Dehydration, adult

Dehydration in adults is a deficit of total body water, with or without sodium loss, that produces a reduction in extracellular and intracellular volume. Strictly, dehydration is loss of water (hypernatraemic), and volume depletion is loss of salt and water (isonatraemic or hyponatraemic); in practice they coexist, and both are managed by restoring volume and correcting sodium.

Clinical relevance. Dehydration is among the commonest reasons for emergency admission in older adults and in gastroenteritis, diabetic hyperglycaemia, heat illness and sepsis. Untreated it causes acute kidney injury, electrolyte disturbance, delirium, falls, thrombosis, shock and death. Over rapid correction of sodium can cause osmotic demyelination.

Physiology. Total body water is about 60% of body weight in men and 50% in women (less in older adults and obesity). Two thirds is intracellular. Osmolality is regulated by thirst and antidiuretic hormone; volume is regulated by the renin-angiotensin-aldosterone system and natriuretic peptides. Daily obligatory losses are about 1.5 litres urine, 0.5 to 1 litre insensible and 0.1 litre faecal.

Classification by sodium.

• Isonatraemic (sodium 135 to 145 mmol/L): proportionate loss, such as acute gastroenteritis and haemorrhage.
• Hypernatraemic (sodium over 145 mmol/L): water loss exceeds sodium loss, such as poor intake in older adults, diabetes insipidus, fever, osmotic diuresis.
• Hyponatraemic (sodium under 135 mmol/L): salt loss exceeds water loss, or water replacement with hypotonic fluid, such as diuretics, adrenal insufficiency, cerebral salt wasting, vomiting with water intake.

Classification by severity (as a percentage of body weight lost).

• Mild: 3% to 5% (thirst, dry mucosa).
• Moderate: 6% to 9% (orthostatic hypotension, tachycardia, oliguria, reduced skin turgor).
• Severe: 10% or more (hypotension, shock, altered consciousness, anuria).

Classification by cause.

• Reduced intake: dysphagia, dementia, immobility, delirium, sedation, poverty, lack of access to water, intentional restriction, nausea.
• Increased gastrointestinal loss: vomiting, diarrhoea, fistula, stoma, intestinal obstruction, laxatives.
• Renal loss: diuretics, osmotic diuresis (hyperglycaemia, mannitol, glycosuria from sodium glucose co-transporter 2 inhibitors), adrenal insufficiency, diabetes insipidus, post obstructive diuresis, recovery phase of acute tubular necrosis, hypercalcaemia, salt losing nephropathy.
• Skin and respiratory loss: fever, heat exposure, exercise, burns, tachypnoea.
• Third space loss: pancreatitis, bowel obstruction, peritonitis, ascites, sepsis.
• Haemorrhage.

History.

• Intake of fluid and food over days; thirst; dry mouth; vomiting, diarrhoea (volume, frequency, blood), polyuria, nocturia, reduced urine volume, dark urine; sweating, fever, hot environment, strenuous exercise.
• Dizziness on standing, falls, weakness, headache, muscle cramps, confusion, drowsiness, palpitations.
• Ability to drink: swallowing, access, dependency, cognition, mood, fear of incontinence.
• Chronic disease: diabetes, kidney disease, heart failure, cirrhosis, Addison disease, cancer, dementia, psychiatric illness.
• Drugs: diuretics, ACE inhibitors, angiotensin receptor blockers, NSAIDs, sodium glucose co-transporter 2 inhibitors, metformin, lithium, laxatives, antihypertensives, digoxin, chemotherapy.
• Symptoms of infection, polyuria with thirst (diabetes mellitus, hypercalcaemia, diabetes insipidus), weight loss, pregnancy.
• Alcohol and substance use.

Examination.

• Vital signs: pulse, blood pressure lying and standing (postural drop 20 mmHg systolic or 10 mmHg diastolic, rise in pulse over 30 beats/minute), temperature, respiratory rate, oxygen saturation, capillary glucose, urine output, weight (compared with baseline).
• Mucous membranes (dry tongue and axillae are more specific than skin turgor, which is unreliable in older adults), sunken eyes, capillary refill (over 2 seconds), jugular venous pressure (flat), skin turgor, mental state, absence of tears.
• Signs of cause: abdominal tenderness, distension, bowel sounds, stoma output, fever, source of infection, ascites, oedema, thyroid, pigmentation (Addison disease).
• Signs of overload in vulnerable patients: crackles, raised jugular venous pressure, oedema.

