Define Intravenous Fluid & Electrolyte Therapy. Describe its types and complications.

Start with the Compartments — It Explains Everything

To prescribe fluids sensibly you must first know where water sits in the body, because this determines where any fluid you infuse will end up. Total body water is about 60% of body weight — roughly 42 litres in a 70 kg man. Of this, two-thirds is intracellular (ICF, ~28 L) and one-third is extracellular (ECF, ~14 L). The ECF is itself split, with three-quarters in the interstitium (~10.5 L) and only one-quarter in the plasma/intravascular space (~3.5 L). That last figure is the key: the intravascular compartment — the one that keeps the circulation going — is small, and different fluids reach it to very different degrees.

Types of Fluid — and Where Each One Goes

This is the heart of the topic. Fluids are either crystalloids or colloids:

• Isotonic crystalloids (0.9% saline, Ringer lactate/Hartmann's) — distribute throughout the whole ECF. Since only about a quarter of the ECF is intravascular, only ~25% of what you infuse stays in the circulation — so you need roughly 3 times the volume of blood lost to refill the intravascular space. These are the fluids for resuscitation and replacement.
• 5% dextrose — the glucose is quickly metabolised, leaving free water that distributes across total body water. Almost none stays intravascular, so it is useless for resuscitation; it is used to provide free water for maintenance.
• Colloids (albumin, gelatins, starches) — contain large molecules that exert oncotic pressure and so stay in the intravascular space longer, expanding plasma volume with a smaller infused volume — but they are costlier and carry specific risks (allergy; starches are now largely avoided).

The Three Purposes of Fluid Therapy

Every fluid prescription serves one of three distinct purposes, and muddling them causes errors:

Maintenance Requirements

A person at rest needs roughly 30–35 mL/kg/day of water, about 1–2 mmol/kg/day of sodium, and about 1 mmol/kg/day of potassium, plus some glucose to limit ketosis. In children the Holliday-Segar ('4-2-1') rule is used: 4 mL/kg/h for the first 10 kg, 2 mL/kg/h for the next 10 kg, and 1 mL/kg/h for each kg thereafter.

Assessing Fluid Status

Good prescribing depends on assessment. Look for dehydration (thirst, dry mucous membranes, reduced skin turgor, tachycardia, low JVP/CVP, oliguria, and in severe cases hypotension) or overload (raised JVP, oedema, basal crackles, gallop rhythm). Combine the history, a careful examination, the urine output, and where needed the CVP and biochemistry, then reassess frequently — fluid therapy is a continuous adjustment, not a single decision.

Composition of Common IV Fluids

Complications of Fluid Therapy

Fluids are drugs and can harm. Under-resuscitation leaves the patient in shock with organ hypoperfusion. Over-resuscitation causes pulmonary and peripheral oedema, and tissue oedema that impairs wound and gut healing; large volumes of 0.9% saline additionally cause a hyperchloraemic metabolic acidosis (which is why balanced solutions like Hartmann's are often preferred). The remedy is careful assessment and frequent reassessment rather than a fixed prescription.

'third-space' Losses

In major surgery, sepsis or bowel obstruction, fluid is sequestered into a non-functional 'third space' (the inflamed gut wall, peritoneal cavity or interstitium). This fluid is lost from the circulation even though it remains in the body, causing hypovolaemia that must be replaced — and it later mobilises back into the circulation during recovery, a shift that must be anticipated to avoid overload.

Total body water is two-thirds intracellular, one-third extracellular.

• Total body water ≈ 60% of weight; 2/3 intracellular, 1/3 extracellular; of the ECF only ~1/4 (≈3.5 L) is intravascular.
• Isotonic crystalloid distributes through the ECF (only ~25% stays intravascular → need ~3× volume lost); 5% dextrose spreads across all body water (useless for resuscitation); colloids stay intravascular longer.
• Resuscitate with isotonic crystalloid/blood, never 5% dextrose.
• Three purposes: maintenance (normal losses), resuscitation (restore volume), replacement (match abnormal losses).
• Maintenance ≈ 30–35 mL/kg/day water + Na 1–2 mmol/kg + K 1 mmol/kg (children: 4-2-1 rule); assess status clinically + urine output/CVP and reassess.