What are the effects of saline infusion on urinary electrolytes?

Effects of Saline Infusion on Urinary Electrolytes

Saline infusion, particularly 0.9% normal saline, causes significant urinary electrolyte changes including increased sodium and chloride excretion, decreased potassium excretion, and can lead to hyperchloremic metabolic acidosis that may negatively impact renal function and patient outcomes. 1

Electrolyte Changes Following Saline Infusion

Sodium and Chloride Effects

• Infusion of saline solutions results in increased urinary sodium and chloride excretion that persists for up to 48 hours after infusion 2
• The magnitude of sodium excretion is proportional to the concentration and volume of saline administered
• 0.9% saline infusion leads to hyperchloremia due to its high chloride content (154 mmol/L) compared to plasma (approximately 100-110 mmol/L) 1
• Hyperchloremia can persist for 24-48 hours after significant saline infusion

Potassium Effects

• Saline infusion typically causes an initial decrease in plasma potassium followed by a compensatory increase in potassium excretion 3
• Hypertonic saline (7.5%) can cause a biphasic potassium response: initial decrease during infusion followed by an increase of approximately 0.3 mmol/L above baseline after 1 hour 3
• Normal saline infusion may reduce urinary potassium excretion compared to balanced solutions 4

Acid-Base Effects

• 0.9% saline infusion leads to hyperchloremic metabolic acidosis characterized by:
  • Decreased pH (typically by 0.05-0.07 units)
  • Decreased serum bicarbonate
  • Decreased base excess (by approximately 1.9 mmol/L)
  • Increased anion gap 1, 3

Mechanisms of Saline-Induced Electrolyte Changes

1. Volume Expansion Effects:

   • Increased glomerular filtration rate
   • Suppression of the renin-angiotensin-aldosterone system for up to 48 hours 2
   • Increased atrial natriuretic peptide (ANP) and urodilatin release 5, 2

2. Direct Renal Effects:

   • Hyperchloremia reduces renal blood flow and glomerular filtration rate
   • Decreased kidney perfusion and urine output
   • Increased vasopressor requirements
   • Risk of acute kidney injury 1, 6

3. Tubular Effects:

   • Altered sodium and chloride reabsorption in the renal tubules
   • Continued reabsorption of osmotically free water from kidney tubular fluid 4
   • Excretion of urine hyperosmotic to plasma 4

Clinical Implications

Adverse Effects of 0.9% Saline

• Hyperchloremic metabolic acidosis
• Decreased kidney perfusion and urine output
• Increased extravascular fluid accumulation
• Increased vasopressor requirements
• Risk of acute kidney injury 1
• Electrolyte derangements and dilutional coagulopathy 1

Recommendations for Clinical Practice

• Balanced crystalloids should be used in preference to 0.9% normal saline for resuscitation and maintenance of intravascular volume 1
• Limit use of 0.9% saline, especially in:
  • Higher-risk patients with existing electrolyte derangements
  • Patients with acidosis or hyperchloremia
  • Patients requiring significant fluid resuscitation 1
• For patients requiring potassium supplementation, infusion should include 20-30 mEq/L potassium (2/3 KCl and 1/3 KPO₄) once renal function is assured 7
• For high-output stoma management, isotonic glucose-saline solutions are preferred over hypotonic/hypertonic fluids 1

Monitoring Recommendations

• Monitor serum electrolytes, particularly sodium, potassium, chloride, and bicarbonate
• Assess acid-base status through arterial or venous blood gases
• Monitor renal function parameters (BUN, creatinine)
• Measure urine output and urinary electrolyte excretion in critical patients
• Assess fluid balance and body weight changes 1, 2

Common Pitfalls and Caveats

• Failure to recognize hyperchloremic metabolic acidosis as a consequence of large-volume saline infusion
• Overlooking the impact of saline-induced acidosis on organ function and patient outcomes
• Inappropriate use of 0.9% saline in patients with pre-existing acidosis or renal dysfunction
• Not considering the duration of electrolyte changes, which can persist for 24-48 hours after infusion 1, 2
• Neglecting to monitor for potassium changes, which can follow a biphasic pattern after saline infusion 3