Adult Dehydration

Etiology

Body water is lost through the skin, lungs, kidneys, and GI tract. The loss of body water without sodium causes dehydration. Water is lost from the skin, lungs, gastrointestinal tract, and kidneys. Dehydration results when water losses from the body exceed water replacement. It may be caused by failure to replace obligate water losses. There are several forms of dehydration.[1] Isotonic water loss occurs when water and sodium are lost together. Causes of isotonic water loss are vomiting, diarrhea, sweating, burns, intrinsic kidney disease, hyperglycemia, and hypoaldosteronism. Hypertonic dehydration occurs when water losses exceed sodium losses. Serum sodium and osmolality are always elevated in hypertonic dehydration. Excess pure water loss occurs through the skin, lungs, and kidneys. Etiologies are fever, increased respiration, and diabetes insipidus. Hypotonic dehydration is mostly caused by diuretics, which cause more sodium loss than water loss. Hypotonic dehydration is characterized by low sodium and osmolality.

The source of water loss may also understand the etiologies of dehydration:

• Failure to replace water loss: altered mentation, immobility, impaired thirst mechanism, drug overdose leading to coma
• Excess water loss from the skin: heat, exercise, burns, severe skin diseases
• Excess water loss from the kidney: medications such as diuretics, acute and chronic renal disease, post-obstructive diuresis, salt-wasting tubular disease, Addison disease, hypoaldosteronism, hyperglycemia
• Excess water loss from the GI tract: vomiting, diarrhea, laxatives, gastric suctioning, fistulas
• Intraabdominal losses: pancreatitis, new ascites, peritonitis
• Excess insensible loss: sepsis, medications, hyperthyroidism, asthma, chronic obstructive pulmonary disease, drugs

Epidemiology

There is no recent data on dehydration rates in the general population, but we know much about dehydration's epidemiology in adults. Healthy adults with access to water rarely become dehydrated. Any adult may develop dehydration as a complication of an illness such as hyperglycemia. Dehydration may cause illness or be caused by an illness, so searches of databases may not capture all cases of dehydration. The data that we do have shows that older adults are more likely to develop dehydration. The elderly population is also 20% to 30% more prone to developing dehydration due to immobility, impaired thirst mechanism, diabetes, renal disease, and falls.[2][3]

Pathophysiology

Water plays a key role in maintaining multiple physiological functions within the body. The human body is 55% to 65% of water. Two-thirds of that water is intracellular, and one-third is extracellular. One-fifth of extracellular water is intravascular. The body has a complex system designed to maintain euvolemia. Water is absorbed through the gastrointestinal tract. The primary control of water homeostasis is through osmoreceptors in the brain. As perceived by these osmoreceptors, dehydration stimulates the thirst center in the hypothalamus, which leads to water consumption. These osmoreceptors can also help the kidneys conserve water. When the hypothalamus detects lower water concentration, it causes the posterior pituitary to release antidiuretic hormone, which stimulates the kidneys to reabsorb more water. Decreased blood pressure, which often accompanies dehydration, triggers renin secretion from the kidney. Renin converts angiotensin I to angiotensin II, which increases aldosterone release from the adrenals. Aldosterone increases the absorption of sodium and water from the kidney. The body uses these mechanisms to regulate volume and sodium and water concentration.

History and Physical

Hypovolemic patients can present with a wide assortment of symptoms and physical exam findings. Some of the most common symptoms of dehydration include but are not limited to fatigue, thirst, dry skin and lips, dark urine or decreased urine output, headaches, muscle cramps, lightheadedness, dizziness, syncope, orthostatic hypotension, and palpitations. The patient's history may elicit factors that could cause dehydration, such as exercise, heat exposure, medications, illness, impaired access to water, fever, or fluid loss.

Vital signs may show hypotension, tachycardia, fever, and tachypnea. Hypotension does not appear until significant dehydration is present. Tachycardia may be absent due to medications such as beta-blockers. A patient may appear lethargic or obtunded upon observation in severe cases of dehydration. The physical examination could show dry mucosa, skin tenting, delayed capillary refill, or cracked lips. A 2015 Cochrane review evaluated predictors of dehydration in the elderly.[4] Historical and physical findings tested were dry axilla, mucous membranes, tongue, increased capillary refill time, poor skin turgor, sunken eyes, orthostatic blood pressure drop, dizziness, thirst, urine color, weakness, blue lips, altered mentation, tiredness, and appetite. Of all these factors, only fatigue and missed drinks between meals predicted the diagnosis of dehydration.

