A 32-year-old woman who was 17 weeks pregnant presented to the emergency department complaining of abdominal pain, nausea, and vomiting. After physical examination, a presumptive diagnosis of appendicitis was made, and an emergency appendectomy was scheduled.
What is the incidence of nonobstetric surgery in pregnant patients?
The incidence of nonobstetric surgery during pregnancy is 0.3%–2%. There are approximately 4 million deliveries per year in the United States, which means that 80,000 pregnant women require surgery annually. This figure may be an underestimate because it does not account for surgery performed before clinical recognition of pregnancy. Appendectomy is the most common nonobstetric operation performed during pregnancy. However, almost every type of surgical procedure has been successfully performed in pregnant patients, including open heart procedures with cardiopulmonary bypass, neurosurgical procedures requiring hypotensive techniques and hypothermia, and liver transplantation.
What are the anesthetic concerns in a pregnant patient?
Anesthetizing a pregnant patient is one of the only times an anesthesiologist must consider two patients simultaneously. Maternal considerations result from the physiologic changes of pregnancy that affect almost every organ system ( Table 57-1 ). To provide safe anesthesia to a pregnant patient, one must not only understand the physiologic changes but also know when they occur during the gestational period and what impact they have on the administration of anesthesia. Fetal concerns include the possible teratogenic effects of anesthetic agents, avoidance of intrauterine fetal asphyxia, and prevention of premature labor.
|Minute ventilation||Increases by 50%|
|Tidal volume||Increases by 40%|
|Respiratory rate||Increases by 10%|
|Oxygen consumption||Increases by 20%|
|PaO 2||Increases by 10 mm Hg|
|Dead space||No change|
|Alveolar ventilation||Increases by 70%|
|PaCO 2||Decreases by 10 mm Hg|
|Arterial pH||No change|
|Serum HCO 3 −||Decreases by 4 mEq/L|
|Functional residual capacity||Decreases by 20%|
|Expiratory reserve volume||Decreases by 20%|
|Residual volume||Decreases by 20%|
|Vital capacity||No change|
|Cardiac output||Increases by 30%–40%|
|Heart rate||Increases by 15%|
|Stroke volume||Increases by 30%|
|Total peripheral resistance||Decreases by 15%|
|Femoral venous pressure||Increases by 15%|
|Central venous pressure||No change|
|Systolic blood pressure||Decreases by 0%–15%|
|Diastolic blood pressure||Decreases by 10%–20%|
|Intravascular volume||Increases by 35%|
|Plasma volume||Increases by 45%|
|Red blood cell volume||Increases by 20%|
|Stomach position||More cephalad and horizontal|
|Pseudocholinesterase||Decreases by 20%|
|Platelet count||Decreases by 20%|
|Renal blood flow||Increases|
|Glomerular filtration rate||Increases|
|Serum creatinine and BUN||Decrease|
|MAC||Decreases by 40%|
Describe the physiologic changes during pregnancy and the impact they have on anesthesia.
As a result of increased progesterone levels during the first trimester, minute ventilation is increased by almost 50% and remains at this level for the remainder of the pregnancy. The increase in minute ventilation leads to a decrease in arterial carbon dioxide tension (PaCO 2 ) to approximately 30 mm Hg. Arterial pH remains unchanged because of a compensatory increase in renal excretion of bicarbonate ions. At term, alveolar ventilation is increased by 70% because anatomic dead space does not change significantly during pregnancy. After the fifth month of pregnancy, the functional residual capacity, expiratory reserve volume, and residual volume all are decreased by about 20% because of the gravid uterus pushing on the diaphragm. Vital capacity is not appreciably changed from prepregnancy levels.
Increased alveolar ventilation and decreased functional residual capacity lead to a more rapid uptake and excretion of inhaled anesthetics. The decrease in functional r esidual capacity in conjunction with increases in cardiac output, metabolic rate, and oxygen consumption make the pregnant patient more susceptible to arterial hypoxemia during periods of apnea or airway obstruction.
Edema, weight gain, and increase in breast size may make tracheal intubation technically difficult. An array of laryngoscope blades and handles and other emergency airway management equipment should be available. Capillary engorgement of the mucosal lining of the upper airway accompanies pregnancy. Extreme care is mandated during manipulation of the airway, and a smaller than normal tracheal tube needs to be used. The use of a nasal airway and nasotracheal intubation should be avoided.
Cardiac output is increased by 30%–40% during the first trimester. This increase in cardiac output is primarily related to an increase in stroke volume (30%) and secondarily related to an increase in heart rate (15%). Cardiac output increases slightly further during the second trimester, and this increase lasts throughout the pregnancy.
