Types of parenteral fluids
Parenteral fluids are divided into Crystalloids (water + electrolytes/small molecules) and Colloids (water + large molecular weight substances).
1. Crystalloids: Isotonic
Do not cause significant fluid shifts between intracellular and extracellular compartments. Primary use: Intravascular volume expansion/Resuscitation.
- 0.9% Normal Saline (NS)
- Composition: 154 mEq/L Na+, 154 mEq/L Cl-.
- Indications: Hypovolemic shock, sepsis, hemorrhage, DKA/HHS (initial fluid of choice), hypercalcemia, hyponatremia (mild/asymptomatic).
- HY Adverse Effects: Large volumes cause hyperchloremic non-anion gap metabolic acidosis (due to supraphysiologic chloride load displacing bicarbonate).
- Lactated Ringer’s (LR) (Balanced Crystalloid)
- Composition: 130 mEq/L Na+, 109 mEq/L Cl-, 4 mEq/L K+, 3 mEq/L Ca2+, 28 mEq/L Lactate.
- Indications: Burn resuscitation (Parkland formula), trauma, acute pancreatitis, surgery, volume depletion. Preferred over NS in large volume resuscitation to avoid acidosis.
- HY Adverse Effects: Can cause metabolic alkalosis (lactate metabolized to bicarb in liver). Do not use with ceftriaxone in neonates (calcium precipitation). Avoid in severe hyperkalemia or end-stage liver disease (cannot clear lactate).
- Plasmalyte / Isolyte (Balanced Crystalloid)
- Composition: Contains acetate/gluconate instead of lactate as buffers. Closest to human plasma.
- Indications: Similar to LR. Increasingly preferred in ICU resuscitation.
2. Crystalloids: Hypotonic
Distribute primarily into the intracellular fluid (ICF) compartment. Primary use: Free water replacement / Maintenance.
- 0.45% Half-Normal Saline (1/2 NS)
- Composition: 77 mEq/L Na+, 77 mEq/L Cl-.
- Indications: Hypernatremia (e.g., euvolemic or hypervolemic), DKA/HHS maintenance (once glucose < 250 mg/dL, usually given as D5 1/2 NS).
- HY Adverse Effects: Risk of causing hyponatremia, cellular edema. Never use for acute resuscitation (distributes out of intravascular space → worsening shock).
- 5% Dextrose in Water (D5W)
- Composition: Isotonic in the bag, but rapidly becomes hypotonic in the body as dextrose is metabolized, leaving pure free water.
- Indications: Severe hypernatremia, administration of certain IV meds (e.g., Amiodarone).
- HY Adverse Effects: Hyponatremia, hyperglycemia, cerebral edema.
3. Crystalloids: Hypertonic
Draw fluid from the intracellular space into the extracellular/intravascular space. Primary use: Osmotherapy / Severe Na+ deficit.
- 3% Hypertonic Saline
- Composition: 513 mEq/L Na+, 513 mEq/L Cl-.
- Indications: Severe, symptomatic hyponatremia (e.g., seizures, coma, neurologic deficits), traumatic brain injury (TBI) with elevated intracranial pressure (ICP).
- HY Adverse Effects: Osmotic demyelination syndrome (central pontine myelinolysis) if Na+ corrected too rapidly (>8 mEq/L in 24h). Volume overload, hypernatremia.
4. Colloids
Remain entirely in the intravascular compartment due to large oncotic molecules.
- Albumin (5% or 25%)
- Indications: Volume expansion in cirrhotic patients (e.g., large volume paracentesis > 5L, Spontaneous Bacterial Peritonitis [SBP], Hepatorenal syndrome [HRS]), burns (> 24h post-injury).
- HY Note: No mortality benefit over crystalloids for general trauma, sepsis, or hypovolemic shock. Avoid in traumatic brain injury (increased mortality).
- Blood Products (PRBCs, FFP, Platelets)
- Indications: Hemorrhagic shock. Transfuse PRBCs if Hb < 7 g/dL (or < 8-9 g/dL in acute coronary syndrome/ortho surgery) or active massive bleeding (1:1:1 ratio of PRBC:FFP:Plt).
- Synthetic Colloids (Hydroxyethyl Starch/Hetastarch, Dextran)
- HY Note: Generally avoided on the USMLE. Associated with increased risk of Acute Kidney Injury (AKI) and coagulopathy in critically ill patients.