Fluid & Electrolytes

Approximately 60% of the weight of a typical adult consists of fluid (water & electrolytes). Body fluid is located in two compartments: theintracellular space (fluid inside cells) & the extracellular space (fluid outside of cells). People who are obese have less fluid than those who are thin, as fat cells contain little water. Muscle, skin, and blood contain the highest amounts of water. Approximately 2/3 of body fluid is intracellular, and 1/3 is extracellular. The extracellular compartment is divided into the 1) intravascular space, 2) interstitial space, and 3) transcellular fluid space.

Intravascular space: contains plasma (approximately 3L of blood volume), RBCs, WBCs, and platelets (approximately 3L of blood volume).

Interstitial space: contains fluid that surrounds cells, totals to about 11-12L in an adult. Lymph fluid is an interstitial fluid.

Transcellular fluid: contains approximately 1L total; cerebrospinal, pericardial, synovial, intraocular, and pleural fluids; also sweat & digestive fluids.

*Loss of extracellular fluid into a space that does not contribute to the equilibrium between the intracellular and extracellular fluid is referred to as "third spacing".

Signs & Symptoms of Third-Spacing

• Decreased urine output
• Increased HR
• Decreased BP
• Decreased CVP
• Edema
• Increased body weight
• and imbalances in I & O.

Common causes of third-spacing include: ascites, burns, peritonitis, bowel obstructions, and massive bleeding into a joint or body cavity.

Kidney Function

Everyday, the kidneys filter about 170L of plasma & excrete only about 1.5L of urine. They act both autonomously and through bloodborne messengers, such as aldosterone & ADH. The kidneys regulate the extracellular volume by selective retention & excretion of body fluids, regulate electrolyte levels in extracellular fluid by selective retention of needed substances and excretion of un-needed substances, regulate pH of the extracellular fluid by retention of hydrogen ions, and excrete metabolic wastes & toxic substances.

Renin-Angiotensin-Aldosterone System

Renin is an enzyme that converts angiotensinogen, an inactive substance formed by the liver, into angiotensin I. Angiotensin-converting enzyme (ACE) converts angiotensin I to angiotensin II. Angiotensin II, with its vasoconstrictor properties, increases arterial perfusion pressure & stimulates thirst. As the sympathetic nervous system is stimulated, aldosterone is released in response to an increase in the release of renin.Aldosterone is a volume regular, and is also released when serum potassium increases, serum sodium decreases, or adrenocorticotropic hormone (ACTH) increases.

Hypovolemia

Clinical manifestations of hypovolemia include: acute weight loss, decreased skin turgor, oliguria, concentrated urine, postural hypotension, weak & rapid HR, flattened neck veins, increased temperature, decreased CVP, cool & clammy skin r/t peripheral vasoconstriction, thirst, anorexia, nausea, lassitude, muscle weakness, and cramps.

Isotonic Solutions:

0.9% Sodium Chloride- expands extracellular fluid volume, used in hypovolemic states, resuscitative efforts, shock, diabetic ketoacidosis, metabolic acidosis, hypercalcemia, and mild sodium deficit. Only solution that can be administered with blood products.

Lactated Ringer's Solution- Contains multiple electrolytes in roughly the same concentration as plasma (Na+, K+, Ca++, Cl-, Lactate), but no Mg++. Provides 9 cal/L. Used to treat hypovolemia, burns, fluid lost as bile or diarrhea, and for acute blood loss replacement. Not given when pH is >7.5 because bicarbonate is formed as lactate breaks down, causing alkalosis. Not used in renal failure because it contains potassium and can cause hyperkalemia.

5% Dextrose in Water- Supplies 170 cal/L and free water to aid in renal excretion of solutes,contains no electrolytes. Used to treat hypernatremia, fluid loss, and dehydration. Shouldn't be used in excessive volumes in the early postoperative period (when ADH secretion is increased due to stress). Contraindicated in head injury because it may cause increased ICP. Not used solely in treatment of fluid volume deficit, because it dilutes plasma electrolyte concentrations. Not used for fluid resuscitation because it may cause hyperglycemia. Converts to a hypotonic solution as dextrose is metabolized as body, which can lead to water intoxication (if not combined with NS).

Hypotonic Solutions:

0.45% Sodium Chloride- supplies Na+, Cl-, and free water. Free water aids the kidneys in excreting solutes. Contains no electrolytes. Used to treat hypertonic dehydration, Na+ & Cl- depletion, and gastric fluid loss. Not indicated for third-spacing fluid loss and increased ICP. Administer cautiously as it can cause fluid shifts from vascular system into cells, resulting in cardiovascular collapse and increased ICP.

