Fluid, Electrolyte, and Endocrine Problems

12 Fluid, Electrolyte, and Endocrine Problems






Pearls




Note: This formula does not reflect the influence of plasma proteins or administered osmotic agents such as mannitol. It may be inaccurate in the presence of severe hyperglycemia and hyperlipemia.





Anatomy and physiology


Body fluids contain water and solutes. These solutes are positively or negatively charged electrolytes (e.g., Na+, K+, Cl) and nonelectrolytes (e.g., glucose, urea). Fluid and electrolyte homeostasis is present when fluid and electrolyte balance is maintained within narrow limits despite significant variations in dietary intake, metabolic rate, and renal function.



Fluid Compartments


Water accounts for 65% to 80% of total body weight. The total body water (TBW) volume and distribution are influenced by factors such as age, gender, adipose content, and skeletal muscle mass (Table 12-1). TBW is divided into two compartments (see Evolve Table 12-1 and Evolve Fig. 12-1 in the Chapter 12 Supplement on the Evolve Website for more information): intracellular fluid (ICF) and extracellular fluid (ECF) compartments (Fig. 12-1).








Developmental Considerations


Renal function, metabolic rate, body surface area (BSA), and fluid requirements change with development. The infant kidney is unable to concentrate urine until approximately 3 months of age, and it is relatively inefficient at concentrating urine until approximately 2 years of age. In the first years of life the kidneys are also inefficient at excreting electrolytes and waste products, and they are unable to effectively conserve or excrete sodium, acidify urine, or handle large quantities of solute-free water. As a result, infants and small children are less able to maintain homeostasis with sudden, acute changes in fluid and electrolyte intake or output.


Energy requirements (per kilogram body weight) and metabolic rate are higher in infancy and childhood (for further information, see Chapter 14). The ratio of BSA to volume is significantly higher in infants and children than in adults. As a result, pediatric evaporative fluid losses and fluid requirements per kilogram body weight are higher than those of adults.


Insensible water loss (IWL) is fluid lost through the skin, via evaporation and sweat, and through the respiratory tract. Normal IWL is approximately 300 to 400   mL/m2 BSA per day (for more detailed information, see Evolve Table 12-2 in the Chapter 12 Supplement on the Evolve Website). Fever increases IWL by approximately 0.42   mL/kg per hour for each degree Celsius increase above 37°   C. Increased IWL can occur with increased air movement across the skin, and with tachypnea, unless inspired air is humidified. IWL decreases when ambient and inspired air are humidified.


Table 12-2 Formulas for Estimating Daily Maintenance Fluid and Electrolyte Requirements for Children





























































  Daily Requirements Hourly Requirements
Fluid Requirements Estimated from Weight*
Newborn (up to 72   hr after birth) 60-100   mL/kg (newborns are born with excess body water)
Up to 10   kg 100   mL/kg (can increase up to 150   mL/kg to provide caloric requirements if renal and cardiac function are adequate) 4   mL/kg
11-20   kg 1000   mL for the first 10   kg + 50   mL/kg for each kg over 10   kg 40   mL for first 10   kg + 2   mL/kg for each kg over 10   kg
21-30   kg 1500   mL for the first 20   kg + 25   mL/kg for each kg over 20   kg 60   mL for first 20   kg + 1   mL/kg for each kg over 20   kg
Fluid Requirements Estimated from Body Surface Area (BSA)
Maintenance 1500   mL/m2 BSA
Insensible losses 300-400   mL/m2 BSA
Electrolytes
Sodium (Na) 2-4   mEq/kg
Potassium (K) 1-2   mEq/kg
Chloride (Cl) 2-3   mEq/kg
Calcium (Ca) 0.5-3   mEq/kg
Phosphorous (Phos) 0.5-2   mmol/kg
Magnesium (Mg) 0.4-0.9   mEq/kg

* The “maintenance” fluids calculated by these formulas must only be used as a starting point to determine the fluid requirements of an individual patient. If intravascular volume is adequate, children with cardiac, pulmonary, or renal failure or increased intracranial pressure should generally receive less than these calculated “maintenance” fluids. The formula utilizing body weight generally results in a generous “maintenance” fluid total.


Fluid and electrolyte requirements will vary with age and clinical condition. Normal baseline fluid and electrolyte requirements are listed in Table 12-2. Critical care practitioners use estimated maintenance fluid requirements as a baseline and individualize administered fluid and electrolytes to meet patient needs.



