Page 530 - ACCCN's Critical Care Nursing

Gastrointestinal, Liver and Nutritional Alterations 507

was described in relation to symptoms, such as gastro-
TABLE 19.1 Enzymes required for digestion intestinal bleeding, mechanical obstruction, and pancre-
17
18
of nutrients 2 atitis resulting from ischaemia. However, the presence
of covert ischaemia has resulted in a heightened interest
Location Enzymes Target substance in the prevention and early detection of gastrointestinal
ischaemia in the critically ill, in an attempt to minimise
Oral cavity Salivary amylase Starch and
(ptyalin) glycogen ischaemia-related dysfunction.
Bromelain Protein
Stomach Pepsin Proteins Gastrointestinal Mucosal Hypoperfusion
Gelatinase Proteoglycans in The gastrointestinal system is particularly susceptible to
meat (gelatine
and collagen) alterations in regional blood flow and oxygen delivery
Gastric amylase Starch because it has a higher critical oxygen delivery (DO 2 )
Gastric lipase Triglyceride than the rest of the body. Splanchnic vasoconstriction is
Chymosin Milk also proportionally greater than other vascular beds and
Pancreas Trypsin, Proteins the countercurrent O 2 exchange between vessels within
chymotrypsin, the villi further contribute to decreased regional oxygen
carboxypeptidase, delivery. 5
elasatases
Pancreatic lipase Triglycerides During shock states, decreased blood flow from vasocon-
Pancreatic amylase Carbohydrates striction occurs in this region first. It is the last place to
19
be restored following successful resuscitation. In shock
Small intestine Sucrase Sucrose states, the gastrointestinal system attempts to maintain
Lactase Lactose
Maltase Maltose (into 2 adequate cellular oxygenation by increasing the amount
molecules of of oxygen extracted from the blood. This increase in
glucose) oxygen extraction may prevent serious compromise of
Isomaltase Maltose into tissue oxygenation even in the presence of reduced oxygen
isomaltose delivery. 20
Intestinal lipase Fatty acid

Practice tip
The gastrointestinal tract also plays a role in immunity. It Remember, assessment of arterial blood pressure, heart rate
has a variety of mechanisms in place that prevent the and urine output provides information about the haemody-
movement of substances (other than nutrients, water and namic and oxygenation status of the whole body. A reduction
electrolytes) into the systemic circulation (see Table 19.2). in regional perfusion and oxygenation may occur despite con-
In the setting of critical illness, where gastrointestinal ventional clinical assessment findings being normal.
hypoperfusion may be present, these protective functions
may be diminished, so it is essential to understand the
alterations in normal gastrointestinal physiology that During periods of ischaemia and hypoxia, oxygen free-
occur during critical illness. radicals are generated as byproducts of anaerobic meta-
bolism. With successful resuscitation of the gastrointestinal
tract, blood flow and oxygen delivery are restored but the
ALTERATIONS TO NORMAL oxygen free-radicals are liberated, contributing to the
GASTROINTESTINAL PHYSIOLOGY IN microvascular and mucosal changes characteristic of isch-
CRITICAL ILLNESS aemia and reperfusion of the gut mucosa. 21
During critical illness, the digestion and absorption of
nutrients may be altered. Gastric acid production is com- Consequences of Gastrointestinal
monly thought to increase in critical illness, although Hypoperfusion
evidence suggests that many critically ill patients do not
hypersecrete gastric acid with increased gastric pH being The consequences of gastrointestinal hypoperfusion are
13
observed in some critically ill patients, even in the absence significant, and include disruption of the physical barrier
of pharmacological inhibition of gastric acid secretion. 14,15 to pathogens; disruption of chemical control of bacterial
The ability of the small intestine to absorb nutrients can overgrowth; decreased peristalsis; and reduced immuno-
be impaired during critical illness, although most criti- logical activities of gastrointestinal-associated lymphoid
16
cally ill patients appear to be able to tolerate enteral tissue. In health, all of these mechanisms work efficiently
nutrition, making the clinical significance of impaired to contain bacteria within the gastrointestinal tract.
absorption unclear. During critical illness, however, reduced oxygenation
contributes to decreased cellular function and failure of
Some alterations to normal gastrointestinal physiology in the protective mechanisms described in Table 19.2. Con-
critical illness relate to hypoperfusion and decreased oxy- sequently, bacterial proliferation and translocation from
genation in this area and have high metabolic demands. the gastrointestinal tract to the systemic circulation may
Historically, gastrointestinal dysfunction in critical illness occur. 22