What causes hyperchloremic metabolic acidosis?

Causes of Hyperchloremic Metabolic Acidosis

Hyperchloremic metabolic acidosis is primarily caused by the excessive use of normal saline or unbalanced colloid solutions containing supra-physiological concentrations of chloride (>154 mmol/L), renal tubular acidosis, gastrointestinal bicarbonate loss, and early renal failure. 1, 2, 3

Classification of Causes

1. Iatrogenic Causes

• IV Fluid Administration:
  • Normal saline (0.9% NaCl) or unbalanced colloid solutions used as priming volume or infusion fluids during cardiopulmonary bypass 1
  • Large volume administration of chloride-rich solutions (especially in perioperative settings) 1, 4
  • Albumin solutions with high chloride concentrations used in therapeutic plasma exchange 4

2. Renal Causes

• Renal Tubular Acidosis (RTA):
  • Distal RTA: Primary defect in renal acidification with insufficient regeneration of bicarbonate 3
  • Proximal RTA: Filtered bicarbonate is lost by kidney wasting 3
  • RTA of renal insufficiency (early renal failure) 2, 3
• Impaired ammonium excretion in early kidney disease 3

3. Gastrointestinal Causes

• Bicarbonate loss from the gastrointestinal tract 2, 3
• Diarrhea (leading to loss of bicarbonate-rich intestinal fluids)
• Cholestyramine-induced acidosis: Reported in patients with liver disease taking bile acid sequestrants 5

4. Other Causes

• Drug-induced hyperkalemia leading to metabolic acidosis 1
• Excessive chloride gain relative to sodium 6
• Excessive sodium loss relative to chloride 6

Diagnostic Approach

To determine the specific cause of hyperchloremic metabolic acidosis:

1. Calculate the anion gap:

   • Normal anion gap (hyperchloremic) vs. high anion gap acidosis 2, 6

2. Assess urinary parameters:

   • Urinary anion gap or osmolal gap to distinguish between renal and extrarenal causes 3
   • Urinary pH (low pH with normal renal acidification, inappropriately high pH with distal RTA) 5

3. Review medication history:

   • IV fluid therapy (especially normal saline)
   • Cholestyramine or other medications affecting acid-base balance 5

4. Evaluate for underlying conditions:

   • Renal function tests
   • Gastrointestinal disorders
   • Recent procedures (cardiopulmonary bypass, plasma exchange) 1, 4

Clinical Implications and Management

Hyperchloremic metabolic acidosis can lead to:

• Tissue injury and organ dysfunction 1
• Neurological symptoms (confusion, lethargy) 5
• Gastrointestinal complaints and muscle weakness 4

Management should focus on:

1. Treating the underlying cause rather than just correcting the acidosis 7, 2
2. Using balanced crystalloid solutions instead of 0.9% saline when possible 7
3. Bicarbonate supplementation when serum bicarbonate falls below 22 mmol/L 7, 5
4. Close monitoring of electrolytes, particularly potassium 7

Common Pitfalls and Caveats

• Don't overlook iatrogenic causes: Normal saline administration is a common but often overlooked cause of hyperchloremic acidosis 1
• Avoid worsening the condition: Using additional chloride-rich solutions in patients with existing hyperchloremic acidosis can exacerbate the condition 7
• Consider special populations: Patients with chronic comorbidities (cardiac dysfunction, liver disease, chronic kidney disease) are at higher risk for fluid and electrolyte disturbances 1
• Remember that acidosis may be multifactorial: Multiple mechanisms may contribute simultaneously to hyperchloremic acidosis 2, 3

In summary, hyperchloremic metabolic acidosis has diverse causes ranging from iatrogenic fluid administration to renal and gastrointestinal disorders. Identifying the specific cause through careful evaluation of the anion gap, urinary parameters, and clinical context is essential for appropriate management.