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Dehydration can range from mild to severe with death occurring when the body loses more that 15% of the water content. Hypovolemia is another term that relates to loss of blood or plasma. It is a different term that is more evident in people with hemorrhage, burn and sometimes dehydration. It can be divided according to the origin into hypernatremic, hyponatremic or isonatremic.
Pathophysiology of Dehydration
Water is distributed in the body in the extracellular space and intracellular space. Fluid from the blood and extracellular space are lost first, then fluids from the intracellular space with cellular shrinkage and metabolic dysfunction.
If the water loss is accompanied with more sodium loss with a serum concentration more than the normal serum sodium concentration it is hypernatremic dehydration.
When the serum sodium concentration is less than the normal serum concentration, it is hyponatremic dehydration.
When the serum sodium concentration is the same with dehydration, it is isonatremic dehydration.
Signs and Symptoms of Dehydration
Dehydration is more clinically evident in the elderly especially in hot weather because of impaired thirst sensation. Most patients with dehydration will present with thirst, headache, fatigue, constipation and low urine volume unless the cause is polyuria. More severe symptoms will progress to confusion and even seizures or death.
Signs will range from dry skin, sunken eyes, dry mucous membranes, oliguria to seizures and coma. Patients may present with signs of the cause as well.
Signs of hypovolemia may also be present, including tachycardia, orthostatic hypotension and flat neck veins. Old or hospitalized patients will show signs of irritability and sometimes delirium. It is important to calculate total fluid intake and output for hospitalized patients.
This is the most common cause of dehydration when both the water and sodium loss are proportional to the serum concentration with normal serum osmolality. Serum osmolality determines the movement of fluids and electrolytes across membranes. Serum osmolality of 285-295 mOsm/kg is normal.
Causes of isotonic dehydration
Dehydration can occur from causes which impair replacement of water loss. In elderly people, impaired thirst sensation, chronic illness, fever and sickness are common reasons for decreased water intake. Other reasons for increased water loss include vomiting, diarrhea, diuresis, and sweating. Working in a hot weather without water replacement is a common reason.
- Vomiting and diarrhea: severe watery diarrhea can be life threatening, especially in children and if accompanied with vomiting. People with gastroenteritis may lose tremendous amounts of fluids in a short time plus their oral replacement will also be limited. Losing fluids and electrolytes will affect the acid base status and will be life threatening in these patients.
- Excessive sweating: vigorous exercise, especially in humid weather, will increase sweating and lead to dehydration. Moreover, if dehydration is not corrected, it will lead to renal injury from muscle breakdown and lactic acidosis.
Lab values in isotonic dehydration
Dehydration can result in elevated liver and pancreatic enzymes with decreased glomerular filtration rate. Dehydration also can result in a variety of electrolyte imbalance that will affect the clinical picture and prognosis of the patients.
Apart from hypernatremia and hyponatremia, other electrolytes that can be affected.
Hypokalemia: results from severe nausea and vomiting.
Hypermagnesemia, hyperphosphatemia can also occur.
Isotonic dehydration will show the same laboratory values as normal serum, including normal osmolality of 285-295 mOsm/kg and normal serum sodium of 135-145 mmol/L.
Urine volume will be decreased with low fractional sodium excretion and increased specific gravity.
Hypertonic dehydration occurs when the water excretion is more than sodium excretion which results in more sodium concentration in the extracellular fluid, increased blood osmolality and shift of water from the intracellular to the extracellular space.
Causes of hypertonic dehydration
- Fever: normally, the body loses water through sweating and breathing. This is called insensible water loss. Fever will increase the respiratory rate and subsequently the water loss. Sweating will also increase during fever to lower the body temperature. Moreover, water intake is commonly decreased during fever which will aggravate dehydration.
- Polyuria: causes of polyuria include diabetes mellitus, diabetes insipidus or use of diuretic medication.
- Less water intake
- Excessive sweating
- End stage renal disease
- Drinking urine or sea water for survival.
Lab values in hypertonic dehydration
Serum osmolality will exceed 300 mOsm/kg while serum sodium will be more than 150 mEq/L.
Bun to creatinine ratio will be more than 20:1.
Urine volume will be decreased unless the cause of dehydration is polyuria or diuretics.
Specific gravity will be high and low fractional excretion of sodium.
Hypotonic dehydration occurs when sodium loss is more than water loss which will decrease the serum osmolality and result in shift of fluids from the extracellular space into the cells. The cells will swell and cerebral edema may occur.
Hyponatremia can be acute or chronic. If sodium loss occurs for a period more than 48 hours it is chronic hyponatremia, which gives the body a good chance to adapt. Sodium imbalance mainly manifests with neurological symptoms ranging from headache, nausea, lethargy and sometimes 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, not dehydration.
Causes of hypotonic dehydration
- Addison’s disease
- Renal tubular acidosis
- Iatrogenic causes; hypotonic fluids or plain water with less salt for patients with heat stroke or diarrhea.
- Diuretics: loop, thiazide and osmotic diuretics especially with prolonged use.
- Cystic fibrosis.
