changes in cardiovascular physiology in pregnancy may result in decompensation in a patient with preexisting heart disease, because of the rise in cardiac output reaching a peak at about 28 weeks, 40% to 50% above baseline levels. Cardiac lesions limiting cardiac output (e.g., mitral stenosis, aortic stenosis) therefore present a significant risk for precipitating pulmonary edema or hypotension in the third trimester. This risk is particularly high during labor because of the tachycardia and volume shifts associated with delivery. Pulmonary hypertension is associated with significant morbidity and mortality because of the limitation of cardiac output and the inability to respond to postpartum fluid shifts. Congenital heart abnormalities are generally better tolerated in pregnancy unless complicated by pulmonary hypertension. Pregnancy may also predispose the patient to the development of cardiomyopathy.

16 Does termination of pregnancy improve the outcome of a critically ill mother?

An understanding of the physiologic effects of late pregnancy may suggest that delivery of the pregnant patient with respiratory failure will improve the mother’s condition. However, this has not been found to be correct; some improvement in oxygenation may occur, but without improvement in positive end-expiratory pressure requirements or compliance. If the fetus is at a viable gestation and is at risk because of severe maternal hypoxia, there may be a benefit from removing the fetus from the intrauterine environment. However, delivery is usually not appropriate solely in an attempt to improve maternal oxygenation or ventilation. Consultation by a neonatologist is essential to evaluate fetal risks or benefits, and obstetric indications should determine the mode of delivery. Although cesarean section allows more rapid delivery, the increased physiologic stress may be associated with a higher mortality in critically ill patients.