Table of Contents
Overview – Fluid Imbalances
Fluid imbalances refer to disruptions in the volume of extracellular fluid within the body, commonly seen as either hypervolaemia (fluid overload) or hypovolaemia (fluid depletion). These imbalances are critical in both acute and chronic medical settings, affecting blood pressure, perfusion, and electrolyte balance. A firm grasp of the causes, symptoms, and treatments of volume disorders is essential for final-year medical students preparing for clinical exams and practice.
Definition
Fluid or volume imbalances involve gains or losses of extracellular fluid, often in association with sodium disturbances, and can significantly affect cardiovascular and renal function.
Hypervolaemia
Definition
- An excess of extracellular fluid, usually accompanied by sodium retention
Causes
- Excessive fluid intake (oral or intravenous)
- Chronic renal failure (↓ urine output)
- Endocrine disorders (e.g., ↑ ADH or aldosterone activity)
Clinical Features
- Hypertension
- Peripheral oedema (due to increased hydrostatic pressure)
- Weight gain
- Jugular venous distension (in advanced cases)
- Pulmonary oedema (if severe)
Treatment
- Diuretics to promote fluid and sodium excretion
- Address underlying endocrine or renal causes
Hypovolaemia
Definition
- A loss of extracellular fluid, often associated with sodium depletion
Causes
- Inadequate fluid intake
- Gastrointestinal losses – vomiting, diarrhoea
- Haemorrhage
- Endocrine dysfunction (e.g., adrenal insufficiency, ↓ aldosterone)
Clinical Features
- Hypotension
- Tachycardia (compensatory)
- Increased respiratory rate
- Thirst
- Dry mucous membranes
- Reduced skin turgor
- Oliguria
Treatment
- Fluid resuscitation:
- IV isotonic saline for rapid correction
- Oral rehydration with electrolyte-rich fluids in milder cases
- Identify and treat the underlying cause
Differential Diagnosis
- Syndrome of inappropriate ADH (SIADH)
- Congestive heart failure (as cause of hypervolaemia)
- Sepsis with third-spacing (as cause of hypovolaemia)
- Nephrotic syndrome
- Cirrhosis (may present with features of both)
Fluid Replacement Therapy
Restoring volume may involve crystalloids, colloids, or blood products, depending on the clinical scenario.
Crystalloid Solutions
1. Normal Saline (0.9% NaCl)
- Isotonic → No fluid shift between compartments
- Use: General extracellular volume replacement
- Caution: Can dilute haemoglobin → ↓ O₂-carrying capacity
2. Dextrose Saline (5% Dextrose in 0.9% NaCl)
- Use: If patient is hypoglycaemic or hypernatraemic
- Becomes hypotonic after metabolism → Risk of fluid overload
3. Lactated Ringer’s / Hartmann’s Solution
- Contains: Na⁺, Cl⁻, K⁺, Ca²⁺, Lactate
- Use: Trauma, burns, haemorrhage, acidosis buffering
Colloid Solutions
1. Albumin
- 4% (40 g/100 mL): Used in sepsis, liver disease, surgery
- 20% (200 g/100 mL): Used in severe hypoalbuminaemia, plasma loss, burns
2. Polygeline (Haemaccel)
- Gelatin-based colloid
- Use: GI-related dehydration (vomiting, diarrhoea)
- Expands plasma volume and maintains oncotic pressure
Blood Products
1. Whole Blood
- Contains: RBCs, WBCs, platelets, plasma, electrolytes
- Use: Massive haemorrhage and oxygen-carrying restoration
2. Packed Red Blood Cells (RBCs)
- Use: Increase haematocrit in anaemia or haemorrhage
3. Plasma
- Contains: Clotting factors, albumin, fibrinogen, electrolytes
- Use: Plasma volume expansion and coagulation support
Summary – Fluid Imbalances
Fluid imbalances, including hypervolaemia and hypovolaemia, are disturbances in extracellular volume regulation that impact blood pressure and tissue perfusion. Hypervolaemia is marked by oedema and hypertension, while hypovolaemia leads to hypotension, tachycardia, and thirst. Treatment includes diuretics or fluid replacement depending on the underlying issue. For a broader context, see our Renal Overview page.