What is the management approach for a patient with an abnormal Base Excess (BE) in an Arterial Blood Gas (ABG)?

Management of Abnormal Base Excess in Arterial Blood Gas

When base excess is abnormal on ABG, immediately identify whether it indicates metabolic acidosis (BE < -2 mEq/L) or metabolic alkalosis (BE > +2 mEq/L), then treat the underlying cause while monitoring serial ABGs to guide therapy. 1

Understanding Base Excess

• Base excess represents the amount of acid or base needed to normalize blood pH to 7.4 at a PCO2 of 40 mmHg, providing an indirect estimation of metabolic disturbances due to impaired perfusion or metabolic derangements 2
• Normal base excess range is -2 to +2 mEq/L; values outside this range indicate metabolic acid-base disturbances 2
• Base excess of the extracellular fluid (ECF) is the physiologically appropriate parameter for clinical use, avoiding potential misinterpretation 3

Systematic Interpretation Approach

Follow this three-step process to interpret ABG results: 4

1. Evaluate pH first to determine acidemia (pH < 7.35) or alkalemia (pH > 7.45) 4
2. Examine PaCO2 to identify the respiratory component: PaCO2 > 45 mmHg with low pH indicates respiratory acidosis, while PaCO2 < 35 mmHg with high pH indicates respiratory alkalosis 4
3. Evaluate base excess/bicarbonate to identify the metabolic component: BE < -2 or HCO3 < 22 indicates metabolic acidosis, while BE > +2 or HCO3 > 26 indicates metabolic alkalosis 4

Management of Metabolic Acidosis (BE < -2 mEq/L)

For metabolic acidosis, treat the underlying cause as the primary intervention: 1

• Base excess < -2 mEq/L is the optimal cut-off to diagnose clinically relevant metabolic acidosis and identify strong ion gap acidosis 5
• Monitor base deficit as a sensitive marker for severity of shock and mortality risk, particularly in trauma patients 1
• Consider sodium bicarbonate therapy only for severe acidosis with arterial pH < 7.1 and base deficit < -10 mEq/L 1

Sodium Bicarbonate Administration (When Indicated)

For severe metabolic acidosis requiring bicarbonate: 6

• In less urgent metabolic acidosis, administer 2 to 5 mEq/kg body weight over 4 to 8 hours depending on severity 6
• Monitor therapy with serial blood gases, plasma osmolarity, arterial lactate, hemodynamics, and cardiac rhythm 6
• Target total CO2 content of approximately 20 mEq/L at the end of the first day rather than attempting full correction, as complete normalization within 24 hours may cause unrecognized alkalosis due to delayed ventilatory readjustment 6
• Plan therapy in a stepwise fashion since the degree of response is not precisely predictable 6

Additional Metabolic Acidosis Considerations

• To analyze components of base excess shifts at bedside, measure anion gap corrected for albumin, apparent strong ion difference, albumin level, and lactate concentration 5
• If change in base excess is smaller than change in lactate concentration, consider recent bicarbonate therapy or infusion solutions containing lactate 7

Management of Metabolic Alkalosis (BE > +2 mEq/L)

For metabolic alkalosis, identify and treat the underlying cause while restoring intravascular volume: 2

• Restore intravascular volume with normal saline if volume depletion is present 2
• Base excess > 30 mEq/L indicates extreme metabolic alkalosis requiring urgent intervention, as this can cause decreased tissue oxygen delivery due to leftward shift of the oxyhemoglobin dissociation curve 2
• Monitor for hypoventilation as a compensatory mechanism 2
• Monitor arterial blood gases frequently to assess response to therapy 2

Critical Monitoring Parameters

Serial ABG monitoring is essential for all patients with abnormal base excess: 1, 4

• Repeat ABG after each therapeutic intervention, particularly after oxygen therapy changes in patients at risk for CO2 retention 4
• In patients with shock or hypotension, monitor blood gases, plasma osmolarity, arterial lactate, hemodynamics, and cardiac rhythm 6
• Use arterial samples rather than venous samples in critically ill patients for accurate assessment 1

Common Pitfalls to Avoid

• Do not attempt full correction of metabolic acidosis within the first 24 hours, as this may cause unrecognized alkalosis due to delayed ventilatory readjustment 6
• Failing to repeat ABG measurements after oxygen therapy changes in patients at risk for CO2 retention is a critical management error 4
• Normal oxygen saturation does not rule out significant acid-base disturbances, so always interpret base excess in the full clinical context 1
• Ensure you are using base excess of the ECF rather than blood sample base excess to avoid misinterpretation 3