Evaluation

There is no gold standard test for dehydration. Serum and plasma osmolality tests are often used to diagnose but may be affected by fluid loss or fluid loss acuity. A reasonable definition of dehydration due to water loss is serum osmolality greater than or equal to 295 mOsm/kg. The 2015 Cochrane review used serum osmolality greater than 294 mOsm/kg to define dehydration. Weight loss equal to or greater than 3% over 7 days may also indicate dehydration if this data is available. The 2015 Cochrane review of diagnostic tests for dehydration in elderly patients, bioelectrical impedance analysis, urine specific gravity, the osmolality of urine, saliva, or tears, tear volume, number of urine voids, and urine volume were not useful as stand-alone tests for dehydration in the elderly. 

Blood urea nitrogen to creatinine ratio should be higher than 10:1 in dehydration. However, this may be mimicked by high urea production, low creatinine due to low muscle loss, and urea reabsorption due to upper gastrointestinal bleeding. Urine tests may suggest volume depletion. Urine sodium concentration should be low, fractional excretion of sodium should be under 1%, and urine osmolality should be greater than 450 mOsm/kg. These tests of renal perception of low blood flow may also be abnormal in heart failure, cirrhosis, nephrotic syndrome, and other causes of kidney disease.

Ultrasound can assess a patient’s fluid volume by measuring the collapsibility of the inferior vena cava (IVC) with respiration. A variation in the diameter of the IVC greater than 50% with respiration indicates a collapsible IVC. IVC collapse with inspiration may correlate with right atrial pressure and intravascular volume.[5] Ultrasound evaluation of the IVC may be influenced by cirrhosis, chronic heart conditions, and ventilation (spontaneous or mechanical). IVC ultrasound has a limited ability to predict fluid responsiveness.[6] It may be used as part of the entire clinical picture.

Treatment / Management

Treatment of dehydration is aimed at rapid fluid replacement and identification of the cause of fluid loss. Patients with fluid deficits should be given isotonic fluid boluses tailored to the individual circumstance. Patients with more severe dehydration get larger boluses of isotonic fluid. A more careful approach is needed in elderly patients and patients with heart failure and kidney failure. Small boluses should be given in these patients, followed by frequent reassessment and additional bolus as needed.[7]

Blood pressure, heart rate, serum lactate, hematocrit (if bleeding, no blood loss), and urine output may be used to assess the volume deficit and response to fluids.

Isotonic crystalloid fluid should be used in most cases of dehydration. Colloids such as albumin may be used in specific situations but do not improve outcomes.[8](A1)

The choice of crystalloid should be customized to the patient. Normal saline lactated Ringer solution and a balanced crystalloid solution may all be used. Normal saline may cause hyperchloremic metabolic acidosis in large volumes. Buffered crystalloids may cause hyponatremia. Lactated Ringer solution also contains potassium, so it should not be used in renal failure or hyperkalemia. No fluid has proved superior in all patients.

Rapid volume repletion may cause a rapid rise in sodium in patients with dehydration and severe hyponatremia, which can cause central pontine myelinolysis. The clinician must weigh the risks of continued dehydration against the risks of central pontine myelinolysis. The patient's volume status and serum sodium must be followed closely.

As the patient is being resuscitated, clinical and laboratory examinations must focus on discovering and correcting the cause of dehydration.

Differential Diagnosis

The principal differential of dehydration in adults is the loss of body water versus the loss of blood. This is important because blood loss should be replaced with blood, while water loss should be replaced with fluid. The next point to consider is the differential diagnosis of the cause of dehydration, as discussed under etiology.

Prognosis

Most patients recover fully when the underlying cause of dehydration is treated, and the patient's volume has been restored. Failure to treat dehydration in older adults may lead to significant mortality.[9]

Complications

Complications of dehydration occur due to inadequate fluid replacement and over-aggressive fluid replacement. Complications of under-resuscitation are due to hypoperfusion of vital organs and complications due to renal efforts to retain fluid. Altered mental status, renal failure, shock liver, lactic acidosis, hypotension, and death are related to organ hypoperfusion. Fluid and electrolyte abnormalities such as uremia, hyponatremia, hypernatremia, hypokalemia, hyperkalemia, metabolic acidosis, and metabolic alkalosis may occur. Excess fluid administration to correct dehydration may cause peripheral edema and pulmonary edema. In patients with severe hyponatremia, volume correction may cause a rapid rise in sodium, which can cause central pontine myelinolysis.