Electrolytes

® Electrolytes are essential for many metabolic and homeostatic functions, including enzymatic and biochemical reactions, maintenance of cell membrane structure and function, neurotransmission, hormone function, muscle contraction, cardiovascular function, bone composition, and fluid homeostasis. The causes of electrolyte abnormalities in patients receiving PN may be multifactorial, including altered absorption and distribution; excessive or inadequate intake; altered hormonal, neurologic, and homeostatic mechanisms; altered excretion via GI and renal losses; changes in fluid status and fluid shifts; and medications. PN should not be used to treat acute electrolyte abnormalities, but electrolytes in PN should be adjusted to meet maintenance requirements and to minimize worsening of underlying electrolyte disturbances.

Electrolytes that are included routinely in PN admixtures include sodium, potassium, phosphorus (as phosphate), calcium, magnesium, chloride, and acetate. When determining electrolytes in PN admixtures, the patient's kidney function always must be taken into account. Typical daily electrolyte maintenance requirements for adults with normal kidney function are listed in Table 100-3.

* For elderly patients (e.g., greater than 60 years old), use 15 mL/kg for every kilogram above 20 kg.

Table 100-3 Approximate Daily Maintenance Electrolyte Requirements for Adults

Electrolyte

Approximate

Electrolyte Salts

Daily Maintenance

Used in PN

Requirements"

Sodium

1-2 mEq/kg

Chloride, acetate,

phosphate

Potassium

1-2 mEq/kg

Chloride, aceiater

phosphate

Phosphorus

20-40 mmol

Sodium phosphate.

potassium

phosphate

Calcium

10 lSmEq

Gluconate

Magnesium

S-2Q mtq

Sulfate

Chloride

b

Sodium, potassium

Acetate

b

Sodium, potassium

Conversions

1 mmol potassium phosphate - 1.47 mEq

potassium

1 mmol sodium phosphate = 133 mEq sodium

,JElectrolyte requirements are adjusted based on serum electrolyte concentrations, and vary depending on kidney function, gi losses, nutritional status, specific metabolic and endocrine functions, and medication therapy that affect electrolyte losses or retention.

''As needed to maintain acid-base balance; linked to amounts of sodium and potassium provided (as chloride and acetate salts).

Sodium

Sodium is the most abundant extracellular cation in the body and is the major osmot-ically active ion in the extracellular fluid. Sodium concentration determines the distribution of water in the extracellular space, and sodium disorders can be caused by many factors. Patients with abnormal GI losses (e.g., gastric, diarrhea, ostomy, and fistula losses) have increased sodium requirements. Patients with fluid overload and hypervolemic hypotonic hyponatremia may require sodium and fluid restriction. Sodium in PN can be provided in the forms of chloride, acetate, and phosphate salts. One millimole (mmol) of sodium phosphate provides 1.33 mEq of elemental sodium. Total sodium concentration in PN should not exceed 154 mEq/L (154 mmol/L, the equivalent of normal saline).

Potassium

Potassium is the second most abundant cation in the body and is found primarily in the intracellular fluid. Potassium has many important physiologic functions, including regulation of cell membrane electrical action potential (especially in the myocardium), muscular function, cellular metabolism, and glycogen and protein synthesis. Potassium in PN can be provided as chloride, acetate, and phosphate salts. One mil-limole of potassium phosphate provides 1.47 mEq of elemental potassium. Generally, the concentration of potassium in peripheral PN (PPN) admixtures should not exceed 80 mEq/L (80 mmol/L). While it is safer to limit potassium solution concentration to 80 mEq/L (80 mmol/L) for infusion through a central vein, the maximum recommended potassium concentration for infusion via a central vein is 150 mEq/L (150 mmol/ L).1 Patients with abnormal potassium losses (e.g., loop or thiazide diuretic therapy, diarrhea, high gastric fluid output) may have higher potassium requirements, and patients with high gastric fluid output, acute kidney injury or chronic kidney disease may require potassium restriction.

Calcium and Phosphorus

Calcium and phosphorus are essential electrolytes for many physiologic processes and biochemical reactions. Phosphorus is provided as sodium or potassium phosphate in PN. Approximately 10 to 15 mmol of phosphate are needed per 1,000 kilocalories to maintain normal serum phosphorus concentrations (provided the patient is well nourished and has normal kidney function).14 Patients with decreased kidney function may require phosphorus restriction.

The FDA published a safety alert in 1994 in response to two deaths associated with calcium-phosphate precipitation in PN.15 Autopsy reports from these patients revealed diffuse microvascular pulmonary emboli containing calcium-phosphate precipitates. Because calcium and phosphate can bind and precipitate in solution, caution must be exercised when mixing these two electrolytes in PN admixtures. Several factors can affect calcium-phosphate solubility, including the following:

• pH. Largely affected by the final amino acid concentration, to a lesser extent by the dextrose concentration (unless the final amino acid concentration is very low); the lower the solution pH, the less chance there is for calcium-phosphate precipitation; monobasic phosphates predominate at low pH, leaving fewer free dibasic phosphates for precipitation with divalent calcium; monobasic calcium phosphate is more soluble than dibasic calcium phosphate.

• Amino acid concentration. Primary factor that affects pH of the PN admixture; the pH of amino acid stock solutions may vary between commercial products and thus differently affects the final pH of PN admixture; however, in general the higher the final amino acid concentration, the lower the pH of the final admixture (see above), and more phosphates likely bind with amino acids leaving fewer phosphates available to bind with calcium.

• Calcium salt. Calcium gluconate is the preferred calcium salt in PN because it is has a low dissociation constant in solution with lesser free calcium available at a given time to bind phosphate (as opposed to calcium chloride, which dissociates rapidly in solution). One gram of calcium gluconate provides 4.5 mEq of elemental calcium.

• Time. The longer calcium and phosphate are in solution, the more calcium and phosphate will dissociate overtime and increase the risk for calcium-phosphate precipitation.

• Temperature. As temperature increases, more calcium and phosphate dissociate and increase the risk of calcium-phosphate precipitation.

• Order of mixing. Calcium and phosphate should be separated when mixing PN admixtures (e.g., add phosphate first, then all other PN components, and then add calcium last); if calcium is added before all other components are in the PN admixture, including lipid emulsion, then the volume in the PN admixture at the time calcium is added must be used to determine the maximum calcium that can be added.

Magnesium

Magnesium is the second most abundant intracellular cation after potassium. Magnesium serves as an essential cofactor for numerous enzymes and in many biochemical reactions, including reactions involving adenosine triphosphate (ATP).16 Magnesium disorders are multifactorial and can be related to kidney function, disease state(s), and medication therapy. Magnesium in PN typically is provided as magnesium sulfate. One gram of magnesium sulfate provides 8.1 mEq of elemental magnesium.

Chloride and Acetate

Concentration limits for chloride and acetate in PN typically are linked to limitations of sodium and potassium. The usual ratio of chloride:acetate in PN is about 1:1 to 1.5:1. Chloride and acetate primarily play a role in acid-base balance. Chloride is primarily eliminated via the kidneys. Serum chloride concentrations that exceed 130 mEq/L (130 mmol/L) may cause hyperchloremic acidosis. Acetate is converted to bicarbonate at a 1:1 molar ratio. This conversion appears to occur mostly outside the liver. Acetate conversion to bicarbonate is rapid but not immediate, and thus acetate salts should not be used to correct acute severe acidosis. Bicarbonate never should be added to or coinfused with PN solutions. This can lead to the release of carbon dioxide and potentially result in the formation of calcium or magnesium carbonate (very insoluble salts).

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