Normal Values for Bicarbonate and CO2 in Arterial Blood Gas Analysis
In arterial blood gas analysis, the normal bicarbonate (HCO3-) level is 22-26 mmol/L, and the normal partial pressure of carbon dioxide (PaCO2) is approximately 35-45 mmHg (or 4.7-6.0 kPa). 1, 2
Understanding the Measurements
Bicarbonate (HCO3-)
- Normal range: 22-26 mmol/L 1, 2
- Bicarbonate represents the metabolic component of acid-base balance 3, 4
- Standard bicarbonate (HCO3 standard) represents the bicarbonate concentration at a normal PCO2 of 40 mmHg, making it a better indicator of the metabolic component independent of respiratory influences 1
Carbon Dioxide (PaCO2)
- Normal range: 35-45 mmHg (4.7-6.0 kPa) 5, 6
- PaCO2 represents the respiratory component of acid-base balance 3, 4
- The normal PCO2 to HCO3 ratio is approximately 1:20 6
Clinical Significance of These Values
When Bicarbonate is Abnormal
- Bicarbonate <22 mmol/L indicates metabolic acidosis and warrants investigation for conditions like diabetic ketoacidosis, renal failure, or sepsis 1, 2
- Bicarbonate >27 mmol/L should prompt consideration for arterial blood gas measurement to confirm hypercapnia, particularly when screening for conditions like Obesity Hypoventilation Syndrome 5, 1, 2
- In chronic kidney disease patients, serum bicarbonate should be maintained at or above 22 mmol/L to prevent complications of metabolic acidosis 1, 2
When PaCO2 is Abnormal
- PaCO2 >45 mmHg indicates respiratory acidosis or compensation for metabolic alkalosis 3, 4
- PaCO2 <35 mmHg indicates respiratory alkalosis or compensation for metabolic acidosis 3, 4
- A rise in PaCO2 of >1 kPa (7.5 mmHg) during oxygen titration may indicate clinically unstable disease 5
Important Measurement Considerations
Serum vs. ABG Bicarbonate
- Serum bicarbonate on a Basic Metabolic Panel is actually measured as total CO2 content, which includes bicarbonate (96% of total), dissolved CO2, and other minor carbon dioxide components 1, 2
- Sequential rather than simultaneous sampling can lead to differences between serum and ABG bicarbonate values, especially in unstable patients 1, 2
- Strong positive correlations exist between calculated ABG and measured BMP bicarbonate across various pH ranges 7
Common Pitfalls to Avoid
- Normal oxygen saturation does not rule out significant acid-base disturbances, making bicarbonate assessment crucial even when oxygenation appears adequate 1
- Arterial blood gas analysis alone may be misleading in estimating the internal milieu of myocardial and cerebral intracellular acid-base status 1, 2
- When interpreting bicarbonate levels, clinicians must consider both the respiratory and metabolic components together to identify mixed disorders 6, 3
Diagnostic Thresholds in Clinical Practice
Screening Applications
- For patients with low-to-moderate probability of Obesity Hypoventilation Syndrome, a serum bicarbonate <27 mmol/L has a very high negative predictive value (99.0%) for ruling out the condition, potentially eliminating the need for arterial blood sampling 5, 2
- A bicarbonate level >27 mmol/L should trigger clinicians to measure PaCO2 as a confirmatory diagnostic test, especially when the pretest probability is 10-20% 5
Therapeutic Thresholds
- Sodium bicarbonate administration should be limited to patients with severe acidosis (arterial pH <7.1 and base deficit <-10) and special circumstances such as hyperkalaemia or tricyclic antidepressant overdose 1, 2
- For metabolic acidosis, the expected compensatory PCO2 can be estimated using the formula: Expected PCO2 = (1.5 × HCO3) + 8 ± 2 mmHg 6