Hypertonic Solutions:

3% Sodium Chloride- Used to increased extracellular volume and to decrease cellular swelling. Highly hypertonic ( Na+: 513 mEq/L, Cl- 513 mEq/L), used only in critical situations to treat hyponatremia. Administer slowly and cautiously, because it can cause intravascular volume overload and pulmonary edema. Supplies no calories.

5% Sodium Chloride- Highly hypertonic solution used to treat symptomatic hyponatremia.Administer slowly and cautiously, because it can cause intravascular volume overload and pulmonary edema. Supplies no calories.

Colloidal Solutions:

Dextran in NS (5% D5W)- Used as a volume/plasma expander for intravascular part of extracellular fluid. Affects clotting by coating platelets and decreasing ability to clot. Remains in circulatory system for up to 24 hours. Used to treat hypovolemia in early shock to increase pulse pressure, cardiac output, and arterial blood pressure. Improves microcirculation by decreasing RBC aggregation. Contraindicated in hemorrhage, thrombocytopenia, renal disease, and severe dehydration.

Hyponatremia

Na+ <>

Sodium can be lost by vomiting, diarrhea, fistulas, or sweating; can also result from the use of diuretics in combination with a low-salt diet. A deficiency of aldosterone that occurs in adrenal insufficiency, can also cause hyponatremia.

Dilutional hyponatremia can be caused from SIADH, hyperglycemia, increased water intake from the administration of electrolyte-poor parenteral fluids, tap-water enemas, and irrigation of NG-tubes with water vs. NS.

S/S: anorexia, nausea/vomiting, headache, lethargy, dizziness, confusion, muscle cramps and weakness, muscle twitches, seizures, papilledema, dry skin, increased HR, decreased BP, weight gain, edema.

Hypernatremia

Na+ > 145

Too much sodium can be caused by fluid deprivation, administration of hypertonic enteral feedings without adequate water supplements, watery diarrhea, insensible water loss, diabetes insipidus. Less common causes are: heat stroke, near-drowning in sea-water, and malfunction of hemodialysis or peritoneal dialysis.

S/S: thirst, elevated body temperature, swollen/dry tongue, hallucinations, lethargy, restlessness, irritability, focal or grand mal seizures, pulmonary edema, hyperreflexia, twitching, nausea/vomiting, anorexia, increased HR, increased BP.

Hypokalemia

K+ <>

Hypokalemia can be caused by diarrhea, vomiting, gastric suction, corticosteroid administration, hyperaldosteronism, carbenicillin, amphotericin B, bulimia, osmotic diuresis, alkalosis, starvation, diuretics, and digoxin toxicity.

S/S: fatigue, anorexia, nausea/vomiting, muscle weakness, polyuria, decreased bowel motility, ventricular asystole/fibrillation, paresthesias, leg cramps, decreased BP, ileus, abdominal distention, hypoactive relexes. ECG: flattened T-waves, prominent U-waves, ST depression, prolonged PR interval.

Hyperkalemia

K+ > 5.0

Caused by oliguric renal failure, use of K+ sparing diuretics in patients with renal insufficiency, metabolic acidosis, Addison's disease, crush injury, burns, stored bank blood transfusions, and rapid IV administration of K+.

S/S: vague muscular weakness, tachycardia -> bradycardia, dysrhythmias, flaccid paralysis, paresthesias, intestinal colic, cramps, irritability, anxiety. ECG: tall, tented T-waves, prolonged PR interval and QRS duration, absent P waves, ST depression.

Hypocalcemia

Ca++ <>

A calcium deficit can result from hypoparathyroidism, malabsorption, pancreatitis, alkalosis, vitamin D deficiency, massive subcutaneous infection, generalized peritonitis, chronic diarrhea, decreased parathyroid hormone, diuretic phase of renal failure, increased phosphorus, fistulas & burns.

S/S: numbness & tingling of toes, fingers, positive Trousseau's & Chvostek's sign, seizures, carpopedal spasms, hyperactive DTR's, irritability, bronchospasm, anxiety, impaired clotting time. ECG: prolonged QT interval and lengthened ST.

Hypercalcemia

Ca++ > 10.5

Too much calcium caused by hyperparathyroidism, malignant neoplastic disease, prolonged immobilization, vitamin D excess, oliguric phase of renal failure, acidosis, corticosteroid therapy, thiazide diuretic use, increased parathyroid hormone, and digoxin toxicity.

S/S: muscular weakness, constipation, anorexia, nausea/vomiting, polyuria & polydipsia, dehydration, hypoactive DTR's, lethargy, deep bone pain, pathological fractures, flank pain, and calcium stones. ECG: shortened ST segment and QT interval, bradycardia, heart blocks.