What does a CO2 (carbon dioxide) level of 19.7 indicate and how is it treated?

Serum CO2 of 19.7 mEq/L: Low Bicarbonate Requiring Metabolic Acidosis Evaluation

A serum total CO2 of 19.7 mEq/L indicates low bicarbonate and requires immediate arterial blood gas analysis to determine if this represents primary metabolic acidosis or compensatory response to chronic respiratory alkalosis. 1

Clinical Interpretation

This value falls below the normal range of bicarbonate (typically 22-28 mEq/L for total CO2), representing either:

• Primary metabolic acidosis with compensatory respiratory alkalosis (most common)
• Compensatory response to chronic respiratory alkalosis (less common) 1

The serum total CO2 primarily reflects bicarbonate concentration, as bicarbonate comprises 70-85% of CO2 carried in blood. 2

Immediate Diagnostic Steps

Obtain arterial blood gas immediately to differentiate the underlying disorder and assess severity: 1

• If pH is low (<7.35) with low bicarbonate and low PaCO2: Primary metabolic acidosis with appropriate respiratory compensation
• If pH is high (>7.45) with low bicarbonate and low PaCO2: Primary respiratory alkalosis with renal compensation 1

Calculate the anion gap: Na+ - (Cl- + HCO3-) to determine the type of metabolic acidosis if present:

• Anion gap >12 mEq/L: Suggests lactic acidosis, ketoacidosis, renal failure, or toxic ingestion
• Normal anion gap: Suggests GI bicarbonate loss (diarrhea), renal tubular acidosis, or early renal failure

Treatment Approach

For Metabolic Acidosis (Most Likely Scenario)

Address the underlying cause first - bicarbonate replacement is rarely indicated and can be harmful: 3

• Lactic acidosis: Improve tissue perfusion and oxygen delivery
• Diabetic ketoacidosis: Insulin and fluid resuscitation (bicarbonate only if pH <6.9)
• Renal failure: Consider dialysis if severe
• Diarrhea: Volume resuscitation with isotonic saline

Bicarbonate Administration (Only in Specific Circumstances)

Sodium bicarbonate should only be administered when pH <7.20 with severe symptoms or in cardiac arrest. 3 The FDA-approved dosing is 2-5 mEq/kg over 4-8 hours, with stepwise titration based on repeated blood gas measurements. 3

Critical caution: Rapid correction risks overshoot alkalosis, as ventilatory compensation lags behind bicarbonate administration. Aim for total CO2 of approximately 20 mEq/L in the first 24 hours, not complete normalization. 3

Monitoring Requirements

• Repeat arterial blood gas every 2-4 hours during active treatment 3
• Monitor serum electrolytes, particularly potassium (hypokalemia predisposes to alkalosis) 3
• Monitor for sodium overload in patients with heart failure or renal dysfunction 3

Critical Pitfalls to Avoid

Do not administer bicarbonate empirically without knowing the pH and underlying cause - this can worsen outcomes in many conditions. 3 Bicarbonate therapy in the absence of severe acidemia (pH <7.20) may cause:

• Paradoxical CNS acidosis (CO2 crosses blood-brain barrier faster than bicarbonate)
• Hypokalemia and hypocalcemia with tetany 3
• Volume overload from sodium load 3
• Overshoot metabolic alkalosis 3

In patients with chronic respiratory disease, low bicarbonate may represent appropriate compensation for chronic respiratory alkalosis from hyperventilation - treating this would be harmful. 2, 1