Electrolytes, Trace Elements, and Vitamins
In patients receiving nutritional support, levels of electrolytes and trace elements should be adjusted to fit the clinical circumstances [see Table 2]. Carbohydrate feeding induces sodium retention, resulting in refeeding edema. In malnourished patients, great care should be taken to prevent salt and water overload.
Body potassium is disproportionately reduced relative to nitrogen in malnourished patients. Positive nitrogen balance does not occur unless potassium, phosphorus, and magnesium are given.58,59 During enteral and parenteral nutrition, serum phosphorus may drop precipitously and cause dangerous neurologic symptoms.
Micronutrients comprise vitamins and trace elements. The former are complex organic compounds; the latter are inorganic elements. Trace elements important to nutritional support include zinc, copper, chromium, and selenium. Diarrhea increases zinc requirements markedly and copper requirements modestly [see Table 2]. Oral chromium requirements have not been precisely determined, but deficiency occurs in patients receiving TPN; in one of my patients, the daily chromium needs were increased to 10 to 20 ^g. Patients receiving parenteral nutrition may develop selenium deficiency, with muscle pains and car-diomyopathy. Increased losses of selenium can occur from the GI tract and from wounds. The recommended dose of selenium for stable patients is 40 ^g/day. Patients depleted of selenium may require as much a 120 ^g/day to regain normal levels.
The current recommendations for vitamins [see Table 3] specify the amounts required to maintain normal plasma or blood levels in patients on long-term home parenteral nutrition. There are no clearly defined recommendations for critically sick or septic patients.
Home parenteral nutrition
Patients with intestinal failure from a short bowel, chronic bowel obstruction, radiation enteritis, or untreatable malabsorption can be nourished by parenteral nutrition given at home. Arteriovenous shunts were initially used for long-term venous access in these patients, but success was limited because of clotting or disruption of the shunt. Long-term success has been achieved with a tunneled silicone rubber catheter or an implanted reservoir. Premixed nutrients are infused overnight. The catheter is then disconnected and a heparin lock applied, leaving the patient free to attend to daily activities. We have used home parenteral nutrition for more than 20 years in two patients with total jejunoileal resection; one continues to receive it after 30 years. Survival of patients with short bowel from treatment for Crohn disease or pseudo-obstruction is excellent. Home parenteral nutrition increases quality-adjusted years of life in these patients and is cost-effective. On the other hand, mean survival in AIDS patients or those with metastatic cancer who receive home parenteral nutrition is about 3 months. There is no evidence that home parenteral nutrition prolongs their survival or enhances their quality of life. Trials are urgently required to justify the use of home parenteral nutrition in terminal cancer and AIDS.
Table 3 Recommendations for Vitamins in Adults on Total Parenteral Nutrition
|
Vitamin |
Recommended Daily Dose |
|
A |
3,300 IU |
|
D2 |
200 IU |
|
E |
10 IU |
|
K1 |
150 mg |
|
Ascorbate |
200 mg |
|
Thiamin |
6 mg |
|
Riboflavin |
3.6 mg |
|
Pyridoxine |
6 mg |
|
Niacin |
40 mg |
|
Pantothenate |
15 mg |
|
Biotin |
 |
|
Folate |
 |
|
Cobalamin |
 |
Complications of Long-term Home Parenteral Nutrition
At the start of nutritional support, patients are vulnerable to complications related to venous and enteral access and to metabolic complications. Careful and frequent monitoring and adjusting of nutrient intake will prevent these complications. Over the longer term, patients receiving TPN are vulnerable to three organ-specific complications: hepatic disease, bone disease, and gallstones.
Hepatic disease The most serious form of hepatic disease related to TPN is chronic cholestasis with fibrosis. This condition is most common in patients with a very short bowel. The exact cause is unknown, but absorption of endotoxin or alteration in bile salts by bacterial dehydroxylation are possible factors. Successful treatment with metronidazole and with ur-sodeoxycholic acid has been reported. In some patients, carni-tine infusions have corrected cholestasis.
Bone disease Bone loss during long-term TPN is a complex issue. In a prospective longitudinal study, patients were noted to have a high bone turnover before the institution of home parenteral nutrition, but during TPN this changed to os-teomalacia and slow bone turnover. This process has been attributed to aluminum toxicity but occurs in its absence61 and seems to respond to withdrawal of vitamin D from the TPN formula. In a prospective 4-year study of patients on home par-enteral nutrition, withdrawal of vitamin D increased spinal bone mass.62 On the other hand, patients on home parenteral nutrition can lose bone mass as a result of factors such as active inflammatory bowel disease, corticosteroid therapy, and inactivity. Some clinicians are treating reduced bone mineral density in these patients with intravenous bisphosphonates such as pamidronate and clodronate (the latter is not available in the United States). Although there are no controlled trials of bis-phosphonates in patients receiving home parenteral nutrition, there are anecdotal reports of improvement of bone mass with this therapy.
Gallstones The short bowel state results in bile salt deficiency and increased biliary cholesterol secretion. In addition, sludge composed of bilirubin and calcium forms in the gallbladder. Consequently, the incidence of gallstones is high in these patients. These stones are mixed cholesterol and pigment.