Red flags.

• Systolic blood pressure under 90 mmHg, pulse over 120, respiratory rate over 24, altered consciousness, capillary refill over 3 seconds.
• Urine output under 0.5 mL/kg/hour for 6 hours or anuria.
• Sodium under 120 mmol/L or over 160 mmol/L, potassium under 3.0 or over 6.0 mmol/L.
• Glucose over 20 mmol/L, ketones, or pH under 7.3.
• Bloody diarrhoea, severe abdominal pain, fever, peritonism.
• Acute kidney injury (creatinine rise of 26 micromol/L in 48 hours or 1.5 times baseline).
• Adrenal insufficiency (hyperpigmentation, hypotension, hyponatraemia, hyperkalaemia, hypoglycaemia).
• Heart failure, cirrhosis or renal failure requiring careful fluid balance.

Gastrointestinal loss. Infective gastroenteritis (viral, Salmonella, Shigella, Campylobacter, Escherichia coli, cholera, Clostridioides difficile, Giardia), inflammatory bowel disease, bowel obstruction, high output stoma or fistula, pancreatitis, malabsorption, laxative abuse, pyloric stenosis, hyperemesis gravidarum.

Renal and endocrine loss. Diuretic effect, diabetes mellitus with osmotic diuresis, diabetic ketoacidosis, hyperosmolar hyperglycaemic state, diabetes insipidus (central and nephrogenic), hypercalcaemia, adrenal insufficiency, hyperaldosteronism, post obstructive diuresis, recovering acute tubular necrosis, chronic kidney disease with salt wasting, sodium glucose co-transporter 2 inhibitors.

Skin and respiratory loss. Heat illness, fever, burns, extensive skin disease, hyperventilation, cystic fibrosis (salt loss in sweat).

Poor intake. Dementia, delirium, depression, stroke, dysphagia, oesophageal obstruction, oral lesions, nausea, anorexia, anorexia nervosa, frailty, poverty, neglect, abuse.

Third space loss and haemorrhage. Sepsis, pancreatitis, peritonitis, burns, trauma, gastrointestinal bleeding, ruptured aneurysm, ectopic pregnancy.

Mimics of dehydration. Heart failure or sepsis with hypotension, hypovolaemia from haemorrhage, adrenal crisis, hyponatraemia from SIADH (euvolaemic), primary polydipsia, cirrhosis with ascites (intravascular depletion), nephrotic syndrome.

1. Bedside assessment of volume status (postural vital signs, mucous membranes, capillary refill, jugular venous pressure, urine output) and identify the cause. Weigh the patient daily.
2. Core tests: urea, creatinine and estimated glomerular filtration rate (urea to creatinine ratio over 100 to 1 by mass suggests prerenal azotaemia), sodium, potassium, chloride, bicarbonate, glucose, calcium, magnesium, phosphate.
3. Also request full blood count (haemoconcentration, rising haematocrit, leucocytosis), lactate and venous gas, urinalysis.
4. Urine studies: urine sodium (under 20 mmol/L suggests prerenal; over 20 mmol/L with salt losing causes).
5. Urine osmolality: over 500 mOsm/kg suggests intact concentrating ability; under 300 with high plasma osmolality suggests diabetes insipidus.
6. Fractional excretion of sodium: under 1% prerenal; unreliable with diuretics, use urea.
7. Serum osmolality (2 x sodium + glucose + urea; effective osmolality 2 x sodium + glucose), anion gap, ketones.
8. Cause directed tests: stool microscopy, culture and Clostridioides difficile toxin, blood cultures, 08:00 cortisol or short synacthen test, thyroid function, HbA1c, calcium and parathyroid hormone, ECG for potassium disturbance, abdominal imaging (CT or ultrasound), chest radiograph, lactate, serum amylase or lipase, creatine kinase.
9. Water deprivation test or desmopressin test under specialist supervision for suspected diabetes insipidus.
10. Monitor serial sodium, potassium, creatinine and urine output during rehydration.

