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|Fluid and Electrolyte Balance: A Must With Nutrition Support|
Kalman E. Holdy, M.D.
Medical Director of Clinical Nutrition, Sharp Memorial Hospital
A major challenge during parenteral and enteral nutrition is to maintain fluid and electrolyte balance (FEB). This article will discuss the basics of normal fluid and electrolyte physiology, alterations which occur with bowel dysfunction, and tips to maintain FEB for patients with short bowel syndrome (SBS).
Normal Fluid and Electrolyte Physiology
Total body water composes about 60 percent of body weight. A variety of key electrolytes are dissolved in body fluids to maintain organ function and fluid balance. The gastrointestinal (GI) tract, the kidneys and the endocrine system (pituitary-thyroid-adrenals) orchestrate precise FEB. In addition, the respiratory system helps to maintain the body’s acid base balance.
In the average adult, intracellular fluid is about 40 percent of body weight and extracellular fluid is about 20 percent of body weight. The electrolytes within these fluid compartments are strikingly different. Within the extracellular fluid, sodium chloride predominates, while potassium with phosphates and negatively charged proteins is the major intracellular electrolyte. Sodium and potassium gradients are essential for muscle and nerve function. Numerous other electrolytes, including bicarbonate, magnesium, calcium, etc., are involved with a variety of essential metabolic functions.
The kidney and endocrine system adjust urine volume and composition to maintain FEB with wide changes in GI function, type of fluid consumed, or metabolic demands. To clear metabolic wastes, the kidneys must excrete a minimum of 700 ml of urine per day. The amount of urine excreted each day is directly proportional to the amount of fluid consumed and inversely related to the amount of fluid lost through sweat and the GI tract. The kidneys very efficiently conserve sodium so that an adult needs as little as 30 mg of daily sodium intake. However, potassium losses through the urine are unavoidable, so at least 40 mEq of potassium must be consumed daily. Normal daily metabolism produces a large amount of acid which is cleared by the lungs and kidneys. Dehydration, especially from increased GI losses of bicarbonate, complicates acid clearance. The resulting acid accumulation contributes to a variety of metabolic disturbances, including bone loss and impaired protein formation.
The normal GI tract processes about 9 liters of fluid each day: approximately 2 to 3 liters of fluid are consumed orally, and about 6 to 7 liters of fluid are secreted by various parts of the GI tract. As this 9 liters passes along a normal GI tract, almost all of it is absorbed: the upper small bowel (duodenum and jejunum) absorbs 4 to 5 liters; the ileum, 2 to 3 liters; and the colon, 1 to 2 liters. Only 100 to 200 ml of this fluid is lost in the stool.
Fluid and Electrolyte Disorders
Fluid and electrolyte balance is upset when there is inadequate fluid and electrolyte intake, or dysfunction of key FEB organs: the GI tract, kidneys, or endocrine system. For example, with various forms of short bowel syndrome the normal 200 ml of daily GI loss can exceed 5 liters and be accompanied by large electrolyte losses! The kidneys cannot maintain FEB under such circumstances. A variety of medications such as diuretics or cathartics (sorbitol, magnesium, caffeine, etc.) may also contribute to fluid and electrolyte imbalance.
Sodium losses are manifested by dehydration and muscle weakness. Potassium losses result in muscle weakness and cardiac arrhythmias. If more water is lost than sodium, hypernatremia occurs which may lead to confusion and impaired consciousness.
Dehydration produces symptoms and can be expressed as a fraction of total body water deficit. A 2 percent (1 liter) fluid loss is associated with thirst and increased drinking, and a decrease in tearing. In addition, the mouth and tongue become increasingly dry, and the skin looses its elasticity. With 5 percent or greater fluid loss, the eyes become sunken. It is not unusual for a person to experience up to 5 percent (2 liters) dehydration from time to time, but it is rare for a person to lose more than this. Very severe dehydration is life threatening and occurs when 10 percent or more (about 4 liters) body water is lost. When this level of dehydration is approached, the symptoms of dehydration mentioned above increase, excessive drinking finally stops, the individual becomes lethargic and his/her level of consciousness decreases.
Urine flow begins to decrease as soon as fluid intake lessens or fluid losses increase. Therefore, measuring the amount of urine flow in 12 or 24 hours is an excellent measure of adequate hydration. The volume can be measured at home and should be at least 1500 ml in a 24 hour period for most adults. Much less than this suggests some degree of dehydration.
