Causes of Low Total CO2 in Blood: Metabolic Acidosis
Low total CO2 in blood primarily indicates metabolic acidosis, which occurs due to either increased acid production, decreased acid excretion, or bicarbonate loss from the body. 1
Primary Causes of Metabolic Acidosis (Low Total CO2)
1. Increased Acid Production
- Lactic acidosis: Results from tissue hypoxia due to decreased oxygen delivery (hypoxemia, low cardiac output) or impaired oxygen utilization (sepsis) 1
- Diabetic ketoacidosis: Accumulation of ketone bodies due to insulin deficiency
- Alcoholic ketoacidosis: From alcohol metabolism
- Toxic ingestions: Ethylene glycol, methanol, salicylates 2
2. Decreased Acid Excretion
- Renal failure: Impaired acid excretion when GFR falls below 60 mL/min/1.73m² 2
- Renal tubular acidosis: Defects in renal tubular handling of acid-base balance
3. Bicarbonate Loss
- Gastrointestinal losses: Chronic diarrhea, intestinal fistulas, or drainage tubes 1
- Renal bicarbonate wasting: Certain forms of renal tubular acidosis
Diagnostic Approach to Low Total CO2
Laboratory Assessment
- Arterial blood gases: Confirms acidosis (pH <7.35) and helps differentiate respiratory from metabolic causes 2
- Anion gap calculation: [Na⁺] - ([Cl⁻] + [HCO₃⁻])
- Normal: 8-12 mEq/L
- Elevated (>12 mEq/L): Indicates presence of unmeasured acids 2
Categorization by Anion Gap
High Anion Gap Metabolic Acidosis
- Lactic acidosis (tissue hypoxia, shock, sepsis)
- Ketoacidosis (diabetic, alcoholic, starvation)
- Renal failure (accumulation of organic acids)
- Toxins (methanol, ethylene glycol, salicylates)
Normal Anion Gap Metabolic Acidosis
- Gastrointestinal bicarbonate loss (diarrhea)
- Renal tubular acidosis
- Early renal failure
- Administration of chloride-containing solutions (hyperchloremic acidosis) 1
Severity Classification
The severity of metabolic acidosis can be classified based on total CO2 levels:
- Mild: ≥19 mmol/L
- Moderate to Severe: <19 mmol/L 2
Special Considerations
Chronic Kidney Disease
Patients with CKD commonly develop metabolic acidosis due to:
- Impaired ammonia excretion
- Reduced tubular bicarbonate reabsorption
- Insufficient renal bicarbonate production 3
Critically Ill Patients
Metabolic acidosis is common in critically ill patients and requires prompt identification of the underlying cause for appropriate management 4
Compensatory Mechanisms
The body attempts to compensate for metabolic acidosis through:
- Respiratory compensation: Hyperventilation to reduce CO2 levels
- Renal compensation: Enhanced renal excretion of H⁺, primarily as ammonium 5
Clinical Implications
Low total CO2 (metabolic acidosis) is associated with:
- Faster progression of kidney disease in CKD patients
- Increased mortality
- Bone demineralization (osteodystrophy)
- Protein catabolism
- Impaired cardiac function 2, 3
Management Approach
Treatment should focus on addressing the underlying cause while monitoring acid-base status:
- For CKD-related acidosis: Oral sodium bicarbonate to maintain serum bicarbonate ≥22 mmol/L 2, 3
- For lactic acidosis: Improve tissue oxygenation and treat underlying cause 6
- For diabetic ketoacidosis: Insulin therapy and fluid resuscitation
- For diarrhea-induced acidosis: Fluid replacement and correction of electrolyte imbalances
Remember that accurate diagnosis of the specific type and cause of metabolic acidosis is essential for appropriate management, as treatment approaches differ significantly based on the underlying etiology.