Fluid, Electrolyte, and Glucose Balance



Role of Osmolality


The term osmolality refers to the concentration of solute (electrolytes and proteins) per liter of fluid. Serum osmolality reflects ECF osmolality. It can be estimated with the following formula*:



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Because sodium is the primary electrolyte that determines serum osmolality, a major increase or decrease in serum sodium concentration will increase or decrease the serum osmolality, respectively.


Changes in the osmolality of one body fluid compartment will affect all other compartments. Water shifts between the ICF and the ECF compartments in response to changes in the osmolality of either compartment, moving from the compartment of lower osmolality to the compartment of higher osmolality until osmolality equilibrates. When the osmolality of the extracellular compartment (including the vascular space) decreases, water will shift from the extracellular compartment into cells (Fig. 12-2). Conversely, when the osmolality of the extracellular compartment (including the vascular space) increases, water will shift from the intracellular to the extracellular compartment (including into the vascular space). The volume and acuity of the water shift, as well as likely clinical significance, is determined by the magnitude and acuity of the osmolality gradient between compartments. Significant water shifts can cause neurologic complications.





Endocrine Influences


Several hormones contribute to regulation of fluid and electrolyte balance.



Antidiuretic Hormone (ADH, Vasopressin)


ADH, also known as vasopressin and as arginine vasopressin, is manufactured in the hypothalamus and stored in and released by the posterior pituitary in response to a rise in serum osmolality, a decrease in circulating blood volume (volume depletion), or a decrease in blood pressure. ADH acts on vasopressor-2 receptors of cells in the renal collecting ducts and distal tubules, increasing the permeability of these cells to water; this results in reabsorption of water from the filtrate and returns water to the circulation. Urine volume decreases and urine concentration increases. ADH secretion does not influence the rate of sodium reabsorption, but the serum sodium concentration typically falls because it is diluted by the reabsorbed water.


Negative feedback mechanisms normally regulate ADH secretion to maintain a serum osmolality of 275 to 295   mOsm/L. Osmolality receptors located in the brain are stimulated by a rise in serum osmolality (e.g., above 285   mOsm/L or a rise of at least 2%). Volume-sensitive receptors (located in the left atrium and thoracic vessels) and baroreceptors (stretch receptors located in the ascending aorta, pulmonary arteries, and carotid sinus) are stimulated by volume depletion and hypotension. Additional causes of ADH secretion include stress, trauma and severe pain (through activation of cholinergic neurotransmitters in the hypothalamus), angiotensin II, and some medications.3 A normal or low serum osmolality, hypertension, and an increase in left atrial stretch should inhibit ADH secretion.


Although endogenous ADH does not affect vascular tone, exogenous (administered) vasopressin can cause vasoconstriction and increase blood pressure. For further information regarding use vasopressin, see Chapters 6 and 14.





Serum Glucose in Critically Ill or Injured Children


Although a serum glucose concentration of 60 to 180   mg/dL is normally maintained over a wide range of conditions, critically ill or injured children often develop hypoglycemia or hyperglycemia. Infants have high glucose needs and low glycogen stores, so they can rapidly become hypoglycemic during critical illness or injury.24 Providers should monitor serum glucose concentration with point-of-care testing, if possible, and treat hypoglycemia as needed. Treatment of hypoglycemia should avoid frequent, intermittent bolus administration of large quantities of glucose; provision of a continuous source of glucose is preferable.


Hyperglycemia can result from steroid administration, stress response, relative hypoinsulinemia, or insulin resistance and has been associated with increased mortality in critically ill children in some studies. A prospective, randomized study of tight control of serum glucose concentration in critically ill children (targeted to age-adjusted normal fasting glucose concentration) reduced critical care unit mortality,37 but was associated with episodes of hypoglycemia. In general, an insulin infusion (0.5-1 unit regular insulin/kg per hour) is often titrated during the first 18 to 24   hours of critical care therapy to maintain the serum glucose concentration less than 150   mg/dL (range will vary; use your unit protocol). Careful monitoring is required to avoid and treat episodes of hypoglycemia. The ultimate value versus risk of this approach is still under investigation.



