How to Interpret Arterial Blood Gas (ABG) Results
Systematic Three-Step Approach
Use a systematic three-step method to interpret every ABG: first evaluate pH to determine acidemia versus 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
- pH < 7.35 indicates acidemia 1
- pH > 7.45 indicates alkalemia 1
- This determines the primary direction of the acid-base disturbance 1, 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
- Normal PaCO2 is 35-45 mmHg 1, 2
Step 3: Assess the Metabolic Component (Bicarbonate/Base Excess)
- Base excess < -2 or HCO3 < 22 mmol/L indicates metabolic acidosis 1
- Base excess > +2 or HCO3 > 26 mmol/L indicates metabolic alkalosis 1
- Normal bicarbonate is 22-26 mmol/L 1, 2
Step 4: Calculate Delta Ratio for Mixed Disorders (When Applicable)
- When metabolic acidosis with elevated anion gap is identified, calculate the delta ratio: (Anion Gap - 12) / (24 - HCO3⁻) 2
- This helps identify coexisting metabolic alkalosis or additional metabolic acidosis 2
- Critical caveat: The delta ratio has limitations in chronic conditions where baseline bicarbonate may differ significantly from 24 mmol/L 2
Assess Oxygenation Status
Evaluate PaO2
- Normal PaO2: >90 mmHg on room air at sea level 3
- Severe hypoxemia: PaO2 <60 mmHg requires immediate intervention 3
Calculate Alveolar-Arterial (A-a) Oxygen Gradient
- Normal P(A-a)O2: <15 mmHg (or <20 mmHg if age ≥65 years) 1, 3
- Elevated P(A-a)O2 indicates pulmonary gas exchange defects from V/Q mismatch, diffusion limitation, or shunt 2
Interpret Oxygen Saturation
- Normal arterial oxygen saturation: >94% 3
- Target SpO2 88-92% for COPD and all causes of acute hypercapnic respiratory failure 4, 1, 2
- Target SpO2 94-98% for all other patients 3
Critical Clinical Contexts Requiring ABG
Mandatory Indications
- All critically ill patients to assess oxygenation, ventilation, and acid-base status 1, 2, 3
- Shock or hypotension (initial sample must be arterial) 1, 2, 3
- SpO2 fall below 94% on room air or supplemental oxygen 1, 2, 3
- Deteriorating oxygen saturation (fall ≥3%) or increasing breathlessness in patients with previously stable chronic hypoxemia 3
Metabolic Emergencies
Respiratory Failure Management
- Before and after initiating non-invasive ventilation (NIV) 4, 1
- Within 60 minutes of starting or changing oxygen therapy in patients at risk for CO2 retention 2, 3
- After each titration of oxygen flow rate in patients with baseline hypercapnia 2, 3
Management Based on ABG Results
Acute Hypercapnic Respiratory Failure
Initiate NIV when pH <7.35 and PaCO2 >6.5 kPa (49 mmHg) persist despite optimal medical therapy. 4, 1, 2
- Start controlled oxygen therapy targeting SpO2 88-92% for all causes of acute hypercapnic respiratory failure 4, 1, 2
- Begin oxygen at 1 L/min and titrate up in 1 L/min increments until SpO2 >90% 2, 3
- Obtain ABG prior to and following starting NIV 4, 1
- Maximize time on NIV in the first 24 hours depending on patient tolerance 4, 1
Monitoring for Worsening Hypercapnia
- A rise in PaCO2 >1 kPa (7.5 mmHg) indicates clinically unstable disease requiring further medical optimization 1, 2
- Monitor for worsening pH and respiratory rate, which indicate need to change management strategy 4, 1
Criteria for Intubation
- Worsening ABG/pH in 1-2 hours on NIV 1
- Lack of improvement after 4 hours of NIV 1
- Respiratory rate >35 breaths/min 1
- Severe acidosis alone does not preclude a trial of NIV in an appropriate area with ready access to intubation 4, 1
Discontinuing NIV
- Discontinue NIV when pH and pCO2 normalize with general improvement in patient condition 4, 1
- Taper daytime NIV use over 2-3 days depending on pCO2 before discontinuing overnight 4, 1
Technical Considerations
Sampling Technique
- Perform Allen's test before radial ABG to ensure dual blood supply to the hand from both radial and ulnar arteries 2, 3
- Use local anesthesia for all ABG specimens except in emergencies 3
- 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 4, 2
- Arterialized earlobe blood gases may be used for non-critical patients to measure acid-base status and ventilation 2, 3
- Transcutaneous CO2 (PtCO2) monitoring may be useful when calibrated and readings are within 10 mm Hg of concurrent arterial PCO2 4
Carboxyhemoglobin Considerations
- Standard two-wavelength pulse oximeters cannot distinguish carboxyhemoglobin from oxyhemoglobin, potentially leading to falsely normal oxygen saturation readings 2
- Either arterial or venous specimens provide comparable carboxyhemoglobin concentrations 2
Common Pitfalls to Avoid
Critical Errors in Interpretation
- A normal oxygen saturation does not rule out significant acid-base disturbances or hypercapnia, especially in patients on supplemental oxygen 2, 3
- Pulse oximetry will be normal in patients with normal oxygen levels but abnormal acid-base status, ventilation, or low blood oxygen content due to anemia 3
- Older blood-gas analyzers lacking a carbon-monoxide oximetry module may miss clinically significant elevations in carboxyhemoglobin 2
Management Errors
- Failing to repeat ABG measurements after changes in oxygen therapy in patients at risk for CO2 retention is a critical error 1, 2, 3
- Chest radiography should not delay initiation of NIV in severe acidosis 4, 1
- NIV use should not delay escalation to invasive mechanical ventilation when more appropriate 4, 1