Dehydration — Isotonic, Hypotonic and Hypertonic Fluid Disorders

Image : “The human body requires a minimum intake of water in order to be able to sustain life before mild and then severe dehydration occurs.” by Pan American Health Organisation. License: CC BY-ND 2.0

Overview

Dehydration is defined as the excessive loss of water from the body. The balance between fluid intake and fluid loss from the body is greatly disproportionate in dehydration. The severity of dehydration ranges from mild to severe, and dehydration can be fatal when fluid loss exceeds more than 15% of the total body water.

Hypovolemia is defined as decreased circulatory volume due to blood or plasma loss.

Pathophysiology of Dehydration

Total body water is distributed into extracellular and intracellular compartments. The extracellular compartment contains one-third of total body water and consists of the intravascular, interstitial, and transcellular spaces. The intracellular compartment contains two-thirds of the total body water and consists of the fluid inside the body’s cells. In dehydration, fluid from the extracellular compartment is depleted first, followed by fluid from the intracellular space. Fluid loss from the intracellular space results in cellular shrinkage and metabolic dysfunction.

Dehydration occurs because of decreased water intake, increased fluid loss, or both. In the elderly, impaired thirst sensation, chronic illness, fever, and sickness are common causes of decreased water intake. Common causes of increased fluid loss include vomiting, diarrhea, diuresis, and sweating. Working in hot weather without water and electrolyte replacement is another common cause of dehydration.

Dehydration can be classified according to serum sodium concentration into hypernatremic, hyponatremic, or isonatremic dehydration.

• If water loss is greater in comparison to sodium loss, then the serum sodium concentration increases, resulting in hypernatremic (or hypertonic) dehydration.
• If water loss is accompanied by excessive sodium loss, then the serum sodium concentration decreases, resulting in hyponatremic (or hypotonic) dehydration.
• If water and sodium are lost at the same rate, then the serum sodium concentration remains the same. This is called isonatremic (or isotonic) dehydration.

Apart from hypernatremia and hyponatremia, other electrolyte imbalances may also occur:

• Hyperkalemia, which occurs in insulin-dependent diabetes mellitus, Addison’s disease, and kidney failure.
• Hypokalemia, which results from increased potassium loss in severe diarrhea and vomiting.
• Hypermagnesemia and hyperphosphatemia can also occur from increased concentrations of magnesium and phosphate, respectively.

Evaluation of Total Body Sodium

1. Decreased total body sodium produces signs of volume depletion, including:

• Dry mucous membranes
• Decreased skin turgor (skin tenting when the skin is pinched)
• Decreased blood pressure and increased pulse (reflex tachycardia) when sitting up from the supine position (i.e., positive tilt test)

2. Increased total body sodium may produce body cavity effusions (ascites) and pitting edema.

• Dependent pitting edema is due to an excess of sodium-containing fluid in the interstitial space (over 2–3 L).
  • • Caused by low protein content in edematous fluid
    • Fluid obeys the law of gravity and moves to the most dependent portion of the body (ankles when standing).
• The Starling equation describes the forces that regulate the production of interstitial fluid; in the situation of increased total body sodium. Starling pressures are altered to produce pitting edema and body effusions.
• An increase in total body sodium increases plasma hydrostatic pressure due to an increase in plasma volume.
• An increase in total body sodium increases the weight of the patient and is the most common cause of weight gain in a hospitalized patient.

3. Normal total body sodium is associated with normal skin turgor and hydration.

• Fluid movement across a capillary wall into the interstitial space is driven by Starling pressures (not osmosis).
• The net direction of fluid movement depends on which Starling pressure is dominant.
• An increase in plasma hydrostatic pressure or a decrease in plasma oncotic pressure (i.e. serum albumin), causes fluid to diffuse out of capillaries and venules and into the interstitial space, resulting in dependent pitting edema and body cavity effusions.

Signs and Symptoms of Dehydration

Most patients with dehydration present with:

• Thirst
• Headaches
• Fatigue

Symptoms of mild dehydration include:

• Constipation
• Dry mouth
• Dizziness
• Low urine volume (unless the cause of dehydration is polyuria)

Symptoms of more severe dehydration include:

• Dry skin
• Sunken eyes
• Dry mucous membranes
• Confusion
• Dizziness
• Hypotension
• Tachycardia
• Fever
• Poor skin elasticity
• Lethargy
• Oliguria
• Seizures
• Shock
• Coma
• Death

Signs of hypovolemia may also be present, including:

• Tachycardia
• Orthostatic hypotension
• Flat neck veins

Dehydration is more clinically evident in the elderly, especially in hot weather, due to impaired thirst sensation. Elderly or hospitalized patients will show signs of irritability and, occasionally, delirium.

