Arterial Blood Gas (ABG) Interpretation: A Systematic Approach
Use a three-step systematic approach to interpret ABG results: first evaluate pH to determine if acidemia or alkalemia is present, then examine PaCO2 to identify the respiratory component, and finally evaluate base excess/bicarbonate to identify the metabolic component. 1
Step 1: Evaluate the pH
- pH < 7.35 indicates acidemia 1
- pH > 7.45 indicates alkalemia 1
- pH 7.35-7.45 is normal (but doesn't rule out compensated disorders) 2
The pH tells you the overall acid-base status and determines which direction the body has shifted. This is your starting point for all ABG interpretation. 3
Step 2: Identify the Respiratory Component (PaCO2)
- PaCO2 > 45 mmHg with low pH indicates respiratory acidosis 1
- PaCO2 < 35 mmHg with high pH indicates respiratory alkalosis 1
- Normal PaCO2 is 35-45 mmHg 3
PaCO2 reflects ventilation status—elevated levels indicate hypoventilation (chronic or acute respiratory failure), while decreased levels indicate hyperventilation. 4 The PaCO2 moves in the opposite direction of pH in primary respiratory disorders. 2
Step 3: Identify the Metabolic Component (Base Excess/Bicarbonate)
- Base excess < -2 or HCO3 < 22 mmEq/L indicates metabolic acidosis 1
- Base excess > +2 or HCO3 > 26 mmEq/L indicates metabolic alkalosis 1
- Normal HCO3 is 22-26 mmEq/L 3
The metabolic component reflects kidney function and metabolic processes. Base excess provides valuable information about the effectiveness of resuscitation efforts in critically ill patients, particularly in trauma, shock, and sepsis. 5
Step 4: Determine Primary Disorder and Compensation
The primary disorder is identified by which component (respiratory or metabolic) matches the pH direction. 2, 6
Primary Disorders:
- Respiratory acidosis: Low pH + High PaCO2 1
- Respiratory alkalosis: High pH + Low PaCO2 1
- Metabolic acidosis: Low pH + Low HCO3/negative base excess 1
- Metabolic alkalosis: High pH + High HCO3/positive base excess 1
Compensation Patterns:
- Partial compensation: pH remains abnormal, but the opposite system is attempting to correct (e.g., respiratory acidosis with elevated HCO3) 2, 6
- Full compensation: pH returns to normal range (7.35-7.45), but both PaCO2 and HCO3 remain abnormal 2
- Mixed disorders: Both respiratory and metabolic components move pH in the same direction 2
The degree of compensation helps determine if the disorder is acute, chronic, or mixed. 5 In acute disorders, compensation is minimal; in chronic disorders, compensation is more complete. 6
Step 5: Assess Oxygenation (PaO2)
Critical pitfall: Always interpret PaO2 in relation to the fraction of inspired oxygen (FiO2) the patient is receiving. 3 A normal oxygen saturation does not negate the need for blood gas measurements, especially if the patient is on supplemental oxygen therapy. 7 Pulse oximetry will be normal in patients with normal oxygen levels but abnormal acid-base status or ventilation. 7
Common Clinical Scenarios
Respiratory Acidosis (High PaCO2, Low pH):
Causes: COPD exacerbation, severe asthma, respiratory muscle weakness, oversedation, pneumonia 3, 4
Management approach: For acute hypercapnic respiratory failure with pH < 7.35 and PaCO2 > 49 mmHg despite optimal medical therapy, initiate non-invasive ventilation (NIV). 1 Use controlled oxygen therapy targeting SpO2 88-92% for COPD and all causes of acute hypercapnic respiratory failure. 1
Respiratory Alkalosis (Low PaCO2, High pH):
Causes: Hyperventilation from anxiety, pain, hypoxemia, pulmonary embolism, early sepsis 3, 4
Metabolic Acidosis (Low HCO3, Low pH):
Causes: Diabetic ketoacidosis, lactic acidosis from shock/sepsis, renal failure, diarrhea 1, 4
Management consideration: In cardiac arrest with severe acidosis (arterial pH < 7.1 and base excess < -10), judicious use of sodium bicarbonate 50 mmol (50 ml of 8.4% solution) may be appropriate, with further administration dependent on repeat ABG analysis. 8
Metabolic Alkalosis (High HCO3, High pH):
Causes: Vomiting, nasogastric suction, diuretic use, hypokalemia 3, 4
Critical Pitfalls to Avoid
Do not rely solely on pulse oximetry when acid-base status and ventilation need assessment—a normal SpO2 does not rule out significant acid-base disturbances or hypercapnia. 7, 5
Do not overlook the need for ABG in patients with metabolic conditions such as diabetic ketoacidosis or metabolic acidosis from renal failure, even if oxygen saturation appears normal. 7, 5
Always repeat ABG measurements after changes in oxygen therapy, especially in patients at risk for CO2 retention. 7 ABG should be performed within 60 minutes of starting oxygen therapy and within 60 minutes of a change in inspired oxygen concentration in COPD patients. 7, 5
In patients with baseline hypercapnia, monitor for respiratory acidosis and worsening hypercapnia after each titration of oxygen flow rate. 8, 1 Patients who develop respiratory acidosis or a rise in PaCO2 > 7.5 mmHg during oxygen titration may have clinically unstable disease and should undergo further medical optimization. 8
When to Obtain ABG
Obtain ABG in all critically ill patients to assess oxygenation, ventilation, and acid-base status. 7, 5, 1
Specific indications include:
- Shock or hypotension 7, 5, 1
- Oxygen saturation fall below 94% on room air or supplemental oxygen 7, 5
- Deteriorating oxygen saturation (fall ≥3%) or increasing breathlessness in patients with previously stable chronic hypoxemia 7, 5
- Suspected diabetic ketoacidosis or metabolic acidosis from renal failure 7, 1
- When starting oxygen therapy in COPD patients, especially with known CO2 retention 7, 1
- After oxygen titration to confirm adequate oxygenation without precipitating respiratory acidosis 8, 7, 5
Technical Considerations
Perform Allen's test before radial ABG to ensure dual blood supply to the hand from both radial and ulnar arteries. 8, 7
Use local anesthesia for all ABG specimens except in emergencies to minimize patient discomfort. 7
Obtain consent and discuss possible risks before performing radial ABG. 8
For most non-critical patients, either arterial blood gases or arterialized earlobe blood gases may be used to measure acid-base status and ventilation. 7 Capillary blood gases can be used for re-measuring PaCO2 and pH during oxygen titration. 8