Physiologic Changes Of Pregnancy

A normal pregnancy creates complex changes in maternal anatomy and physiology, many of which begin as early as the first trimester and continue throughout its term. Significant alterations to the maternal reproductive, cardiovascular, respiratory, and gastrointestinal systems as well as maternal anatomy make the evaluation of an acutely injured pregnant patient more difficult. An understanding of these physiologic changes is essential to the appropriate diagnosis and management in such cases. These are discussed in detail in Chap 99, "Normal Pregnancy," but will be noted here in brief.

Maternal blood volume begins to expand at approximately week 10 of gestation and peaks at 45 to 50 percent increase from baseline at week 28, resulting in a state of hypervolemia. Red cell mass increases to a lesser extent, leading to the relative physiologic anemia of pregnancy. Cardiac output is increased by 1.0 to 1.5 L/min at week 10 of pregnancy and remains elevated until the end of pregnancy. Heart rate in the mother is generally increased by 10 to 20 beats per minute in the second trimester, accompanied by decreases in systolic and diastolic blood pressures of 10 to 15 mmHg.

These changes can be frequently misleading during maternal resuscitation in trauma and make clinical findings difficult to interpret. A pregnant woman may lose 30 to 35 percent of circulating blood volume before manifesting hypotension or clinical signs of shock. 2 Uterine arteries vasoconstrict, resulting in diminished fetal blood flow and tissue oxygenation before significant evidence of maternal hypovolemia.

After week 12 of gestation, the uterus becomes an intraabdominal organ, removing it from the relative protection of the maternal pelvis and making it more susceptible to direct injuries. The bladder also moves anteriorly into the abdomen in the third trimester of pregnancy, increasing its susceptibility to injury. Uterine blood flow may increase to upward of 600 mL/min, making severe maternal hemorrhage from uterine injury possible. The gravid uterus also causes passive stretching of the abdominal wall and peritoneum as it enlarges and may lead to diminished sensitivity to injury and irritation from intraperitoneal blood. 4 At or about week 20 of gestation, the expanding mass of the gravid uterus may lead to the supine hypotension syndrome in which venous return and cardiac output are diminished by compression of the maternal inferior vena cava in the supine position. The enlarging uterus may additionally cause engorgement of lower extremity and lower abdominal vessels, predisposing the patient to severe retroperitoneal hemorrhages in acute injuries to these areas.

As pregnancy progresses, the diaphragm is raised by as much as 4 cm and tidal volume increases by 40 percent as residual volume diminishes by 25 percent. Functional residual capacity is similarly decreased, and the compensatory increase in ventilation results typically in respiratory alkalosis. Serum pH is usually maintained at normal values by renal compensation. These changes may significantly impair the ability of a pregnant trauma patient to compensate for respiratory compromise.

The gastrointestinal tract demonstrates diminished motility, and there is delayed gastric emptying during pregnancy. This increases the likelihood of gastroesophageal reflux and the potential for aspiration from acute injuries as well as from resuscitative interventions, including endotracheal intubation. The small bowel is moved upward in the abdomen by the enlarging uterus, protecting the small bowel to some degree from lower abdominal injuries. It does, however, increase the chance of complex bowel injuries in penetrating trauma of the upper abdomen.4 The liver is typically unaffected by pregnancy, and the most common etiology of abdominal hemorrhage remains splenic injury, as in nonpregnant patients.

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