Causes of Hypernatremia in the ICU
Hypernatremia in the ICU is primarily caused by impaired thirst or access to water, excessive fluid losses, and treatment with sodium-containing fluids. 1
Definition and Prevalence
- Hypernatremia: Plasma sodium concentration >145 mEq/L
- Represents a decrease in total body water relative to sodium
- Associated with plasma hyperosmolality
- Occurs in critically ill patients with prevalence increasing with severity of illness
Major Causes of Hypernatremia in ICU Patients
1. Impaired Water Access or Intake
- Inability to drink water due to:
- Mechanical ventilation/intubation (80% of cases) 2
- Altered mental status
- Sedation
- Impaired thirst mechanism
2. Increased Water Losses
Renal Water Losses
- Osmotic diuresis:
- Hyperglycemia
- Mannitol administration
- High protein feeding
- Diuretic therapy (34% of cases) 2
- Post-vasopressin withdrawal (rebound effect after discontinuation) 3
- Diabetes insipidus:
- Central (neurogenic)
- Nephrogenic (renal resistance to ADH)
- Renal dysfunction with impaired concentrating ability 4
Extra-renal Water Losses
- Gastrointestinal losses:
- Diarrhea
- Vomiting
- Nasogastric drainage
- Fistula output
- Insensible losses:
- Fever (25% of cases) 2
- Increased respiratory rate
- Burns
- Open wounds
3. Iatrogenic Causes
- Sodium-rich fluid administration:
- Hypertonic saline
- Sodium bicarbonate
- Multiple isotonic saline infusions
- Inadequate free water replacement:
- Medication-related:
- Certain antibiotics
- Lithium
4. Miscellaneous Causes
- Hypervolemic hypernatremia:
- Excessive sodium administration with inadequate free water
- Acute kidney injury with reduced sodium excretion 3
- Endocrine disorders:
- Cushing's syndrome
- Hyperaldosteronism
Risk Factors for Developing Hypernatremia in ICU
- Mechanical ventilation
- Vasopressor therapy
- Acute kidney injury
- Sepsis
- Neurological injury
- Advanced age
- Prolonged ICU stay
Clinical Implications
- Osmotic movement of water from intracellular to extracellular space
- Cellular dehydration, particularly affecting brain cells
- Cognitive and neuromuscular dysfunction
- Risk of hemorrhagic complications in severe cases
- Independent risk factor for increased mortality in critically ill patients 5
Prevention Strategies
- Regular monitoring of serum sodium levels
- Routine assessment of free water requirements 1
- Appropriate fluid prescription with attention to electrolyte-free water needs
- Monitoring of urinary electrolytes in high-risk patients
- Early identification and treatment of underlying causes
Hypernatremia in the ICU represents a serious electrolyte disorder that requires vigilant monitoring and prompt intervention to prevent adverse outcomes. Understanding the multifactorial etiology is essential for effective prevention and management.