The fluids lost by an abnormal GI tract depend on the actual disease process, the part of the bowel involved, and how long the bowel has recuperated from the initial insult. Loss of fluid may be due to infection such as infectious diarrhea (viral, bacterial or parasitic), inflammation (Crohn’s disease), loss of bowel length (short bowel syndrome) or certain drugs (magnesium, drugs suspensions with sorbitol). Loss of the upper small bowel (jejunum) is better tolerated than loss of the lower small bowel (ileum), because overtime the ileum can assume much of the work done by the jejunum; however, the jejunum can never assume the specialized functions of the ileum in respect to B12 and active bile salt absorption. The more small intestine that is lost, the more important becomes the fluid absorptive ability of the colon. Table 1 summarizes the types of short bowel syndrome and likely modes of nutrition and fluid management. It is amazing how resilient the bowel is and how much functional recovery occurs over time. Therefore, efforts to promote bowel adaptation should continue even with persistent short bowel syndrome.
Tips to Maintain FEB
Fluid and electrolyte balance can be maintained with intravenous (IV) treatment and/or with oral and tube enteral methods. In experienced hands, IV hydration is precise and predictable. You estimate losses, measure electrolytes, develop a treatment plan and infuse the solution. What you infuse, is what you get! The drawbacks of IV hydration are that it requires IV access and is expensive. Further, the use of indwelling catheters can lead to serious complications such as infection.
The oral/tube enteral method is much more a “try and see” method and success improves with repeated attempts. Success depends upon the exact part of the available GI tract and the composition of the fluid used. The composition of fluid and amount given is based not only needs but on how the GI tract is able to process the fluid. Table 1 gives examples of short bowel syndrome likely to tolerate oral/enteral hydration. If your first attempts at oral rehydration don’t meet with success, try again. Over time normal bowel adaptation may pave the way for future success.
IV=Intravenous Fluids; PN=Parenteral Nutrition; ORT=Oral Rehydration Therapy. Adopted from S. Sial, T. Kousayer, S. Klein, Nutrition, p 39, v10, 1994.
Oral/enteral fluid replacement has been advanced by knowledge gained from rehydration treatment of victims of infectious diarrheal epidemics in third world countries. The term applied is “oral rehydration therapy.” As long as there is sufficient functioning GI tract to absorb fluid, oral rehydration therapy can be accomplished despite ongoing diarrheal losses.
Oral rehydration therapy solutions are specifically designed to promote water and sodium absorption. Water by itself is not effective because the GI tract requires sodium, sugar and/or amino acids to absorb the water. In fact, water alone may cause more loss. “Sport drinks” (see Table 2) promote water absorption by providing sodium and sugar, but in the face of major GI losses, their sodium content is too low for them to be used for oral rehydration therapy. Water containing appropriate sodium, sugar and amino acids must be given in specific amounts for optimal absorption. The exact best composition is still unsettled but the amount of sodium is very important. Simple ingredients available in the grocery store can be used. For example, the World Health Organization (WHO) formula can be mixed using 1 liter of water, 3/4 tsp. table salt, 1/2 tsp. baking soda, 1 cup orange juice, 4 Tbs. table sugar or 2 Tbs. honey; sugar free flavoring may be added to make this salty drink more palatable. (Alpers, Manual of Nutritional Therapeutics, 1995). The WHO formula and several other hydration products are available commercially (see Table 2). However, before choosing a commercial hydration product, be sure to check the sodium concentration. This should approach or exceed 90 mEq/L. The sugar concentration should exceed 20 gm sugar/L, preferably around 40 gm/L. Rice-based formulas may have additional absorption advantages because they contain amino acids in addition to sugar and sodium.
Table 2. Examples of commercially available hydration solutions.
Adopted from Alpers, et. al. Manual of Nutritional Therapeutics, p. 201; and Chan & Klein in Enteral and Tube Feeding, ed. Robbeaue and Rolandelli, p. 582, 1997. *Available from Jiannas Brothers Packaging Co. (816) 421-2880.
While intravenous hydration and electrolyte treatment is a critical aspect of acute care medicine and indispensable to parenteral nutrition, oral rehydration therapy is emerging as an integral part of the long term management of many patients with short bowel syndrome since it may cut back on ostomy output and reduce IV replacement. In fact, only patients with extreme SBS (less than 30 cm) may not tolerate such an approach.