THE FETUS

The circulations of the mother and fetus amalgamate in the placenta; this organ has a variety of functions, including endocrine support of pregnancy and transport of nutrients. The pharmacologic and physiologic interactions at this site can be explained in terms of the maternal-placental-fetal unit. This model, as seen in Figure 58-2 , can be compartmentalized into maternal, placental, and fetal components. The following section will outline the placental and fetal factors.

The placenta is composed of both maternal and fetal tissues that consist of a basal and chorionic plate. In its simplest description, it can be viewed as a semipermeable membrane that provides an interface for the maternal and fetal circulation. The intervillous space separates the plates and is subdivided by decidual tissue. Chorionic villi and spiral arteries protrude into this intervillous space. Maternal blood flows into the intervillous space from the spiral artery while placental transfer from the mother to the fetus occurs. Approximately 80% of the uterine blood flow passes through the intervillous space.

Figure 58-2 The maternal-placental-fetal unit. A, Maternal component. B, Placental component. C, Fetal component. (From Birnbach DJ [ed]: Ostheimer's Manual of Obstetric Anesthesia, 3rd ed. New York, Churchill Livingstone, 2000, p 45.)

2312

Figure 58-3 The fetal circulation demonstrating the major blood flow patterns and oxygen saturation values (circled numbers highlight percent saturation). Ao, aorta; DA, ductus arteriosus; DV, ductus venosus; IVC, inferior vena cava; Li, liver; Lu, lung; P, placenta; PA, pulmonary artery; PV, pulmonary vein; RA and LA, right and left atria; RHV and LHV, right and left hepatic veins; RV and LV, right and left ventricles; SVC, superior vena cava; UA, umbilical artery; UV, umbilical vein. (From Birnbach DJ, Gatt SP, Datta S [eds]: Textbook of Obstetric Anesthesia. New York, Churchill Livingstone, 2000, p 51.)

Approximately 40% to 50% of fetal cardiac output goes to the placenta, and a similar amount returns to the heart through the umbilical vein ( Fig. 58-3 ). Fetal blood enters the placenta through the two umbilical arteries, which arise from the internal iliac arteries. These arteries subdivide and eventually form umbilical capillaries that traverse the chorionic villi. Fetal blood flow is approximately 75 mL/kg/min, a rate far less than maternal flow. Although fetal and parturient pressures are uneven, placental transfer occurs rapidly for most drugs.

The umbilical-placental circulation is regulated by physiologic reflex changes and the neuroendocrine axis. Numerous substances, including prostaglandins, endorphins, catecholamines, and vasopressin, all play roles in fine-tuning of umbilical-placental perfusion.