Isotonic Dehydration

Isotonic dehydration is a condition in which both water and sodium are lost proportionally and the serum sodium concentration maintains normal serum osmolality. Serum osmolality determines the movement of fluids and electrolytes across membranes. The normal serum osmolality is 285–295 mOsm/kg.

Image: “Electron micrographs of viruses that cause gastroenteritis in humans. A = Rotavirus (Rotavirus), B = Adenovirus (Adenoviridae), C = Norovirus (Norovirus) and D = Astrovirus (Astroviridae). They are shown at the same magnification of approximately x 200,000” by Dr Graham Beards at en.wikipedia. License: CC BY 3.0

Causes of isotonic dehydration

• Vomiting and diarrhea: severe watery diarrhea and vomiting can be life-threatening conditions, especially in children. People with gastroenteritis may lose notable amounts of fluids and electrolytes within a short time and their oral replacement is limited due to recurrent vomiting, which can result in severe dehydration.

• Excessive sweating: vigorous exercise, especially in humid weather, will increase sweating and lead to fluid and electrolyte loss.

If dehydration is not corrected, it will lead to renal injury from muscle breakdown and lactic acidosis.

Laboratory values in isotonic dehydration

Isotonic dehydration will show normal serum laboratory values, including normal osmolality (285–295 mOsm/kg) and normal serum sodium (135–145 mmol/L).

Urine volume will be decreased with low fractional sodium excretion and increased specific gravity.

Isotonic dehydration can result in elevated liver and pancreatic enzymes and a decreased glomerular filtration rate. Dehydration also can result in various electrolyte imbalances that will affect the clinical picture and prognosis.

Hypertonic Dehydration

Hypertonic dehydration occurs when water excretion from the body exceeds that of sodium excretion, resulting in increased sodium concentration in the extracellular fluid (hypernatremia). Blood osmolality is increased, causing water to shift from the intracellular to the extracellular space.

Causes of hypertonic dehydration

• Fever: Fever will increase the respiratory rate and therefore, water loss. Sweating also increases to lower the body temperature. Water intake is commonly decreased during a fever which will aggravate dehydration.

Image: “Micrograph of spironolactone bodies. H&E stain.” by Nephron – Own work. License: CC BY-SA 3.0

• Polyuria: Increased water loss in the urine causes hypertonic dehydration and may occur in diabetes mellitus, diabetes insipidus, or with diuretic use.
• Decreased water intake
• Excessive sweating
• End-stage renal disease
• Drinking urine or seawater for survival

Laboratory values in hypertonic dehydration

Serum osmolality will exceed 300 mOsm/kg and serum sodium will exceed 150 mEq/L.

Urine volume will decrease unless the cause of dehydration is polyuria or diuretic use.

Specific gravity will be high and fractional excretion of sodium will also increase.

ECF, extracellular fluid; ICF, intracellular fluid; TBNa⁺, total body sodium; TBW, total body water

Hypotonic Dehydration

Hypotonic dehydration occurs when sodium loss is greater than water loss, resulting in a decrease in serum osmolality. This causes a shift of water from the extracellular space into the intracellular space. The cells swell and cerebral edema may occur.

Hyponatremia can be acute or chronic. If sodium loss occurs for more than 48 hours, it becomes chronic hyponatremia, and the body may adapt to this state. Sodium imbalance mainly manifests as neurological symptoms ranging from headaches, nausea, lethargy, and potentially confusion, coma, and death.

The term hyponatremia should be used with caution in cases of dehydration, as most cases of hyponatremia imply excess water retention rather than dehydration.

Causes of hypotonic dehydration

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• Addison’s disease
• Renal tubular acidosis
• Iatrogenic causes: hypotonic fluids or regular saline used for IV hydration in patients with heat stroke or diarrhea
• Diuretics: loop, thiazide, and osmotic diuretics, especially with prolonged use
• Cystic fibrosis

Laboratory values in hypotonic dehydration

Serum sodium and serum osmolality will be less than the normal range.

Urine specific gravity will be decreased

Urine sodium excretion will be decreased.

