Base Excess on Arterial Blood Gas Analysis
Base excess on an arterial blood gas (ABG) is a sensitive marker that measures the metabolic component of acid-base disturbances, indicating the amount of excess or deficit of buffer in the blood compared to normal. 1
Definition and Interpretation
Base excess (BE) is calculated using the Henderson-Hasselbach equation with measured values of arterial pH and PaCO2. It represents:
- The amount of acid or base needed to return blood pH to 7.40 at a PaCO2 of 40 mmHg
- Normal range: -2 to +2 mEq/L
- Negative values (base deficit) indicate metabolic acidosis
- Positive values indicate metabolic alkalosis
Clinical Significance
Stratification of Base Deficit
Base deficit can be stratified into three categories 1:
- Mild: -3 to -5 mEq/L
- Moderate: -6 to -9 mEq/L
- Severe: < -10 mEq/L
Prognostic Value
- Base deficit is a potent independent predictor of mortality in patients with traumatic-hemorrhagic shock 1
- It correlates significantly with:
- Transfusion requirements within the first 24 hours
- Risk of post-traumatic organ failure or death
- Severity of injury and mortality in both adult and pediatric patients 1
Advantages Over Other Measurements
- Base deficit is a better prognostic marker of death than arterial pH 1
- Unlike lactate, base deficit is not affected by alcohol consumption, making it a more reliable marker in alcohol-associated trauma 1
- Base deficit can be obtained from either arterial or peripheral venous blood 1
Clinical Applications
Shock Assessment
- Base deficit provides an indirect estimation of global tissue acidosis due to impaired perfusion 1
- It serves as a sensitive diagnostic marker of the degree and duration of inadequate perfusion 1
- Particularly valuable in trauma patients, where it helps estimate and monitor the extent of bleeding and shock 1
Comparison with Lactate
- Both base deficit and serum lactate levels correlate with shock and resuscitation
- However, these parameters do not strictly correlate with each other in severely injured patients 1
- Independent assessment of both parameters is recommended for comprehensive evaluation of shock 1
Practical Considerations
Calculation Options
Most blood gas analyzers offer two options for calculating base excess 2:
- Base excess of the blood sample
- Base excess of the extracellular fluid (ECF)
The base excess of ECF is generally considered the more appropriate metabolic blood gas parameter for clinical use 2.
Common Pitfalls
- Relying solely on base excess without considering other parameters (pH, PaCO2, HCO3-)
- Not accounting for respiratory component when interpreting metabolic acidosis
- Failing to recognize that normal base excess doesn't exclude significant acid-base disturbances if compensatory mechanisms are active
Conclusion
Base excess is a valuable tool for diagnosing metabolic acid-base disturbances, with a cut-off value of <-2 mEq/L being most useful for identifying clinically relevant metabolic acidosis 3. When used alongside other ABG parameters, it provides crucial information about a patient's metabolic status and helps guide resuscitation efforts, particularly in trauma and critically ill patients.