Lab values in hypotonic dehydration
Serum sodium and serum osmolality will be less than normal range. Urine specific gravity will be decreased and urine sodium will be increased.
Complications in hypotonic dehydration
- Hypovolemic shock: severe dehydration will lead to low blood volume and hypovolemic shock. It can lead to major end organ damage with acidosis and acute kidney injury and might be fatal.
- Seizures: electrolyte imbalance due to dehydration can result in abnormal neuronal excitability which will result in seizures. Potassium imbalance specifically increases muscle twitches which may rise the potential for seizure episodes and muscle breakdown. Another cause of seizure episodes in dehydration is iatrogenic when using hypotonic saline for rapid correction of hypernatremic dehydration. Hypotonic saline will decrease plasma osmolality and water will shift to the intracellular space. Brain edema and resultant seizures will follow.
- Heat stroke: during exercise or working in a hot environment, intake of fluids is recommended in order to avoid heat exhaustion or even heat stroke.
- Coma and death: low blood pressure in severe dehydration will decrease the blood supply to the brain and end up in coma specially elderly patients. Sometimes it can result in death.
- Kidney failure: possible causes of kidney injury from dehydration include hypovolemic shock with low blood supply to the kidney, acidosis from hypovolemia, muscle breakdown and electrolyte disturbance.
- Thrombosis: increased blood viscosity from dehydration will lead to venous thrombosis. Patients will present with DVT, portal vein thrombosis in case of diarrhea and pancreatitis. Fever also will increase thrombosis and limit water intake.
Prevention of Dehydration
Adequate hydration is recommended during all activities to prevent dehydration. Water intake is the key to replace water loss during exercise, hot weather, in hospitalized patients and elderly people with impaired thirst sensation.
Hospitalized patients should be monitored for water intake and total output carefully for early detection of any fluid imbalance.
Children with vomiting and diarrhea should not be replaced for fluid loss with plain water. Electrolyte imbalance with hyponatremia and water intoxication will develop.
Intravenous fluids can be used for unconscious patients while plain water, sports drinks with electrolytes and oral rehydration solutions are used for prevention and treatment of mild dehydration. Electrolyte monitoring is mandatory for patients under prolonged diuretic use.
Treatment of Dehydration
Treatment of patients can vary according to the severity of dehydration and according to the age.
Treatment of the cause should always be considered along with symptomatic treatment and fluid replacement. Urine output should be followed in hospitalized patients with severe dehydration as an indication for efficiency of treatment and restoration of kidney function.
For all causes of severe dehydration, restoration of the blood volume is a priority with fluid bolus of 20ml/kg isotonic saline or lactated ringer. 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 lead to severe dehydration with neurological manifestations and electrolyte imbalance.
Treatment options include fluid replacement orally if the child is conscious and able to drink. Water, fluids and oral rehydration solution can be used. In severe cases, IV fluids should be used.
Electrolytes and acid base status should be monitored as well. In case of diabetic coma, rapid correction of dehydration with IV fluid bolus should be started followed by correction of acidosis, hyperkalemia/hypokalemia and hyperglycemia.
Breast feeding and normal diet should be continued as long as the treatment with fluid replacement is going to avoid losing weight or developmental delay.
Adults can use oral fluids if they are conscious and able to drink, otherwise intravenous fluids should be used.
In elderly patients, mild dehydration can progress to delirium and mood irritability. Fluid replacement should be started and it will be enough to control the symptoms. Sedatives or antipsychotic medications are not advised for these cases as simple fluid correction can be the only treatment needed.
Treatment of cases with isonatremic dehydration
Fluid bolus is started for restoration of the blood volume according to severity of the case followed by maintenance therapy with 5% dextrose in 9% saline. Oral intake should be encouraged as early as possible.
Treatment of cases with hyponatremia
Patients may present with acute cerebral edema. Early steps should be towards stabilization of the patient, securing the airway, breathing, and circulation. Management of hyponatremia follows with hypertonic saline 3% in severe cases with seizures or coma. Correction of hyponatremia should start at a rate of 4-6 mEq/L/hour.
In chronic hyponatremia, correction of sodium concentration should follow a rate of 10 mEq/L in the first 2 days. Rapid correction of hyponatremia with more than 2 mEq/L/h will lead to central pontine myelinolysis where permanent injury to the brain stem will occur and result in quadriplegia and cranial nerve paralysis.
Partial rapid correction is not associated with cerebral edema or pontine myelinolysis. Therefore, if a patient with severe symptomatic hyponatremia fluid bolus with 3% hypertonic saline is given to control symptoms of seizures or disturbed conscious level.
To calculate the sodium deficit:
Sodium deficit = (normal sodium level – serum level) * volume of distribution* weight.
Treatment of cases with hypernatremia
First step is volume restoration in cases with severe dehydration. Fluid bolus with isotonic saline or lactated ringer can be used. Next step is slow correction of the hypernatremia in a rate of 10 mEq/L/24hours to avoid cerebral edema and death as a complication. 5% dextrose in 0.9% sodium chloride can be used with frequent monitoring of the serum sodium every 4 hours.