Limits. Skin turgor and dry mouth are poor markers in older adults and mouth breathers. Blood tests may be normal early. Urea rises with gastrointestinal bleeding, steroids and high protein intake independent of volume.

Assessment of severity and route.

• Mild to moderate with ability to drink and no vomiting or high risk comorbidity: oral rehydration.
• Moderate with persistent vomiting, severe diarrhoea, comorbidity, unable to drink: intravenous fluid in hospital.
• Severe or shock: emergency resuscitation.

Oral rehydration. Oral rehydration solution (sodium 75 mmol/L, glucose 75 mmol/L, osmolarity 245 mOsm/L): 50 to 100 mL/kg over 3 to 4 hours for moderate dehydration. Then give maintenance with ongoing loss replacement (about 200 mL after each loose stool). Use small frequent sips (5 mL every 1 to 2 minutes) if vomiting.

Prepare locally if sachets are unavailable: 1 litre clean water with 6 level teaspoons (about 27 g) sugar and half a level teaspoon (about 2.5 g) salt. Continue normal diet and breastfeeding.

Intravenous resuscitation (shock or severe dehydration). Give balanced crystalloid (Ringer lactate or Plasma-Lyte) or 0.9% sodium chloride 500 mL over 15 minutes (250 mL in older adults, heart failure or kidney disease). Reassess after each bolus.

Repeat to a maximum of about 2 litres in the first hour while looking for the cause (sepsis, bleeding). Stop when perfusion restores. Give blood for haemorrhage. Insert a urinary catheter when output monitoring is needed.

Rehydration plan (after resuscitation).

1. Deficit estimation: percentage dehydration x body weight (kg) x 10 = deficit in mL (for example 8% in a 70 kg adult is about 5.6 litres).
2. Maintenance: 25 to 30 mL/kg/day water (about 1.5 to 2.5 litres), sodium 1 mmol/kg/day, potassium 1 mmol/kg/day, with glucose 50 to 100 g/day.
3. Ongoing losses: replace measured losses (diarrhoea, stoma, vomit, nasogastric) volume for volume with a similar fluid.
4. Replace the deficit over 24 to 48 hours (about half in the first 8 to 12 hours), more slowly in older adults, heart disease and kidney disease.
5. Reassess every 4 to 6 hours: pulse, blood pressure, capillary refill, urine output (target over 0.5 mL/kg/hour), weight, sodium, potassium, creatinine.

Sodium disorders.

• Hypernatraemia (sodium over 145 mmol/L): replace deficit with oral water or 5% glucose or 0.45% sodium chloride after restoring perfusion with isotonic fluid.
• Free water deficit (litres) = 0.6 x body weight (kg) x (sodium/140 minus 1) (use 0.5 in women and older adults).
• Lower sodium by no more than 10 mmol/L in 24 hours (0.5 mmol/L/hour). Monitor sodium every 4 to 6 hours.
• Hyponatraemia with volume depletion (sodium under 135 mmol/L): isotonic saline.
• In chronic hyponatraemia (over 48 hours) raise sodium by no more than 8 to 10 mmol/L in the first 24 hours (and 18 mmol/L over 48 hours). In high risk patients (alcohol use disorder, malnutrition, hypokalaemia, liver disease) raise it by no more than 8 mmol/L.
• When the cause (hypovolaemia) is corrected, water diuresis can overcorrect sodium. Check sodium every 2 to 4 hours, and if it rises too fast give 5% glucose and desmopressin 2 micrograms intravenously.
• Severe symptoms (seizure, coma): hypertonic saline 3% 150 mL over 20 minutes, repeated up to twice, aiming to raise sodium by 5 mmol/L in the first hours.

Potassium. Potassium chloride 20 to 40 mmol in each litre of fluid, never faster than 10 mmol/hour peripherally (20 mmol/hour with monitoring centrally); dose to serum level. Treat hyperkalaemia (potassium 6.5 mmol/L or ECG changes): calcium gluconate 10% 30 mL intravenously over 10 minutes, insulin 10 units with glucose 25 g (50 mL of 50%) over 15 to 30 minutes, salbutamol 10 to 20 mg nebulised.