Blood pressure normally decreases during pregnancy because of a 15% decrease in systemic vascular resistance. Near term, 10%–15% of patients have a dramatic reduction in blood pressure in the supine position, often associated with diaphoresis, nausea, vomiting, pallor, and changes in cerebration. This condition is known as supine hypotensive syndrome and is caused by compression of the inferior vena cava and aorta by the gravid uterus. Other manifestations of the syndrome are decreases in renal and uteroplacental blood flow from compression of the aorta.
Intravascular volume is increased by 35% during pregnancy. Because plasma volume increases by a greater percentage than red blood cell volume (45% and 20%), there is a relative anemia during pregnancy. Nevertheless, a hemoglobin concentration <11 g/dL is considered abnormal.
Increases in cardiac output hasten the speed of intravenous induction of anesthesia. Also, it is critical to displace the uterus 15–30 degrees toward the left to avoid supine hypotensive syndrome and hypotension.
Traditionally, gastric emptying was considered prolonged in a pregnant woman by the end of the first trimester. This delayed emptying was thought to be related to progesterone and mechanical anatomic changes as the stomach is displaced upward by the enlarging uterus. More recent data using acetaminophen absorption ( , 2002) do not support this conclusion, and gastric emptying may not be prolonged until the woman is in active labor. Although gastric emptying is not delayed, gastric pressure increases secondary to the gravid uterus, gastric secretions are more acidic, and lower esophageal sphincter tone is impaired.
Although studies suggest that pregnant women are not at increased risk for pulmonary aspiration, any woman with symptoms of acid reflux such as heartburn, a common finding in pregnancy, should be considered at risk for pulmonary aspiration. If the patient is considered at risk, a nonparticulate antacid, H 2 receptor blocker, and metoclopramide should be used to decrease the acidity and volume of the gastric contents, and general anesthesia with tracheal intubation should be conducted with a rapid-sequence induction, cricoid pressure, and tracheal intubation.
Tests of liver function (aspartate aminotransferase, lactic acid dehydrogenase, alkaline phosphatase, and cholesterol) are commonly increased during pregnancy. These increases do not indicate abnormal liver function. Pseudocholinesterase activity declines 20% during the first trimester and remains stable during the remainder of the pregnancy.
Decreased pseudocholinesterase activity could prolong the effect of succinylcholine but apnea is rarely a problem after a standard dose used for tracheal intubation. Similarly, prolonged activity of ester-linked local anesthetics has not been a problem.
Hematologic system and blood constituents
Pregnancy does not significantly alter the lymphocyte count, but lymphocyte function is depressed, which can decrease maternal resistance to infection. The risk of upper respiratory infections is increased, which may complicate airway management during general anesthesia.
The platelet count decreases by about 20% during pregnancy, but this is not usually of clinical significance. Circulating levels of coagulation factors increase significantly during pregnancy leading to the hypercoagulable state of pregnancy.
The increased risk of upper airway infections may complicate airway management during general anesthesia. Increased coagulability may predispose a pregnant patient to thromboembolic events including pulmonary embolism. Venous thromboembolic prophylaxis should be considered in the perioperative period.
Renal blood flow and glomerular filtration rate are increased during the first trimester, leading to an increase in creatinine clearance and a decrease in serum creatinine. During the third trimester, renal blood flow and glomerular filtration rate decrease toward prepregnant levels because of compression of the aorta by the enlarging uterus. As a result of progesterone, renal calyces and pelves dilate during the third month of pregnancy. During the third trimester, they dilate further because of ureteral compression. This dilation may lead to stasis and urinary tract infections.
Bladder catheterization, which may predispose a patient to urinary tract infections, should be avoided, if possible. Although not generally related to the anesthesia, care should be taken not to overhydrate the patient to try to obviate the need for catheterization.
Central nervous system
The minimum alveolar concentration for inhaled anesthetics is decreased by 40% during pregnancy; this is related to a progesterone and endorphin effect. Compression of the inferior vena cava by the gravid uterus leads to dilation of the azygos system and the epidural veins. Epidural venous engorgement decreases the size of the epidural and intrathecal spaces.
The decrease in minimum alveolar concentration along with an increase in alveolar ventilation places a pregnant patient at risk for anesthetic overdose. The decreased size of the epidural and intrathecal spaces as a result of epidural venous engorgement explains why the doses of drugs used during a major conduction block must be decreased. An alternative explanation is that progesterone may increase the sensitivity of nerve cells to local anesthetics because neuraxial drug requirements decrease before uterine enlargement.