Hypovolaemia

Hypovolaemia, or hypovolaemia in British spelling, is a clinical condition defined by a reduced circulating blood volume due to loss of intravascular fluid or a shift of fluid out of the vascular space. The decreased intravascular volume impairs venous return to the heart, lowers cardiac output, and can lead to organ hypoperfusion if not corrected. Hypovolaemia can beAbsolute, from actual loss of body fluids (haemorrhage, burns, vomiting or diarrhoea with fluid loss, prolonged fever with sweating, or severe diuretic use), or Relative, from redistribution of fluids into the interstitial or third-space compartments (sepsis, pancreatitis, anaphylaxis, or extensive burns).

Causes include acute blood loss from trauma or surgery; gastrointestinal losses such as vomiting or diarrhoea; skin losses from burns or excessive sweating; polyuria from diabetes or diuretic therapy; and conditions causing fluid shifts into the interstitial space. Relative hypovolaemia occurs with vasodilation and fluid sequestration in third spaces during severe infection or allergic reactions.

Pathophysiology centers on diminished venous return and reduced stroke volume, triggering compensatory tachycardia and peripheral vasoconstriction. If fluid loss is not promptly corrected, tissue perfusion deteriorates and shock can ensue. Activation of the renin–angiotensin–aldosterone system and sympathetic nervous system attempts to preserve perfusion.

Clinical features commonly include tachycardia, hypotension, orthostatic dizziness, dry mucous membranes, reduced skin turgor, decreased urine output, and altered mental status. In children, signs may be subtler but capillary refill can be delayed.

Diagnosis relies on history and examination, with supporting data from laboratory tests (raised haematocrit or BUN-to-creatinine ratio in dehydration) and continuous monitoring of urine output and vital signs.

Management focuses on identifying and treating the underlying cause and restoring intravascular volume with isotonic crystalloids (and blood products in haemorrhagic shock). Target is adequate tissue perfusion, guided by clinical response and urine output, while avoiding fluid overload.