What are the steps to interpret an Arterial Blood Gas (ABG) and identify its causes?

ABG Interpretation: Systematic Step-by-Step Approach

Use a three-step systematic method to interpret ABG results: 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, 3

Step 1: Assess Oxygenation Status

• Evaluate PaO2 (normal >80 mmHg) and oxygen saturation (normal >94% in most patients) to determine if hypoxemia is present 1
• Calculate the PaO2/FiO2 ratio to assess severity of hypoxemia in critically ill patients 1
• Critical pitfall: Normal oxygen saturation does not rule out significant acid-base disturbances or hypercapnia—you must still complete the full ABG interpretation 1, 3
• Be aware that pulse oximetry cannot differentiate carboxyhemoglobin and may give falsely normal readings in carbon monoxide poisoning 1

Step 2: Determine pH Status

• pH < 7.35 indicates acidemia 1, 2
• pH > 7.45 indicates alkalemia 1, 2
• Normal pH range is 7.35-7.45 1

Step 3: Identify the Respiratory Component

• Assess PaCO2 (normal range: 35-45 mmHg) 1
• PaCO2 > 45 mmHg with low pH indicates respiratory acidosis 2
• PaCO2 < 35 mmHg with high pH indicates respiratory alkalosis 2
• The direction of PaCO2 change determines if the respiratory system is contributing to acidosis or alkalosis 1, 2

Step 4: Identify the Metabolic Component

• Assess HCO3- (normal range: 22-26 mEq/L) and base excess (normal: -2 to +2) 1, 2
• Base excess < -2 or HCO3 < 22 indicates metabolic acidosis 2
• Base excess > +2 or HCO3 > 26 indicates metabolic alkalosis 2
• Base deficit serves as a sensitive marker for severity of shock and mortality risk, particularly in trauma patients 1

Step 5: Calculate Delta Ratio for Mixed Disorders (When Applicable)

• Calculate delta ratio as (Anion Gap - 12) / (24 - HCO₃⁻) when metabolic acidosis with elevated anion gap is identified 3
• This calculation helps identify mixed acid-base disorders where multiple pathophysiologic processes coexist 3
• Delta ratio interpretation:
  • Ratio <1: Concurrent normal anion gap metabolic acidosis
  • Ratio 1-2: Pure high anion gap metabolic acidosis
  • Ratio >2: Concurrent metabolic alkalosis 3
• Limitation: The delta ratio has reduced accuracy in chronic conditions where baseline bicarbonate differs significantly from 24 mmol/L 3

Common Causes by Primary Disorder

Respiratory Acidosis (↑PaCO2, ↓pH)

• COPD exacerbation with inadequate ventilation 2
• Acute respiratory failure from pneumonia, pulmonary edema, or severe asthma 1
• Neuromuscular disorders affecting respiratory muscles 1
• Central nervous system depression from drugs or injury 1

Respiratory Alkalosis (↓PaCO2, ↑pH)

• Hyperventilation from anxiety, pain, or hypoxemia 1
• Mechanical overventilation 1
• Pulmonary embolism or early sepsis 1

Metabolic Acidosis (↓HCO3, ↓pH)

• Diabetic ketoacidosis 2
• Lactic acidosis from shock, sepsis, or tissue hypoperfusion 1
• Renal failure with inability to excrete acid 2, 4
• Toxin ingestion (methanol, ethylene glycol, salicylates) 4

Metabolic Alkalosis (↑HCO3, ↑pH)

• Vomiting or nasogastric suction with loss of gastric acid 5
• Diuretic use with volume contraction 5
• Excessive bicarbonate administration 5

Additional Markers for Clinical Context

• Lactate levels provide information about tissue oxygenation and perfusion, with elevated levels indicating shock 1
• Serial lactate measurements help predict survival and evaluate response to therapy 1
• Monitor lactate in all patients with suspected sepsis, shock, or severe metabolic acidosis 1

Critical Management Principles Based on ABG Results

For Acute Respiratory Acidosis:

• Initiate non-invasive ventilation (NIV) when pH <7.35 and PaCO2 >6.5 kPa (49 mmHg) despite optimal medical therapy 1, 2, 3
• Address the underlying cause and provide ventilatory support 1
• Start with CPAP 4-8 cmH2O plus pressure support 10-15 cmH2O 2

For Hypercapnic Respiratory Failure:

• Use controlled oxygen therapy targeting SpO2 88-92% for COPD and all causes of acute hypercapnic respiratory failure 2, 3
• Start oxygen at 1 L/min and titrate up in 1 L/min increments until SpO2 >90% 3
• Repeat ABG after each titration to monitor for worsening hypercapnia 1, 3
• A rise in PaCO2 >1 kPa (7.5 mmHg) indicates clinically unstable disease requiring further optimization 2, 3

For Severe Metabolic Acidosis:

• Treat the underlying cause as primary intervention 1
• Consider sodium bicarbonate therapy only for severe acidosis (arterial pH <7.1 and base deficit <10) 1
• Identify and correct shock states, diabetic ketoacidosis, or renal failure 2, 4

For Respiratory Alkalosis:

• Identify and treat the underlying cause 1
• Address pain, anxiety, hypoxemia, or mechanical ventilation settings 1

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, 3
• Use local anesthesia for all ABG specimens except in emergencies 1
• Use arterial samples rather than venous samples in critically ill patients 1
• Capillary blood gases can replace ABG for re-measuring PaCO2 and pH during oxygen titration in stable patients 3

Monitoring Criteria for Intubation

• Worsening ABG/pH in 1-2 hours on NIV 2
• Lack of improvement after 4 hours of NIV 2
• Respiratory rate >35 breaths/min despite NIV 2
• PCO2 rises >1 kPa (7.5 mmHg) despite NIV 2