How do you interpret and manage arterial blood gas (ABG) results in patients with acid-base disturbances?

Systematic Approach to ABG Interpretation and Management

Core Interpretation Framework

Use a three-step systematic method: first evaluate pH to determine acidemia (pH <7.35) or alkalemia (pH >7.45), then examine PaCO2 to identify the respiratory component (>45 mmHg indicates respiratory acidosis, <35 mmHg indicates respiratory alkalosis), and finally evaluate base excess/bicarbonate to identify the metabolic component (base excess <-2 or HCO3 <22 indicates metabolic acidosis, base excess >+2 or HCO3 >26 indicates metabolic alkalosis). 1, 2

Step-by-Step Analysis

• Step 1: Assess pH - Determine if acidemia or alkalemia is present, as this identifies the primary disorder 1, 2
• Step 2: Evaluate PaCO2 - Values >45 mmHg with low pH indicate respiratory acidosis; values <35 mmHg with high pH indicate respiratory alkalosis 1, 2
• Step 3: Evaluate Metabolic Component - Base excess and bicarbonate levels reveal metabolic contributions to the acid-base disturbance 1, 2
• Step 4: Calculate Delta Ratio - When metabolic acidosis with elevated anion gap is identified, calculate (Anion Gap - 12) / (24 - HCO₃⁻) to detect mixed disorders 1

Critical Indications for ABG Testing

All critically ill patients require ABG testing to assess oxygenation, ventilation, and acid-base status. 1, 2

Specific Clinical Scenarios Requiring ABG

• Shock or hypotension - Initial blood gas must be from arterial source 3, 1
• SpO2 <94% on room air or supplemental oxygen - Pulse oximetry alone cannot detect hypercapnia or acid-base disturbances 1, 2
• Suspected diabetic ketoacidosis, metabolic acidosis from renal failure, trauma, sepsis 2
• Post-cardiopulmonary resuscitation - Essential to guide ongoing oxygen therapy 4
• COPD patients starting oxygen therapy - Especially those with known CO2 retention 2

Common Pitfall to Avoid

• Normal oxygen saturation does NOT rule out significant acid-base disturbances or hypercapnia - Pulse oximetry will appear normal in patients with normal PO2 but abnormal pH or PCO2 1, 4

Management Based on ABG Results

Acute Hypercapnic Respiratory Failure

Initiate non-invasive ventilation (NIV) for pH <7.35 and PaCO2 >6.5 kPa (49 mmHg) despite optimal medical therapy. 1, 2

• Start with CPAP 4-8 cmH2O plus pressure support 10-15 cmH2O 2
• Target SpO2 88-92% for COPD and all causes of acute hypercapnic respiratory failure 1, 2
• Monitor for intubation criteria: worsening ABG/pH in 1-2 hours, lack of improvement after 4 hours, respiratory rate >35 breaths/min 2

Oxygen Therapy Titration

Start oxygen at 1 L/min and titrate up in 1 L/min increments until SpO2 >90%, then perform ABG to confirm target PaO2 ≥8 kPa (60 mmHg) at rest. 3, 1

• For patients WITHOUT pre-existing COPD - High concentrations of oxygen (35% or greater) are indicated and can be safely used 3
• For patients WITH COPD or CO2 retention - Initial treatment with low oxygen concentrations (24-28%) should be progressively increased based on repeated ABG measurements, keeping SaO2 >90% without causing arterial pH to fall below 7.35 3

Monitoring Hypercapnia During Oxygen Therapy

Patients with baseline hypercapnia must have ABG monitoring after each flow rate titration. 3, 1

• Critical threshold: A rise in PaCO2 >1 kPa (7.5 mmHg) indicates clinically unstable disease requiring further medical optimization and reassessment after 4 weeks 3, 1
• If respiratory acidosis and/or PaCO2 rise >1 kPa occurs on two repeated occasions while apparently clinically stable, only order domiciliary oxygen in conjunction with nocturnal ventilatory support 3

Timing of Repeat ABG Measurements

Check ABG within 60 minutes of starting oxygen therapy and within 60 minutes of any change in inspired oxygen concentration in patients at risk for hypercapnic respiratory failure. 2

• If oxygen prescribed during an exacerbation, recheck ABG in 30-90 days 3
• Perform ABG after oxygen titration is complete to determine whether adequate oxygenation has been achieved without precipitating respiratory acidosis 3

Technical Considerations for ABG Sampling

Pre-Procedure Assessment

Perform Allen's test before radial ABG to ensure dual blood supply to the hand from both radial and ulnar arteries. 3, 1, 4

• Obtain informed consent with discussion of possible risks 3, 1
• Use local anesthesia for all ABG specimens except in emergencies 4

Alternative Sampling Methods

• Capillary blood gases (CBG) - Can replace ABG for re-measuring PaCO2 and pH during oxygen titration 3, 1
• Cutaneous capnography - Can replace ABG for re-measuring PaCO2 alone but NOT pH 3, 1
• Arterial samples preferred - Over capillary samples in critically ill patients 4

Essential Documentation

Knowledge of the inspired oxygen concentration is essential to interpretation of blood gas measurements and must be clearly recorded with the blood gas result. 3

Long-Term Oxygen Therapy (LTOT) Assessment

Confirmation Requirements

Patients assessed for LTOT during apparent clinical stability should undergo two ABG measurements at least 3 weeks apart before the need for LTOT can be confirmed. 3

• Physiological indication: PaO2 <7.3 kPa (55 mmHg) 3
• Therapeutic goal: Maintain SpO2 ≥90% during rest, sleep, and exertion 3
• LTOT should be ordered for minimum 15 hours per day, up to 24 hours per day may provide additional benefit 3

Flow Rate Adjustments

• Non-hypercapnic patients should increase flow rate by 1 L/min during sleep in absence of contraindications 3
• Patients active outdoors should receive ambulatory oxygen assessment to determine if flow rate needs increasing during exercise 3
• Patients with cognitive, visual, or coordination impairments should be maintained on a single flow rate 3

Special Population Considerations

ECMO Patients

• ABG samples should come from right radial arterial line as this best represents cerebral perfusion 4
• Helps identify "Harlequin syndrome" or "North-South syndrome" in VA-ECMO where differential oxygenation occurs between upper and lower body 4

Hepatopulmonary Syndrome

• Use P(A-a)O2 ≥20 mmHg cutoff instead of ≥15 mmHg for diagnosis in patients aged ≥65 years 2
• Patients with PaO2 <60 mmHg should be evaluated for liver transplantation 2
• Perform ABG analysis every 6 months in these patients 2

Cardiogenic Shock

• ABG analysis identifies metabolic acidosis associated with poor outcomes 4
• Helps differentiate between cardiac and pulmonary causes of respiratory distress in acute heart failure 4
• Lactate levels obtained with ABG provide information about tissue oxygenation and perfusion 4

Critical Management Errors 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. 1

• Never withdraw oxygen based on improved PaO2 in patients whose need was determined when in stable state - this may be detrimental 3
• Reversal of hypoxemia supersedes concerns about CO2 retention 3
• Flow rates may be increased at 20-minute intervals during oxygen titration until target PaO2 is achieved 3