Extracellular Fluid Balance

Also written as extracellular compartment.

Extracellular fluid (ECF) volume and composition are tightly regulated by sodium, water, and hormones. Disruption leads to oedema, dehydration, or electrolyte disorders, which are common in low‑resource settings.

Definition and diagnosis

ECF includes plasma and interstitial fluid that bathes cells. Clinically we assess volume (hypovolaemia, euvolaemia, hypervolaemia) and composition (sodium, potassium, chloride, bicarbonate, calcium, phosphate).

• Measure blood pressure, pulse, skin turgor, and urine output.
• Check serum Na+, K+, Cl‑, HCO3‑, Ca2+, PO4‑ and osmolality.

Severity and staging

Volume disturbances are staged by clinical signs and simple labs:

• Mild: orthostatic drop ≤10 mmHg, urine <0.5 mL/kg/h.
• Moderate: orthostatic drop 10‑20 mmHg, urine 0.3‑0.5 mL/kg/h.
• Severe: orthostatic drop >20 mmHg, urine <0.3 mL/kg/h, altered mental status.

Causes, most likely first

In East African hospitals the common precipitants are:

• Gastro‑intestinal loss (cholera, dysentery, malaria‑related diarrhoea).
• Renal loss (diuretic over‑use, acute tubular necrosis from severe malaria).
• Third‑space loss (peritoneal TB, severe pneumonia, burns).
• Excess intake (hypertonic saline infusions, herbal decoctions).

How it presents

Hypovolaemia presents with tachycardia, low BP, dry mucosa, decreased skin turgor, and oliguria. Hypervolaemia shows peripheral oedema, raised JVP, crackles, and possibly hypertension.

• Rapidly falling BP suggests severe dehydration or haemorrhage.
• Pitting oedema in dependent limbs points to fluid overload.

Investigations

• Serum sodium: Is the patient hyponatraemic, normonatraemic or hypernatraemic? Guides fluid type (isotonic vs hypotonic).
• Serum potassium: Detects K+ loss (diarrhoea, diuretics) or excess (renal failure). Influences choice of rehydration solution.
• Serum creatinine & urea: Assesses renal contribution to volume status; rising creatinine signals acute kidney injury.
• Urine output (catheter or bedside collection): Quantifies renal perfusion; <0.5 mL/kg/h suggests hypovolaemia or renal failure.
• Point‑of‑care ultrasound (if available): Looks for IVC collapsibility (hypovolaemia) or pleural effusions (hypervolaemia).
• If ultrasound unavailable – bedside assessment of neck veins and lung sounds: Provides surrogate for volume status; flat neck veins and clear lungs favour hypovolaemia.

Red flags and complications

Uncorrected ECF disturbances can rapidly become fatal.

• Severe hypovolaemia → shock, acute kidney injury, myocardial ischaemia.
• Severe hypernatraemia → cerebral dehydration, seizures, coma.
• Rapid correction of chronic hyponatraemia → osmotic demyelination.

Management

Treat the underlying cause first, then restore volume and electrolyte balance.

• Isotonic crystalloid (0.9 % saline) for hypovolaemia due to loss of Na+ and water.
• Balanced solutions (Ringer's lactate) when large volumes needed and metabolic acidosis present.
• Hypertonic saline (3 % NaCl) only for severe symptomatic hyponatraemia; avoid rapid >10 mmol/L rise in 24 h.
• Potassium replacement (IV KCl) when serum K+ <3.5 mmol/L and urine output adequate.
• Diuretics (loop) for hypervolaemia with pulmonary oedema; avoid if renal failure or severe hypovolaemia.

Follow‑up and prevention

Re‑assess vitals, urine output, and electrolytes every 4‑6 h until stable. Educate patients on safe fluid intake, early treatment of diarrhoea, and avoiding excessive herbal fluids.

• Provide oral rehydration salts (ORS) for mild‑moderate dehydration.
• Vaccinate against cholera and rotavirus where available.
• Monitor chronic diseases (heart failure, CKD) that predispose to fluid overload.

Often confused with

Conditions that mimic ECF disturbances:

• Anemia – can cause tachycardia but does not change fluid volume.
• Septic shock – may present with warm extremities and low BP but is driven by vasodilation, not pure volume loss.

Where results can mislead

Lab values may be deceptive in certain settings.

• Serum sodium may appear normal in hyperglycaemia; correct using Na_corrected = Na_measured + 1.6 × [(glucose‑100)/100].
• Urine output can be low in early hypovolaemia despite adequate intravascular volume if renal perfusion is delayed.

Questions an examiner may ask

• A 45‑year‑old man with severe cholera presents with 12 L diarrhoea in 24 h – what is your immediate fluid plan and why? — Give isotonic saline bolus 20 mL/kg, reassess vitals, then switch to ORS once stable; sodium replacement is essential to restore ECF volume.
• A patient on anti‑TB therapy develops peripheral oedema and hypertension – which electrolyte abnormality should you suspect and how would you confirm it? — Suspect hypernatraemia from excessive saline; check serum Na+; treat by adjusting fluid type.
• How does point‑of‑care ultrasound differentiate hypovolaemia from hypervolaemia? — IVC collapsibility >50 % indicates low intravascular volume; a plethoric IVC with hepatic vein congestion suggests fluid overload.
• Why is balanced crystalloid preferred over normal saline in large‑volume resuscitation for severe malaria? — Balanced solutions contain less chloride, reducing risk of hyperchloraemic metabolic acidosis common in massive saline infusions.
• A child with sickle cell disease presents with vaso‑occlusive crisis and low urine output – what fluid strategy avoids worsening sickling? — Use isotonic saline cautiously; avoid rapid over‑hydration that may cause pulmonary oedema, monitor electrolytes closely.
• What red‑flag sign would prompt you to call senior staff in a patient with suspected hypernatraemia? — New onset seizures or decreased consciousness indicates severe hypernatraemia requiring urgent specialist input.

When to seek help

• Unresponsive patient with suspected severe electrolyte disturbance.
• Persistent hypotension despite fluid bolus.
• Rapidly rising serum sodium (>150 mmol/L) or falling (<120 mmol/L).

Quick questions

Why is sodium the principal extracellular cation?

Sodium determines plasma osmolality, drives water movement, and maintains ECF volume.

Can oral rehydration replace IV fluids?

Yes for mild‑moderate dehydration; IV is required for severe hypovolaemia, shock, or when oral intake is impossible.

What is the danger of correcting chronic hyponatraemia too fast?

Rapid rise can cause osmotic demyelination syndrome, leading to irreversible neurologic injury.