Isotonic and Hypotonic Fluid Disorders: Summary

ECF, extracellular fluid; ICF, intracellular fluid; TBNa⁺, total body sodium; TBW, total body water

Complications of dehydration:

• Hypovolemic shock: Severe dehydration will lead to low blood volume and hypovolemic shock. It can lead to major end-organ damage through acidosis and can cause acute kidney injury which can be fatal.
• Seizures: Sodium imbalance can cause abnormal neuronal excitability, resulting in confusion, seizures, delirium, and coma. Seizures in dehydrated patients can be iatrogenic or caused by the rapid correction of underlying serum sodium abnormalities. Hypotonic saline, if used in hypernatremic patients, will rapidly decrease the plasma osmolality and water will shift to the intracellular space, resulting in brain edema and seizures.
• Cardiac arrhythmias: Potassium imbalances caused by dehydration can affect muscles and cause life-threatening cardiac arrhythmias, fatigue, weakness, and muscle breakdown.
• Heatstroke: During exercise or while working in a hot environment, fluid intake is recommended to avoid heat exhaustion or even heatstroke.
• Kidney failure: Possible causes of kidney injury include hypovolemic shock with low blood supply to the kidneys, acidosis due to hypovolemia, muscle breakdown, and electrolyte disturbances.
• Thrombosis: Increased blood viscosity from dehydration will lead to venous thrombosis. Patients may present with DVT, portal vein thrombosis, or pancreatitis. Fever will also increase thrombosis risk and limit water intake.
• Coma and death: Low blood pressure in severe dehydration will decrease blood supply to the brain and could cause coma or death, particularly in elderly patients.

Prevention of Dehydration

Adequate hydration is recommended during all activities to prevent dehydration. Water intake is key to replacing fluid lost during exercise, in hot weather, during hospitalization, and in elderly patients with impaired thirst sensation.

Hospitalized patients should be carefully monitored for water intake and total fluid output for early detection of any fluid imbalances.

Children with vomiting and diarrhea should not be given plain water to replace fluid lost as this could lead to hyponatremia and water intoxication.

Intravenous fluids can be used for fluid replacement in unconscious patients with severe dehydration while plain water, sports drinks with electrolytes, and oral rehydration solutions are used for the prevention and treatment of mild dehydration. Electrolyte monitoring is mandatory for patients using diuretics for prolonged periods.

Treatment of Dehydration

Treatment approaches vary according to patient age and severity of dehydration.

Treatment of the cause should always be considered, along with the treatment of symptoms and fluid replacement. Urine output should be monitored in hospitalized patients as an indicator of treatment efficacy and renal function recovery.

In severe dehydration, restoration of the blood volume is the main goal and is achieved with a fluid bolus of 20 mL/kg isotonic saline or Ringer’s lactate. Adults can use oral fluids if they are conscious and able to drink, otherwise, intravenous fluids should be used. Correction of electrolyte abnormalities should follow.

Infants and children with dehydration

Children are vulnerable to the effects of dehydration. Water deprivation can complicate gastroenteritis or fever and can lead to severe dehydration, with neurological manifestations and electrolyte imbalances.

Treatment options include fluid replacement orally if the child is conscious and able to drink. Water, fluids, and an oral rehydration solution can be used. In severe cases, intravenous fluids should be used.

Breastfeeding and a normal diet should be continued, as long as the treatment with fluid replacement prevents weight loss or developmental delays in infants.

Treatment of isonatremic dehydration

A fluid bolus should be given to restore the blood volume according to severity, followed by maintenance therapy with 0.9% normal saline; 20 mL/kg of isotonic sodium or Ringer’s lactate is given to restore hydration. Oral intake should be encouraged as early as possible.

Treatment of hyponatremia

Image: “Loss of myelinated fibers at the basis pontis in the brainstem (Luxol-Fast blue stain)” by Jensflorian – Own work. License: CC BY-SA 3.0

Patients may present with acute cerebral edema. Early steps should include stabilization of the patient, securing of the airway, and maintenance of breathing and circulation.

In severe acute cases involving seizures and coma, management of hyponatremia is accomplished with 3% hypertonic saline. Correction of hyponatremia should start at a rate of 4–6 mEq/L/hour.

In chronic hyponatremia, correction of sodium concentration should be done using 0.9% normal saline at a rate of 10–12 mEq/L/day during the first 2 days. The rapid correction of hyponatremia will lead to central pontine myelinolysis, resulting in permanent injury to the brain stem, quadriplegia, and cranial nerve paralysis.

To calculate the sodium deficit:

Sodium deficit = (normal sodium level – serum level) × volume of distribution ×  weight

Treatment of hypernatremia

The first step in treating hypernatremia is volume restoration with isotonic saline or Ringer’s lactate.

Following this, a slow correction of the hypernatremia at a rate of 10 mEq/L/24hours is required to avoid complications (cerebral edema and death). 5% dextrose in 0.9% sodium chloride can be used with frequent monitoring of the serum sodium every 4 hours.

Hyperglycemia and hypocalcemia occasionally follow hypernatremic dehydration; thus, serum glucose and calcium levels should be monitored closely.

Note: Antidiarrheal agents, routine antibiotics, and antiemetics should be avoided in this situation as they may worsen the condition.