Indications for Isotonic, Hypertonic, and Hypotonic Fluids
Isotonic Fluids (280-310 mOsm/L)
Isotonic crystalloids are the first-line fluid therapy for volume depletion, hypovolemic shock, and most resuscitation scenarios, with examples including 0.9% normal saline, Ringer's lactate, and balanced crystalloid solutions like Plasma-Lyte. 1
Primary Indications:
Volume depletion/hypovolemia: Patients with fluid and electrolyte losses from bleeding, vomiting, diarrhea, or excessive blood loss should receive isotonic fluids orally, nasogastrically, subcutaneously, or intravenously 1
Acute brain injury: Isotonic crystalloids (0.9% NaCl, Plasma-Lyte, Isofundine) are recommended as first-line therapy to reduce mortality and improve neurological prognosis, as they prevent cerebral edema by maintaining osmotic balance 1
Septic shock resuscitation: Isotonic fluids are the fluids of choice for resuscitation from hypovolemia and shock 2
Pediatric maintenance therapy: Isotonic fluids prevent hospital-acquired hyponatremia in children, with significantly lower risk compared to hypotonic solutions 1
Burn therapy: Beyond 24 hours post-burn, isotonic albumin solutions (25%) can maintain plasma colloid osmotic pressure after initial crystalloid resuscitation 3
Specific Examples and Routes:
- 0.9% Normal Saline: The most commonly available isotonic crystalloid and current crystalloid of choice in brain injury (osmolality ~308 mOsm/kg) 1
- Balanced crystalloids (Ringer's lactate, Plasma-Lyte): Increasingly preferred over 0.9% saline to prevent hyperchloremic acidosis and acute kidney injury 2
- Oral rehydration therapy: Isotonic solutions with sodium, potassium, and glucose for mild-moderate volume depletion 1
Hypertonic Fluids (>310 mOsm/L)
Hypertonic saline (3% NaCl) is specifically indicated for acute management of raised intracranial pressure with impending herniation and severe symptomatic hyponatremia, NOT for routine volume resuscitation.
Primary Indications:
Raised intracranial pressure (ICP): Hypertonic saline (2 ml/kg of 3% saline) is effective for short-term management of impending uncal herniation and mannitol-refractory intracranial hypertension 1, 2
Traumatic brain injury with focal neurological signs: In situations combining hemorrhagic shock with severe head trauma, a hypertonic saline bolus is recommended due to its osmotic effect 1
Severe symptomatic hyponatremia: Hypertonic solutions for acute correction when patients have severe neurological symptoms 4
Important Contraindications and Cautions:
NOT recommended for hemorrhagic shock resuscitation: Hypertonic saline solutions are not recommended as volume resuscitation solutions for patients with hemorrhagic shock, despite earlier theoretical advantages 1
Risk of hypernatremia: Small volume resuscitation with hypertonic solutions carries the disadvantage of hypernatremia 2
Avoid in routine perioperative care: Meta-analyses show no clear benefit and potential harm when used for routine surgical fluid therapy 1
Hypotonic Fluids (<280 mOsm/L)
Hypotonic fluids should be avoided in most acute resuscitation scenarios and are contraindicated in patients with or at risk for cerebral edema.
Absolute Contraindications:
DO NOT use for fluid resuscitation in sepsis: Hypotonic fluids (e.g., glucose solutions) have minimal effect on intravascular volume but carry high risk of tissue edema, brain edema, and dyselectrolytemia 1
DO NOT use in acute brain injury: Hypotonic solutions like Ringer's lactate (when measured by real osmolality) increase mortality in traumatic brain injury patients (HR 1.78, p=0.035) and should be avoided due to risk of cerebral edema 1
DO NOT use in children for maintenance therapy: Hypotonic fluids significantly increase risk of hospital-acquired hyponatremia (RR 0.46 for isotonic vs hypotonic) 1
Limited Appropriate Uses:
Maintenance fluids in specific stable patients: Isotonic or slightly hypotonic fluids may be acceptable for routine maintenance in stable, euvolemic patients without brain injury or hyponatremia risk 1
Chronic hyponatremia correction: Gradual correction with careful monitoring, though fluid restriction is the mainstay of management 4
Critical Pitfall:
Some solutions labeled as "isotonic" by theoretical osmolarity (mosmol/L) are actually hypotonic when real osmolality (mosmol/kg) is measured—Ringer's lactate and Ringer's acetate fall into this category and should be avoided in brain injury 1
Special Populations and Considerations
Patients with Edematous States:
Patients with congestive heart failure, cirrhosis, or nephrotic syndrome have impaired ability to excrete both free water and sodium. Isotonic saline at typical maintenance rates will likely cause volume overload; fluids should be restricted with close monitoring 1
Albumin Solutions:
- 25% albumin (hypertonic): Indicated for hypovolemic shock (draws ~70 mL additional fluid per 20 mL administered), hypoproteinemia, ARDS with fluid overload, and cardiopulmonary bypass 3
- Avoid in traumatic brain injury: 4% albumin increased mortality in TBI patients (RR 1.63, p=0.003) 1
- Tonicity matters: Hypotonic albumin solutions (278 mOsm/kg) increase ICP by 8.5 mmHg, while isotonic albumin solutions (288 mOsm/kg) do not 5
Geriatric Patients:
Older adults with volume depletion should receive isotonic fluids via any available route (oral, nasogastric, subcutaneous, or intravenous), with assessment based on clinical signs including postural pulse changes ≥30 bpm or presence of ≥4 dehydration signs 1