Hypovolemia

Hypovolaemia is a reduction in effective circulating blood volume, from loss of extracellular fluid (volume depletion) or blood, with or without sodium loss.

• Causes:
  • Gastrointestinal: vomiting, diarrhoea, fistulae, bowel obstruction, ileus.
  • Renal: diuretics, osmotic diuresis (hyperglycaemia), salt-losing nephropathy, post-obstructive diuresis, adrenal insufficiency, cerebral salt wasting.
  • Skin and respiratory: burns, heat, sweating, fever, hyperventilation.
  • Third space: pancreatitis, peritonitis, sepsis, burns, bowel obstruction.
  • Haemorrhage: trauma, GI bleeding, obstetric.
  • Reduced intake: elderly, dysphagia, dementia.
• Distinguish isotonic volume depletion from pure water loss (hypernatraemia) and dehydration.
• Complications: prerenal acute kidney injury, shock, electrolyte disturbance, lactic acidosis, organ failure, thrombosis.

• Thirst, postural dizziness, weakness, oliguria, dark urine, dry mucous membranes, reduced skin turgor (unreliable in elderly), low JVP, tachycardia, postural hypotension (rise in heart rate over 30 or fall in systolic over 20 mmHg), cool extremities, delayed capillary refill.
• Haemorrhagic shock stages (ATLS): class I under 15% (minimal changes), II 15–30% (tachycardia, narrow pulse pressure), III 30–40% (hypotension, confusion), IV over 40% (obtunded, imminent arrest).
• Weight loss, infants: sunken fontanelle, absent tears, reduced urine output.
• Signs of the cause.

• Clinical examination, vital signs and orthostatic measurements.
• Laboratory: urea and creatinine (urea:creatinine over 100:1 or BUN:Cr over 20), sodium, potassium, bicarbonate, glucose, lactate, haemoglobin and haematocrit (may be normal in acute bleeding), urinalysis, urine sodium under 20 mmol/L, FENa under 1% (unreliable on diuretics; FEurea under 35%), urine osmolality over 500 mOsm/kg.
• Point-of-care ultrasound: small collapsing IVC (diameter under 1 cm, collapse over 50%), passive leg raise response.
• Determine cause: stool studies, glucose, cortisol, imaging (CT for bleeding, obstruction), endoscopy.
• Dynamic assessment: stroke volume variation, pulse pressure variation in ventilated patients.

• Mild-moderate and tolerating oral intake: oral rehydration solution (WHO ORS: sodium 75 mmol/L, glucose 75 mmol/L) 50–100 mL/kg over 4 hours.
• Moderate-severe: IV isotonic crystalloid (0.9% saline or balanced solution such as Ringer lactate) 250–500 mL boluses (20 mL/kg in children) reassessing after each; for ongoing losses, replace volume for volume.
• Shock: rapid fluid resuscitation (1–2 L) while identifying cause; blood products for haemorrhage (1:1:1 transfusion ratios in massive haemorrhage, tranexamic acid 1 g IV over 10 minutes then 1 g over 8 hours within 3 hours of trauma), permissive hypotension in trauma, vasopressors only when fluid-refractory (norepinephrine 0.05–0.5 micrograms/kg/min).
• Prefer balanced crystalloids to saline in large volumes to avoid hyperchloraemic acidosis; use albumin 4–5% in cirrhosis; avoid starches.
• Correct electrolytes (potassium, magnesium), glucose (insulin for DKA after fluids), acidosis.
• Treat the cause: antiemetics (ondansetron 4–8 mg), antidiarrhoeals (when appropriate), stop diuretics, control bleeding, antibiotics for sepsis, hydrocortisone 100 mg IV for adrenal insufficiency.
• Monitor urine output (0.5–1 mL/kg/h), heart rate, blood pressure, lactate, daily weight, serum sodium and creatinine.
• Avoid overcorrection and pulmonary oedema, especially in heart failure and renal impairment.