Malnourished patients should be reexamined regularly to assure abatement of deficiency symptoms following treatment. Weight should be measured daily in acute care patients receiving nutrition support since body weight is a simple, non-invasive monitor
of hydration status, and a reasonable predictor of body fat and caloric balance.
Repletion of protein and energy reserves is associated with improvement in functional abilities. Patient monitoring is enhanced by a subjective evaluation of grip strength, activity level, and physical endurance.
1. Basic Test Schedule
Laboratory tests are useful for monitoring individual patient responses to parenteral and enteral nutrition support. Laboratory tests are specifically used to assess metabolic state (response to injury or infection, inflammation, starvation-adapted, etc.)
, protein-energy balance, at-risk micronutrient status, fluid, electrolyte and acid-base balance, and organ function. Selection of laboratory tests and testing schedules necessarily depend on individual patient circumstances. A basic menu and schedule for
laboratory testing is provided in Table V.
Table V. Laboratory Test Recommendations for Patients Receiving Nutrition Therapy
| Parameter | ||
|---|---|---|
| PN and Tube Feeding | Baseline | Follow up |
| Mini panel (Na, K, Cl, CO2, Glu, BUN, Creat) | x | daily until stable |
| Albumin | x | weekly |
| Transthyretin | x | weekly |
| C-Reactive protein | x | as needed |
| PO4* | x | q o d until stable |
| Mg* | x | q o d until stable |
| Vitamin C | x | as needed |
| Zinc | x | as needed |
| 24 hour TUN | x | as needed |
| Parameter | ||
| PN only | Baseline | Follow up |
| Chemsticks | x | q 6 hr |
| iCa | x | q o d until stable |
| Alk Phos | x | weekly |
| AST | x | weekly |
| Tbili | x | weekly |
| Triglyceride | x | as needed |
| PT | x | as needed |
2. Nitrogen Balance
Nitrogen balance (NB) can be useful in determining individual protein requirements. NB is calculated as follows:
NB = Nitrogen Intake - Nitrogen Output
Nitrogen intake is estimated from protein intake where g nitrogen equals g intact protein/6.25 or g crystalline amino acids/6.0. Nitrogen output is assessed as 24-hr total urinary nitrogen + 2 g nitrogen to account for normal losses via feces, skin, et
c. One gram of nitrogen should be added to nitrogen output for each 500 mL of diarrhea, fistula, or gastric output (any collectable body fluid can be analyzed for total nitrogen content if extra-urinary losses are in question). To accurately assess NB a s
teady metabolic state needs to be achieved. Therefore, at least 3 days of steady nutrient intake are desirable prior to obtaining the TUN. Ideally, three consecutive 24 hour urine collections are analyzed and averaged for a best estimate of nitrogen outpu
t. Careful and complete urine collection and intake records are imperative.
Example NB calculation:
NB = (125/5.95) - (15 + 2 +2) = 21 - 19 = +2 g nitrogen
Nitrogen equilibrium (NB = 0) is the goal for weight maintenance in healthy individuals. A positive nitrogen balance of 2.4 g nitrogen per day is recommended for growth and other anabolic processes such as wound healing. It is unusual for patients that are critically ill (septic, MOF, burns or trauma) to achieve positive nitrogen balance.
3. Protein-Energy Balance Markers
Several plasma proteins of hepatic origin have been suggested as dynamic indices of protein-energy balance. During periods of inadequate dietary protein or energy, a reduction in hepatic synthesis and secretion of these proteins causes plasma levels to fa
ll. Reinstitution of an adequate diet induces protein synthesis, returning plasma concentrations to normal. Transthyretin (TTHY, previously known as prealbumin) is the currently preferred plasma protein for monitoring protein-energy balance. TTHY has a bi
ological half-life of 2 days allowing it to rapidly respond to recent changes in nutrient balance. Transferrin (half-life = 8-10 days) and retinol binding protein (half-life = 12 h) can also reflect protein-energy balance, but are confounded by other nutr
itional factors (transferrin levels increase in iron deficiency and
retinol binding protein levels decrease in vitamin A deficiency).
4. Evaluating Acid/Base Balance
The routine lab tests recommended for monitoring nutrition support provide information useful in the evaluation of acid-base status. A serum bicarbonate concentration is obtained with the "mini-panel" and a venous pH with ionized calcium. Venous
pH parallels arterial pH, provided the patient is not in shock or shunting blood from the arterial to the venous system. A mild decrease in serum bicarbonate (but >20 mEq/dL) or venous pH (but >7.30) does not merit an exhaustive diagnostic workup.
5. Vitamins and Minerals
A wide variety of laboratory tests for evaluating vitamin and mineral status
are available through the Department of Laboratory Medicine (see
AppendicesD and
H). In general, these tests are
used to confirm suspected nutrient deficiencies. Additionally, some
micronutrients that are involved in wound healing or immune function (e.g.
vitamin C, copper and zinc) may need to be monitored to assure the adequacy
of supplementation.
6. Liver Dysfunction
Liver dysfunction can occur in refeeding malnourished patients as well as in routine PN therapy. Liver function tests, therefore, must be monitored periodically. Abnormal results may indicate the need to modify PN prescriptions (decrease calories, alter l
ipid versus glucose intake) or to discontinue PN.
D. Short Gut Syndrome
Patients with an absence of the terminal ileum should be evaluated for fat soluble vitamin and fat malabsorption by measurement of either serum beta-carotene concentration and/or 72 hr quantitative fecal fat. Patients with resected ileum are at risk for v
itamin B12 malabsorption requiring i.m. or aerosol supplementation and periodic serum vitamin B12 monitoring.
E. Refeeding Syndrome
Starved or severely malnourished patients can undergo life-threatening fluid and electrolyte shifts following the initiation of aggressive nutritional support therapies. This phenomenon is known as the "refeeding syndrome" and can occur in patients receiv
ing either enteral or parenteral nutrition support.
Although not completely understood, the physiological basis of the "refeeding syndrome" is believed to stem from the following: 1) Carbohydrate repletion and insulin release enhance cellular uptake of glucose, phosphate, potassium and magnesium. Since total body stores of these minerals are depleted, blood levels fall. 2) Rapid expansion of the extracellular fluid volume occurs with carbohydrate refeeding and may predispose patients to fluid overload. 3) The reduction in cardiac mass and high energy phosphate reserves associated with malnutrition lead to cardiac i nsufficiency during fluid resuscitation. Alterations in cardiac function also occur as a result of severe hypophosphatemia, hypokalemia and hypomagnesmia. 4) Similarly, respiratory muscle, reduced in mass and ATP content by malnutrition, is unable to resp ond to the increased workload imposed by aggressive nutrition support leading to hypercarbia and in some cases respiratory failure. 5) Alterations in red blood cell shape and function occurs in hypophosphatemia which is believed to contribute to tissue hy poxia and increased respiratory drive. 6) Deficiency of B-vitamins, especially thiamin, are speculated to have a role in the refeeding syndrome since these vitamins are required in carbohydrate metabolism.
To avoid the development of the refeeding syndrome, nutrition support in patients at risk should be increased slowly while assuring adequate amounts of vitamins and minerals. In this situation, it is reasonable to start at 20 kcal/kg for the first thre e days and then increase to 25 kcal/kg. Carbohydrate in PN should be initiated at 2 mg/kg/minute or 150 to 200 mg/day. Nutrient can gradually be increased and be up to requirements by the end of week one. Organ function, fluid balance and serum electrolyt es (especially phosphorus, potassium and magnesium) need to be monitored daily during the first week and less often thereafter.