How do I analyze an arterial blood gas (ABG)?

How to Analyze an Arterial Blood Gas (ABG)

Use a systematic three-step method: first evaluate pH to determine acidemia or alkalemia, then examine PaCO2 to identify the respiratory component, and finally evaluate base excess/bicarbonate to identify the metabolic component. 1, 2

Step 1: Evaluate pH Status

• pH < 7.35 indicates acidemia 1
• pH > 7.45 indicates alkalemia 1
• This is your starting point that determines the primary direction of the acid-base disturbance 2

Step 2: Assess the Respiratory Component (PaCO2)

• PaCO2 > 45 mmHg with low pH indicates respiratory acidosis 1
• PaCO2 < 35 mmHg with high pH indicates respiratory alkalosis 1
• The respiratory component moves in the opposite direction from pH in primary respiratory disorders 3

Step 3: Assess the Metabolic Component (Base Excess/HCO3-)

• Base excess < -2 or HCO3 < 22 mmol/L indicates metabolic acidosis 1
• Base excess > +2 or HCO3 > 26 mmol/L indicates metabolic alkalosis 1
• The metabolic component moves in the same direction as pH in primary metabolic disorders 3
• Base excess is particularly useful in trauma, shock, sepsis, and diabetic ketoacidosis to quantify metabolic acidosis and guide resuscitation 4

Step 4: Evaluate Oxygenation

• PaO2 < 60 mmHg indicates severe hypoxemia requiring immediate intervention 1, 2
• Normal PaO2 is >90 mmHg on room air at sea level 2
• A normal oxygen saturation does NOT rule out significant acid-base disturbances or hypercapnia 2
• Evaluate the P(A-a)O2 gradient: normal is <15 mmHg (or <20 mmHg if age ≥65 years) 2

Step 5: Determine Compensation Status

• In primary respiratory disorders, base excess should remain normal initially 4
• In chronic respiratory disorders, base excess will change to compensate 4
• For patients with baseline hypercapnia, base excess helps distinguish chronic respiratory acidosis from acute-on-chronic respiratory failure 4
• The degree of compensation helps determine if the disorder is acute, chronic, or mixed 4

Step 6: Calculate Delta Ratio for High Anion Gap Metabolic Acidosis

• Delta ratio = (Anion Gap - 12) / (24 - HCO3-) 2
• Calculate this when metabolic acidosis with elevated anion gap is identified to detect mixed acid-base disorders 2
• This is particularly important in critically ill patients where multiple pathophysiologic processes may coexist 2

Critical Clinical Management Points

Oxygen Therapy Targets

• Target SpO2 88-92% for COPD and all causes of acute hypercapnic respiratory failure 1, 2
• Target SpO2 94-98% for all other patients 1, 2
• Start oxygen at 1 L/min and titrate up in 1 L/min increments until SpO2 >90% 2

When to Repeat ABG

• Repeat ABG within 60 minutes after starting oxygen therapy or changing FiO2 in patients at risk for CO2 retention 1, 2
• Patients with baseline hypercapnia must have ABG monitoring after each flow rate titration 2
• A rise in PaCO2 > 1 kPa (7.5 mmHg) indicates clinically unstable disease requiring further medical optimization 2

Indications for Non-Invasive Ventilation

• Initiate non-invasive ventilation for pH < 7.35 and PaCO2 > 6.5 kPa (49 mmHg) despite optimal medical therapy 2

Technical Considerations for ABG Sampling

• Perform Allen's test before radial ABG to ensure dual blood supply to the hand from both radial and ulnar arteries 1, 2
• Use local anesthesia for all non-emergency ABG sampling 1
• Obtain informed consent with discussion of possible risks 2

Alternative Sampling Methods

• Capillary blood gases (CBG) can replace ABG for re-measuring PaCO2 and pH during oxygen titration 2
• Cutaneous capnography can replace ABG for re-measuring PaCO2 alone but not pH 2
• For non-critical patients, arterialized earlobe blood gases may be used to measure acid-base status and ventilation 2
• Venous blood gas can be transformed to estimate arterial values using validated formulas, though ABG remains the gold standard 5, 6

Common Pitfalls to Avoid

• Failing to repeat ABG measurements after changes in oxygen therapy, especially in patients at risk for CO2 retention, is a common management error 2
• Simply measuring arterial blood gas tensions may be misleading and bear little relation to myocardial or cerebral intracellular values during cardiorespiratory arrest 7
• The delta ratio has limitations in chronic conditions where baseline bicarbonate may differ significantly from 24 mmol/L 2