Cause directed treatment.

• Gastroenteritis: oral rehydration, antiemetic ondansetron 4 to 8 mg orally or intravenously (maximum 16 mg single dose), loperamide 4 mg then 2 mg after each loose stool (maximum 16 mg daily) only without fever or blood.
• Antibiotics for cholera: doxycycline 300 mg single dose or azithromycin 1 g single dose.
• Severe Shigella or Campylobacter: ciprofloxacin 500 mg twice daily for 3 days or azithromycin 500 mg daily for 3 days. Giardia: tinidazole 2 g single dose.
• Clostridioides difficile: vancomycin 125 mg orally four times daily for 10 days or fidaxomicin 200 mg twice daily for 10 days.
• Diabetic hyperglycaemia: fixed rate intravenous insulin 0.1 units/kg/hour with fluid and potassium for ketoacidosis.
• Adrenal insufficiency: hydrocortisone 100 mg intravenously then 200 mg over 24 hours, with saline.
• Diabetes insipidus: desmopressin 0.1 to 0.2 mg orally two to three times daily for central diabetes insipidus (10 to 20 micrograms intranasally once or twice daily); thiazide with low salt diet for nephrogenic diabetes insipidus; monitor sodium.
• Hypercalcaemia: 0.9% sodium chloride 3 to 4 litres in 24 hours, zoledronic acid 4 mg intravenously over 15 minutes.
• Drug induced: stop or reduce diuretics, ACE inhibitors, angiotensin receptor blockers, NSAIDs, metformin and sodium glucose co-transporter 2 inhibitors temporarily during acute illness ("sick day rules") and restart after 24 to 48 hours of normal eating and drinking.
• Heat illness: rapid cooling to under 39 degrees Celsius with fluid replacement.
• Sepsis: antibiotics within 1 hour, 30 mL/kg crystalloid for hypotension, vasopressors if mean arterial pressure under 65 mmHg after fluids.
• Poor intake in dementia and frailty: assisted drinking, scheduled fluid rounds, preferred drinks, treat dysphagia (thickened fluids), and review of goals of care.
• Give subcutaneous fluid (0.9% sodium chloride or glucose saline 1 litre over 8 to 12 hours) when oral intake fails and intravenous access is difficult.

Monitoring. Strict fluid balance chart, daily weight, sodium, potassium, creatinine every 12 to 24 hours initially, and consider telemetry for severe electrolyte disturbance. Watch for fluid overload (crackles, oedema, rising jugular venous pressure) in older adults and heart or kidney disease. Refeeding syndrome in malnourished patients (check phosphate, magnesium, potassium; thiamine 100 mg daily).

Referral and escalation. Critical care for shock, severe electrolyte disturbance or multi organ failure. Nephrology for acute kidney injury not improving in 24 to 48 hours. Endocrinology for adrenal insufficiency and diabetes insipidus. Surgery for obstruction or intra-abdominal cause. Geriatrics and dietetics for recurrent dehydration.

Admission. Inability to maintain oral intake, severe or moderate dehydration with comorbidity, electrolyte disturbance (sodium under 125 or over 155 mmol/L, potassium abnormal), acute kidney injury, hypotension, altered consciousness, suspected sepsis, unsafe circumstances for discharge, or diabetic emergency.

Safety netting. Return if unable to drink, passing little urine for 8 hours, dizziness or fainting, confusion, worsening diarrhoea or vomiting, blood in stool, fever, severe abdominal pain, or no improvement within 48 hours.

Special groups. Older adults: reduced thirst, kidney concentrating ability and mobility; fluid is given cautiously and reassessed often. Pregnancy: hyperemesis gravidarum needs intravenous fluids, thiamine and antiemetics. Heart failure and cirrhosis: balance fluid and salt, and give boluses of 250 mL. Kidney disease: avoid nephrotoxins, watch potassium. Athletes and workers: heat illness, replace sodium.

Cholera and epidemics: aggressive early oral and intravenous rehydration saves lives; give 100 mL/kg over 3 to 4 hours for severe dehydration, half in the first 30 minutes where intravenous access exists.