Electrolyte homeostasis and common imbalances


Table 12-3 presents a summary of electrolyte imbalances and associated clinical manifestations in critically ill infants and children. In the following sections, approximate ranges for normal and abnormal serum electrolyte concentrations are listed, but providers should use normal ranges referenced by the clinical laboratory in their practice settings.





Sodium Imbalance: Hyponatremia



Etiology


Hyponatremia is a low serum sodium concentration, typically less than 130 to 135   mEq/L. It often develops as a complication of disease or therapy. The critically ill infant or child can develop hyponatremia from excessive water intake relative to sodium, excess water retention, increased sodium loss, or a combination of these factors.10


Hyponatremia can occur in conjunction with hypervolemia, euvolemia, or hypovolemia. Hypervolemic hyponatremia is associated with water intoxication, nephrotic syndrome, cardiac failure, renal failure, and the syndrome of inappropriate antidiuretic hormone (SIADH). Hypovolemic hyponatremia can occur with renal losses (e.g., osmotic diuresis, renal tubular acidosis) or extrarenal losses (e.g., diarrhea, vomiting, burns).10 Other potential causes include adrenal insufficiency, excessive use of diuretics, and cerebral salt-wasting syndrome.


A laboratory report of a low serum sodium concentration can be misleading. These pseudohyponatremic states are associated with hyperlipidemia, hyperproteinemia, or hyperglycemia. In hyperlipidemia or hyperproteinemia, the lipid or protein displaces fluid from serum, decreasing the relative volume of water and electrolytes. As a result, the reported serum sodium concentration will be low. The total body sodium actually may be normal, although its concentration (in milliequivalents per liter of plasma, or mEq/L) is reduced. Hyperlipidemia of this degree usually produces a milky white plasma.


Because the serum osmolality is determined by the combined effects of particles (solutes) in the serum—especially the sodium, glucose, and blood urea nitrogen—the serum osmolality can be normal if a fall in the concentration of one solute is accompanied by a commensurate (in osmotic effect) increase in the concentration of another solute. With significant hyperglycemia, the high glucose concentration increases serum osmolality, drawing fluid into the vascular space; this may artificially reduce the serum sodium concentration. The significance of this dilutional effect is debatable—the serum sodium concentration may still influence osmotic changes to which the cells are exposed. To estimate the potential effect of severe hyperglycemia on the serum sodium concentration, for every 100   mg/dL rise in serum glucose above normal, the serum sodium concentration is likely to be depressed approximately 1.6   mEq/L below 135   mEq/L.




Management


If the child is at risk for hyponatremia, providers should closely monitor the child’s serum sodium concentration to detect and promptly treat hyponatremia before it becomes severe. Frequent neurologic assessments are indicated. Notify an on-call provider immediately if the child develops altered level of consciousness, seizures, or signs of increased intracranial pressure.


Hyponatremia associated with neurologic symptoms is a neurologic emergency. Urgent treatment includes administration of 2 to 4   mL/kg of 3% saline (513   mEq sodium/L, or 0.513   mEq/mL); this will typically raise the child’s serum sodium concentration approximately 1 to 2   mEq/L and raise serum osmolality sufficiently to slow the intracellular water shift. If the SIADH produces seizures or other severe symptoms, hyponatremia and water intoxication can be treated acutely by administration of hypertonic saline (2-4   mL/kg of 3% saline) and furosemide (1-2   mg/kg). These medications will increase the serum sodium concentration and eliminate excess free water (see section, Specific Diseases, Syndrome of Inappropriate Antidiuretic Hormone).


Management of hyponatremia includes restoration of appropriate intravascular volume, replacement of the sodium deficit, and identification and treatment of the underlying cause. Symptomatic hypovolemia is treated with boluses of normal saline or lactated Ringer’s solution. Management of hypervolemic hyponatremia may include fluid restriction and administration of loop diuretics.


Once the child’s neurologic status is stable and perfusion is adequate, plans are made to replace the sodium deficit. The following formula36 is used to calculate the sodium deficit:



During treatment of hyponatremia, providers must closely monitor the serum sodium concentration and the rate of rise in the concentration. If the serum sodium and osmolality are raised too rapidly, the resulting water shifts from the cellular to the extracellular (including intravascular) compartments can produce neurologic complications including intracranial bleeding (Box 12-1). In general, the serum sodium should be raised no faster than approximately 0.5-1.0   mEq/L per hour.35 Additional important assessments include strict monitoring of intake and output, urine specific gravity, serum electrolytes, serum osmolality, and daily weights.



