Physiology of normal pregnancy
Section snippets
General changes
Normal values of routine laboratory tests differ for gravid and nonpregnant states. Pregnant women adapt quickly to the gravid state because of changes in hormones such as human chorionic gonadotropin and progesterone. In general, pregnancy is characterized by progesterone-mediated hyperemia and edema of mucosal surfaces. This change is evident in the nasopharynx and oropharynx. Pregnant women tend to have more nasal congestion. Accordingly, endotracheal and nasogastric tube size should be
Respiratory changes
Spirometry remains normal throughout pregnancy [3]. Flow volume loops, and peak flows remain unchanged. Total lung capacity decreases by about 4% to 5% [4]. This change mostly is caused by the upward displacement of the diaphragm. Functional residual capacity (FRC) decreases by 20% because of decreases in expiratory reserve volume and residual volume [1], [5]. Diffusion capacity may remain the same or increase slightly in early pregnancy and subsequently return to normal values [1]. These
Cardiac changes
Maternal cardiac output begins to increase at about 6 weeks' gestation. Maternal blood volume increases progressively during pregnancy by about 2 L, or 30% to 50% more than the volume during the nongravid state [7]. Maternal red cell mass increases only 20% to 30%, which results in hemodilution and the relative anemia of pregnancy. The increase in blood volume of 1000 to 1500 mL partially offsets peripartum blood loss. The average blood loss of 0.6 L after an uncomplicated vaginal delivery and
Renal changes
By 16 weeks' gestation, the glomerular filtration rate increases by 50% and remains elevated throughout pregnancy [11]. Creatinine clearance also increases, resulting in lower levels of serum creatinine, blood urea nitrogen, and uric acid. Plasma levels of creatinine and blood urea nitrogen that exceed 0.8 mg/100 mL and 14 mg/100 mL, respectively, may indicate renal impairment [11]. Preexisting renal disease portends increased risks for premature delivery and for worsening renal function [12].
Gastrointestinal changes
Gastroesophageal reflux is a common symptom in most pregnant women. Progesterone causes smooth muscle relaxation and results in a decrease in lower esophageal sphincter pressure, beginning in the first trimester. As abdominal girth increases, the stomach is displaced, causing a further decrease in the effectiveness of the sphincter. Pregnant women always should be considered at high risk for aspiration.
In terms of the liver, increased plasma volume leads to hypoalbuminema. At the end of the
Fetal physiology
The care of pregnant patients in the ICU requires a formal understanding of the uteroplacental unit. The placenta serves the following three main functions: (1) respiratory and gas exchange, (2) nutrition for the fetus, and (3) waste elimination. The maternal and fetal circulations interact by way of a concurrent exchange mechanism (Fig. 2). Maternal oxygen delivery to the placenta is affected by uterine artery blood flow, oxygen content of the uterine artery blood, and hemoglobin concentration
Summary
The care of the critically ill pregnant patients requires a multidisciplinary approach between the obstetrician, critical care physician, and labor and delivery and ICU nurses. Knowledge of normal physiology in pregnancy is important in understanding the pathophysiologic states that are discussed in other articles in this issue. ICU physicians must consider the mother and the fetus in diagnostic and therapeutic interventions.
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