GASTROINTESTINAL SYSTEM
GI problems in the ICU include organ dysfunction and organ failure
from acquired disease, as well as from congenital anatomic malformations and dysfunction.
In addition, delivery of adequate nutrition is perhaps the most critical concern
in the care of a critically ill patient.
Structural and Functional Development of the Intestine
An understanding of fetal midgut development makes it easier to
understand a number of severe congenital anomalies. Although the intestine begins
as a hollow tube that is occluded by rapidly growing epithelial cells by 7 to 10
weeks' gestation, this central lumen is later reconstituted when vacuoles within
the epithelial cells coalesce. Some of the neonatal intestinal atresias result from
abnormalities of this recanalization process. At 3 to 10 weeks' gestation, the midgut
lies outside the abdominal cavity, with only the hindgut fixed in the left side of
the abdomen. The gut rotates 270 degrees counterclockwise and reenters the abdominal
cavity at 10 weeks' gestation. If the midgut fails to migrate back into the abdominal
cavity, an omphalocele results. Abnormalities in midgut rotation result in abnormal
intra-abdominal relationships, the most important being malrotation volvulus of the
intestine.[265]
Development of the Liver
The liver begins as an outgrowth of the foregut ectoderm at approximately
3 weeks' gestation. During fetal life, the liver is relatively large in comparison
to the adult liver. Although the fetus relies on the maternal liver and placenta
for detoxification and excretory function in utero, the fetal liver is necessary
for both prenatal and postnatal survival. As early as 10 to 12 weeks' gestation,
the fetal liver is involved in glucose regulation, protein synthesis, lipid synthesis,
and some drug metabolism.
Fetal stored hepatic glycogen is approximately three times that
of the adult, but it is nearly completely released within several hours of birth
to compensate for interruption of the placental supply of nutrients.[266]
It takes several weeks for the newborn to restore the liver's stores of glycogen,
thus putting the infant at risk for hypoglycemia if any stressful event occurs during
this vulnerable period.
