Arterial Blood Gas (ABG) Interpretation
Use a systematic three-step approach: 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
Step 1: Assess Oxygenation Status
- Check PaO2: Normal is >90 mmHg on room air at sea level 2
- Evaluate P(A-a)O2 gradient: Normal is <15 mmHg (or <20 mmHg if age ≥65 years) 2
- Interpret oxygen saturation: Normal arterial oxygen saturation is >94% 2
- Recognize critical hypoxemia: PaO2 <60 mmHg indicates severe hypoxemia requiring immediate intervention 2
Step 2: Determine Acid-Base Status
Evaluate pH First
- pH <7.35: Acidemia present 1
- pH >7.45: Alkalemia present 1
- pH 7.35-7.45: Normal or fully compensated disorder 3, 4
Identify the Primary Disorder
If acidemia (pH <7.35):
If alkalemia (pH >7.45):
Step 3: Assess for Compensation
- Respiratory compensation for metabolic disorders: PaCO2 changes by approximately 1.2 mmHg for every 1 mmol/L change in HCO3- 3, 4
- Metabolic compensation for respiratory disorders: HCO3- changes by 1 mmol/L for every 10 mmHg change in PaCO2 in acute conditions 3, 4
- Full compensation: pH returns to normal range (7.35-7.45) but never crosses midpoint of 7.40 3, 6
- Partial compensation: pH remains abnormal but is moving toward normal 3, 6
Step 4: Calculate Delta Ratio for Metabolic Acidosis
When anion gap is elevated (>12 mmol/L):
- Calculate delta ratio: (Anion Gap - 12) / (24 - HCO3-) 1
- Delta ratio <1: Concurrent normal anion gap metabolic acidosis 1
- Delta ratio 1-2: Pure anion gap metabolic acidosis 1
- Delta ratio >2: Concurrent metabolic alkalosis 1
Critical Clinical Contexts
Suspected Perforated Peptic Ulcer
- Obtain ABG analysis routinely along with laboratory studies in all suspected cases 2
- Look for metabolic acidosis which indicates peritonitis and systemic inflammation 2
Hepatopulmonary Syndrome
- Measure PaO2 and P(A-a)O2: PaO2 <80 mmHg and P(A-a)O2 ≥15 mmHg (≥20 mmHg if age ≥65) confirms diagnosis 2
- Perform ABG in upright position: Orthodeoxia is characteristic 2
Acute Ischemic Priapism
- Obtain corporal blood gas at initial presentation: PO2 <30 mmHg, PCO2 >60 mmHg, pH <7.25 confirms ischemic priapism 2
- Compare to normal arterial values: PO2 >90 mmHg, PCO2 <40 mmHg, pH 7.40 2
Adrenal Crisis (Addison's Crisis)
- Expect metabolic acidosis: pH <7.35, HCO3- <22 mmol/L 5
- Check for hyperkalemia and hyponatremia: Classic triad with metabolic acidosis 5
- Assess for hypoglycemia: Particularly in children but can occur in adults 5
Oxygen Therapy Titration
- Target SpO2 88-92% for COPD and hypercapnic respiratory failure 2, 1
- Target SpO2 94-98% for all other patients 2, 1
- Repeat ABG within 60 minutes after starting or changing oxygen therapy in patients at risk for CO2 retention 2, 7
- Monitor for respiratory acidosis: pH <7.35 with PaCO2 >6.0 kPa (45 mmHg) indicates need for non-invasive ventilation 2, 1
Common Pitfalls to Avoid
- Never assume normal SpO2 excludes acid-base disturbances: Pulse oximetry cannot detect hypercapnia or metabolic acidosis 1, 7
- Do not discontinue oxygen to obtain room air measurement in patients who clearly require oxygen therapy 2
- Always repeat ABG after oxygen titration in patients with baseline hypercapnia or at risk for CO2 retention 1, 7
- Recognize mixed disorders: Calculate delta ratio when anion gap is elevated to identify concurrent metabolic processes 1
- Consider clinical context: ABG interpretation must integrate patient history, physical examination findings, and other laboratory values 2, 3, 6
Technical Considerations
- Perform Allen's test before radial artery puncture to ensure dual blood supply to the hand 1, 7
- Use arterial samples rather than venous in critically ill patients, especially those with shock or on vasopressors 1, 7
- Apply local anesthesia for all ABG specimens except emergencies 7
- Consider capillary blood gas as alternative for re-measuring PaCO2 and pH during oxygen titration 1