Box 12-1 Advanced Concepts: Potential Central Nervous System Complications of Rapid Changes in Serum Sodium and Osmolality


The brain does not tolerate rapid or significant water shifts into and out of the cells. A shift of water into brain cells—such as that occurring with an acute fall in serum sodium concentration and serum osmolality—is likely to produce cerebral edema. A rapid shift of water from cells, including brain cells, can cause the cells to shrink and can result in cerebral dysfunction. Cells in the gray matter and tissue in the white matter in the brain swell and shrink at different rates when water shifts occur. As a result, significant water shifts between intracellular and extracellular compartments in the brain can cause tearing of cerebral bridging veins and intracranial bleeding.


Rapid correction of hyponatremia has been shown to result in cerebral dysfunction, linked with damage to the myelin sheath of neurons. This myelinolysis is called central pontine myelinolysis if it occurs in the pons (brainstem) and osmotic demyelinization syndrome if it occurs elsewhere in the brain. Signs of brain dysfunction can include decreased level of consciousness, lack of coordination, paralysis, and dysphagia. Because there is no known treatment for such cerebral dysfunction, and it can cause permanent disability, prevention is critical. In general, unless neurologic symptoms indicate the need for more aggressive treatment, providers should aim to correct hyponatremia or hypernatremia no faster than approximately 0.5   mEq/L per hour (or 10-12   mEq/L per day). For more information, consult the National Institutes of Health Web site: http://www.ninds.nih.gov/disorders/central_pontine/central_pontine_myelinolysis.htm.



Sodium Imbalances: Hypernatremia




Pathophysiology and Clinical Signs and Symptoms


Hypernatremia typically increases the serum osmolality. The rise in osmolality stimulates the posterior pituitary to release ADH, which increases renal water reabsorption until the osmolality returns to normal. The increased osmolality associated with hypernatremia also leads to a shift in water from the intracellular to the extracellular compartment, including into the vascular space. This water movement from the cells can cause cellular dehydration and central nervous system dysfunction. Complications including subdural, subarachnoid, and intracerebral bleeding and sinus vein thrombosis can develop with acute and severe increases in serum sodium concentration and the resulting water shift (see Box 12-1). Permanent central nervous system dysfunction can result when the serum sodium concentration is extremely high (e.g., >165-170   mEq/L).12 When serum osmolality is chronically elevated, brain cells will generate idiogenic osmoles to maintain cell volume (Box 12-2).



Patients who are unable to produce and/or respond to ADH (e.g., patients with DI) are at risk for development of significant hypernatremia and hypovolemia. These patients must be closely monitored to detect and treat these complications before they become severe.




Potassium Homeostasis


Potassium is the primary intracellular cation (i.e., a positively charged ion). The magnitude of the transmembrane (i.e., between intracellular and extracellular) potassium gradient determines the excitability of nerve and muscle cells (including skeletal and heart muscle) and the rate of conduction of nerve impulses. Potassium also plays a significant role in the maintenance of acid-base balance.


The normal serum potassium (K+) concentration is 3.0 to 5.0   mEq/L. The intracellular potassium concentration is much higher, approximately 150   mEq/L.28 Small alterations in serum potassium concentration can significantly affect the transmembrane gradient and therefore neuromuscular and cardiac function. The serum K+ concentration is affected by potassium intake and excretion, renal regulation, and serum pH. Because the intravascular (serum) potassium concentration represents only a small proportion of the total body potassium, accurate interpretation of the child’s potassium balance requires consideration of the child’s clinical status and acid-base balance.


Potassium ions normally shift between the intra- and extracellular compartments with changes in the serum pH (see Evolve Fig. 12-2 in the Chapter 12 Supplement on the Evolve Website). When the serum hydrogen ion concentration increases (i.e., with acidosis or a fall in pH), hydrogen moves from the extracellular to the intracellular compartment, where it is buffered. To maintain a balance of cation movement across cell membranes, the intracellular movement of hydrogen ions is associated with an extracellular shift of potassium. Thus, acidosis or a fall in pH (e.g., with correction of alkalosis) is associated with a rise in serum potassium concentration. In contrast, alkalosis or a rise in serum pH (e.g., with treatment of acidosis) is associated with a fall in serum potassium, because hydrogen ion shifts out of cells and is replaced by potassium.


