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Fluids and Electrolytes

Disorders affecting the body fluids and electrolytes are treated by supplying maintenance requirements, correcting volume and electrolyte deficits, and by replacing ongoing abnormal losses dehydration.

Dehydration

Maintenance fluid and electrolytes

Sensible losses, primarily urinary, account for approximately 50% of daily fluid requirements. Caloric requirements for growth can be estimated as equivalent on a kcal-for-mL basis to water requirements.

Factors that increase the requirements for calories and water are fever (10% for each degree), physical activity, ongoing gastrointestinal losses, hyperventilation, and hypermetabolic states.

 

Maintenance Requirements for Fluid and Electrolytes

Body Weight

0 to 10 kg

10 to 20 kg

>20 kg

Water Volume

100 mL/kg

1000 mL + 50 mL/kg for each kg >10 kg

1500 mL + 20 mL/kg for each kg >20 kg

Sodium

3 mEq/kg

3 mEq/kg

3 mEq/kg

Potassium

2 mEq/kg

2 mEq/kg

2 mEq/kg

Chloride

5 mEq/kg

5 mEq/kg

5 mEq/kg

 

Abnormal losses, such as those arising from nasogastric aspiration, prolonged diarrhea or burns, should be measured, and replaced on a volume.

Estimation of deficit

Estimation of volume depletion should assess fever, vomiting, diarrhea, and urine output. Recent feeding, including type and volume of food and drink, and weight change.

 

Estimation of Dehydration

Degree of Dehydration

Mild

Moderate

Severe

Weight Loss--Infants

5%

10%

15%

Weight Loss--Children

3-4%

6-8%

10%

Pulse

Normal

Slightly increased

Very increased

Blood Pressure

Normal

Normal to orthostatic, >10 mm Hg change

Orthostatic to shock

Behavior

Normal

Irritable

Hyperirritable to lethargic

Thirst

Slight

Moderate

Intense

Mucous Membranes

Normal

Dry

Parched

Tears

Present

Decreased

Absent tears, sunken eyes

Anterior Fontanelle

Normal

Normal to sunken

Sunken

External Jugular Vein

Visible when supine

Not visible except with supraclavicular pressure

Not visible even with supraclavicular pressure

Skin

Capillary refill <2 sec

Delayed capillary refill, 2-4 sec (decreased turgor)

Very delayed capillary refill (>4 sec), tenting; cool, acrocyanotic, or mottled skin

Urine Specific Gravity (SG)

>1.020

>1.020; oliguria

Oliguria or anuria

 

The percent dehydration is used to calculate the milliliters of body water deficit per kilogram of body weight.

Isonatremic dehydration

The most common cause of dehydration in infants is diarrhea. Children who have a brief illness and anorexia usually present with isotonic dehydration.

Oral rehydration

Moderate volume depletion should be treated with oral fluids. The majority of patients who have gastroenteritis can be treated with oral rehydration therapy.

Small aliquots of oral hydration solution ( Ricelyte, Pedialyte, Resol, Rehydralyte) are

Parenteral rehydration

Parenteral fluids should be given for severe volume depletion, altered states of consciousness, intractable vomiting, and abdominal distention or ileus.

The first phase of treatment rapidly expands the vascular volume. Intravenous normal saline or Ringers lactate (10-20 mL/kg) should be given over 1 hour.

The next phase of treatment is aimed at correcting the deficit, providing maintenance, and replacing ongoing abnormal losses. In severe depletion, half of the calculated deficit is

Hyponatremia and hyponatremic dehydration

The signs and symptoms of hyponatremia correlate with the rapidity and extent of the fall in serum Na+ concentration. Symptoms include apathy, nausea, vomiting, cramps, weakness, headache, seizures, and

Management of hyponatremia

Hypovolemic patients who have hyponatremia first require volume repletion with normal saline, then a solution containing salt is given to correct the Na+ deficit (10 to 12 mEq/kg of body weight or 15 mEq/kg in severe hyponatremia) and to provide the

Symptomatic hyponatremia (headache, lethargy, disorientation) requires urgent therapy to prevent seizures or coma.

Hypertonic saline (3% saline solution), with or without a loop diuretic and water restriction, should be used to raise the serum Na+ by 1 to 2 mEq/L per hour or halfway toward normal during the

 

Hypernatremia and hypernatremic dehydration

The hypernatremic patient is usually also dehydrated. Total body Na+ most commonly is decreased. Affected patients frequently exhibit lethargy or confusion, muscle twitching, hyperreflexia, or

Differential Diagnosis

Diarrhea, which usually results in isonatremic or hyponatremic dehydration, may cause hypernatremia in the presence of persistent fever, anorexia, vomiting, and decreased fluid

Potassium disorders

Abnormalities of serum K+ are potentially life-threatening because of effects on cardiac function.

Hypokalemia

Hypokalemia (serum K+ concentration <3 mEq/L) is most frequently caused by gastrointestinal K+ losses or renal losses (nasogastric suction, protracted vomiting, diuretics, renal tubular disease).

Hyperkalemia

The most common cause of hyperkalemia (K+ >5.5 mEq/L) is "pseudohyperkalemia" from hemolysis of the blood sample. This cause should be excluded by repeating the measurement on a free-flowing venous sample. Children may display hyperkalemia in metabolic acidosis, tissue catabolism, renal failure, volume depletion, or hypoaldosteronism.

In salt-losing congenital adrenal hyperplasia, due to complete deficiency of the enzyme 21-hydroxylase, the symptoms in affected male infants appear in the first weeks of life and include dehydration and failure

Acid-base disorders

The pH of the body fluids normally is between 7.35 and 7.45.

Metabolic acidosis

Acidosis results from the addition of acid or the removal of alkali from body fluids, and it causes a compensatory increase in ventilation (respiratory alkalosis) and a fall in Pco2. Manifestations of acidosis include depressed myocardial contractility, arrhythmias, hypotension, and pulmonary edema.

Diagnosis

Addition of a fixed acid to the extracellular fluid causes the formation of unmeasured anions. These unmeasured anions are referred to as the anion gap, which can be estimated as:

Normal anion gap (hyperchloremic) acidosis

This disorder occurs when HCO3- is lost from the body, either through the

Metabolic alkalosis

Alkalosis results from a gain of base or a loss of acid. The common clinical manifestations are lethargy, confusion, neuromuscular irritability, arrhythmias, and 

Respiratory acidosis

Respiratory acidosis is induced by an increase in Pco2, which lowers plasma pH. Causes of respiratory acidosis include airway obstruction, and 

Respiratory alkalosis

Respiratory alkalosis is caused by a decrease in Pco2, secondary to hyperventilation, resulting in dizziness, confusion, and