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Intravenous (IV) solutions are sterile fluids administered directly into the bloodstream to replace fluid and electrolytes, deliver medications, or provide nutrition. Selecting the correct IV fluid requires understanding tonicity — the osmotic effect of the fluid on cells. Isotonic fluids (osmolarity ~285–310 mOsm/L) do not cause fluid shifts; hypertonic fluids draw water from cells; hypotonic fluids cause cellular swelling. The most commonly used IV fluids are normal saline (0.9% NaCl), lactated Ringer's solution, and dextrose solutions (D5W, D5NS).

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Common IV Solutions and Tonicity

  • Normal saline (0.9% NaCl): 308 mOsm/L; isotonic; 154 mEq/L Na⁺ and Cl⁻ each; resuscitation, medication diluent, post-surgical hydration
  • Lactated Ringer's (LR): 273 mOsm/L; isotonic (slightly hypotonic); contains Na⁺, K⁺, Ca²⁺, Cl⁻, lactate; more physiologic than NS; preferred for large-volume resuscitation; not for liver failure (impaired lactate metabolism)
  • D5W (5% dextrose in water): 252 mOsm/L; isotonic in bag → becomes hypotonic in body (dextrose is rapidly metabolized → free water remains); maintenance fluid; not for hyponatremia
  • D5NS (D5W + 0.9% NaCl): 560 mOsm/L; hypertonic; maintenance with sodium supplementation
  • 3% NaCl (hypertonic saline): 1026 mOsm/L; hypertonic; treats severe hyponatremia and cerebral edema
  • 0.45% NaCl (half-normal saline): 154 mOsm/L; hypotonic; free water + electrolytes; maintenance fluids

Tonicity Effects on Cells

Isotonic: no net water movement; cells maintain normal volume. Hypertonic: water leaves cells → crenation (shrinkage). Hypotonic: water enters cells → swelling; in red blood cells → hemolysis; in brain → cerebral edema.

Fluid Compartments

After IV infusion, fluid distribution depends on composition: NS/LR → mostly extracellular (75% interstitial, 25% intravascular). D5W → distributes throughout total body water (only 8% remains intravascular). Blood products → stay mostly intravascular.

Glossary

Normal Saline (0.9% NaCl)
Isotonic IV solution: 308 mOsm/L; 154 mEq/L Na⁺ and Cl⁻; used for resuscitation and drug dilution; large volumes cause hyperchloremic metabolic acidosis.
Tonicity
The osmotic effect of a solution on cell volume: isotonic (270–310 mOsm/L) = no cell volume change; hypertonic = cell shrinkage; hypotonic = cell swelling; determines appropriate IV fluid selection.
Lactated Ringer's (LR)
Balanced isotonic IV solution with Na⁺, K⁺, Ca²⁺, Cl⁻, and lactate; 273 mOsm/L; preferred for large-volume resuscitation; avoids hyperchloremic acidosis compared to normal saline.

Frequently Asked Questions

Normal saline (0.9% NaCl): 154 mEq/L each of Na⁺ and Cl⁻; 308 mOsm/L; isotonic. Lactated Ringer's: 130 mEq/L Na⁺, 4 mEq/L K⁺, 3 mEq/L Ca²⁺, 109 mEq/L Cl⁻, 28 mEq/L lactate; 273 mOsm/L; more physiologically balanced. LR is preferred for large-volume resuscitation (≥2 L) because NS causes hyperchloremic metabolic acidosis due to high chloride load. LR is avoided in liver failure (impaired lactate metabolism) and hyperkalemia (contains K⁺). NS is preferred for drug diluent compatibility and head trauma (slightly hypertonic relative to LR).

Tonicity describes the osmotic effect of a solution on cells relative to plasma (280–295 mOsm/L). Isotonic solutions (270–310 mOsm/L) cause no net water movement across cell membranes — cells maintain normal volume. Hypertonic solutions (>310 mOsm/L) draw water out of cells; used for hyponatremia (3% NaCl) and cerebral edema (20% mannitol). Hypotonic solutions (<270 mOsm/L) allow water into cells; can cause cellular swelling and in the brain, potentially dangerous cerebral edema; used carefully for free water replacement in hypernatremia. D5W is isotonic in the bag but becomes hypotonic in vivo as glucose is metabolized.

D5W (5% dextrose, 252 mOsm/L): used primarily for free water replacement (hypernatremia, dehydration without electrolyte deficit), drug diluent when sodium restriction is needed, and providing minimal calories during short-term NPO. D5W should NOT be used for resuscitation (distributes to total body water — only ~8% stays intravascular), and not for hyponatremia (worsens sodium dilution). Normal saline: resuscitation, volume expansion, medication dilution, post-surgical hydration, hyponatremia treatment (isotonic or hypertonic saline). In general: NS and LR are the workhorses for resuscitation; D5W and hypotonic solutions are used for maintenance and electrolyte correction.

Large volumes of 0.9% NaCl (NS) deliver a Cl⁻ load (154 mEq/L) well above plasma Cl⁻ (98–107 mEq/L). Excess Cl⁻ competes with HCO₃⁻ for renal reabsorption — the kidney excretes HCO₃⁻ to maintain electroneutrality → serum HCO₃⁻ falls → non-anion gap metabolic acidosis. The anion gap is normal (hyperchloremia increases the measured Cl⁻, mathematically reducing the gap). Seen after large NS resuscitations (≥2–3 L). Clinical importance: hyperchloremic acidosis may worsen outcomes and impair renal function. Lactated Ringer's and Plasma-Lyte are chloride-balanced alternatives that avoid this complication.