The change in serum potassium concentration associated with changes in serum pH will always be in the direction opposite the change in pH. That is, if the pH falls (as in acidosis) the serum potassium usually rises; as acidosis is corrected (the pH rises) the serum potassium should fall. Hypokalemia with acidosis is particularly dangerous: as acidosis is treated and the serum pH rises, the serum potassium concentration will fall further. Thus, correction of acidosis in the child with hypokalemia and acidosis may result in a dangerously low serum potassium concentration. The potassium concentration should always be evaluated in light of the patient’s present acid-base status, and likely changes in potassium concentration should be anticipated in response to changes in acid-base status that will result from planned therapy.


The kidneys play a critical role in potassium homeostasis. Renal failure limits the kidney’s ability to excrete potassium and may result in hyperkalemia (see Chapter 13).



Potassium Imbalances: Hypokalemia




Pathophysiology and Clinical Signs and Symptoms


The large quantity of intracellular potassium serves as a reservoir to help maintain intravascular potassium concentration despite potassium loss. When the serum potassium concentration begins to decline, some intracellular potassium, known as the exchangeable potassium, moves from the intracellular space to the extracellular compartment (including the vascular space). Only when this amount of exchangeable potassium is depleted will further extracellular potassium loss produce a fall in serum potassium concentration. Thus, the serum potassium concentration may be normal or even elevated when a total body potassium deficit is present.


Evaluation of the patient’s potassium balance is further complicated by potassium shifts between the intracellular and extracellular compartments that result from administration of some medications and with changes in acid-base balance. β-Adrenergic agonists (e.g., inhaled albuterol) can cause a fall in serum potassium, whereas α-adrenergic agents can cause a rise in serum potassium. β-Adrenergic agonists increase intracellular sodium ion movement that stimulates the sodium-potassium pump to move sodium out of the cells. For every three sodium ions pumped out of cells, two potassium ions are allowed to enter cells. Thus, potassium shifts from the extracellular (including vascular) to the intracellular space.


Alpha-adrenergic agents can cause a rise in serum potassium, because potassium shifts from the intracellular to the extracellular (including vascular) space. Insulin administration can produce a fall in the serum potassium concentration, because insulin stimulates cellular uptake of potassium.


Acute alkalosis or a rise in serum pH (e.g., during treatment of acidosis) will cause a fall in serum potassium concentration, because hydrogen ion shifts out of the cell and is replaced by potassium (i.e., potassium moves into the cells). If the serum potassium concentration is within the normal range and the child’s serum pH then rises (e.g., as occurs with treatment of acidosis) hypokalemia may result.


Hypokalemia may also result from increased renal potassium excretion. This hypokalemia can be associated with metabolic alkalosis, renal tubular acidosis, and DKA. Hypokalemia can perpetuate metabolic alkalosis, particularly if either condition is chronic. When the serum potassium concentration is low, the kidney vigorously reabsorbs potassium and must excrete hydrogen ions. As a result, it may be necessary to treat the child’s hypokalemia to correct significant alkalosis.


Excessive renal potassium losses can also result from increased mineralocorticoid activity and increased delivery of sodium to the distal nephron. Excessive renal potassium losses have been associated with administration of antibiotics such as carbenicillin (and other penicillins), amphotericin B, gentamicin and aminoglycosides. A common cause of hypokalemia in the critically ill patient is the use of diuretics, especially loop and thiazide diuretics.


Chronic hypokalemia can change renal concentrating ability and cause polyuria. The kidney has little ability to conserve potassium when body potassium stores become low; as a result, urinary potassium excretion will remain greater than 20   mEq/L once hypokalemia persists for 10 to 20 days.


Although some potassium is lost with vomiting, diarrhea, and loss of other gastrointestinal fluids, potassium losses are exacerbated by intravascular volume contraction. Hypovolemia stimulates aldosterone secretion, which produces sodium and water retention, but it increases hydrogen ion and potassium excretion.


Hypokalemia can cause hyperpolarization of nerve and muscle cells, leading to muscle weakness, slowed nerve impulse conduction, and decreased muscle contraction. Hypokalemia can cause characteristic electrocardiogram (ECG) changes, including development of a U-wave (Fig. 12-3).

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Dec 3, 2016 | Posted by in NURSING | Comments Off on Fluid, Electrolyte, and Endocrine Problems

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