What is the role of arterial blood gas (ABG) in clinical practice?

Arterial Blood Gas (ABG) in Clinical Practice

ABG analysis is an essential diagnostic tool that directly measures pH, PaCO2, and PaO2 to assess oxygenation, ventilation, and acid-base status, with mandatory use in all critically ill patients and those with suspected respiratory or metabolic derangements. 1, 2

Core Components and What They Measure

ABG analysis provides direct measurement of three critical parameters 3, 4:

• pH: Determines acidemia (pH < 7.35) or alkalemia (pH > 7.45), reflecting overall acid-base status 1, 5
• PaCO2: Indicates ventilation status, with values > 45 mmHg suggesting respiratory acidosis and < 35 mmHg indicating respiratory alkalosis 1
• PaO2: Assesses oxygenation adequacy, with target ≥ 60 mmHg (8 kPa) on supplemental oxygen for hypoxemic patients 5
• Bicarbonate (HCO3) and Base Excess: Calculated values identifying metabolic components, with HCO3 < 22 or base excess < -2 indicating metabolic acidosis 1

Systematic Three-Step Interpretation Approach

Use this algorithmic method for every ABG 1, 5:

1. Evaluate pH first: Determine if acidemia or alkalemia is present
2. Examine PaCO2: Identify the respiratory component (PaCO2 moves opposite to pH in respiratory disorders)
3. Assess HCO3/base excess: Identify the metabolic component (HCO3 moves in same direction as pH in metabolic disorders)

Mandatory Indications for ABG Testing

Order ABG immediately in these clinical scenarios 1, 2:

• All critically ill patients requiring assessment of oxygenation, ventilation, and acid-base status 1, 2
• Shock or hypotension of any etiology (initial blood gas must be arterial, not venous) 2, 5
• Oxygen saturation < 94% on room air or supplemental oxygen, or any unexpected fall in SpO2 2
• Deteriorating chronic hypoxemia: SpO2 fall ≥ 3% or increasing breathlessness in previously stable patients (e.g., severe COPD) 2
• Suspected metabolic emergencies: Diabetic ketoacidosis, renal failure, sepsis, trauma-related metabolic acidosis 1

Critical Timing for ABG Measurements

Obtain ABG at these specific timepoints 2, 5:

• Within 60 minutes of starting oxygen therapy in any patient 2
• Within 60 minutes after each change in inspired oxygen concentration, especially in COPD patients 2
• After each titration of oxygen flow rate in patients with baseline hypercapnia to monitor for worsening CO2 retention 2, 5
• Before and after initiating NIV to assess response and guide ongoing management 1

Management Based on ABG Results

Acute Hypercapnic Respiratory Failure

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

• Start with CPAP 4-8 cmH2O plus pressure support 10-15 cmH2O 1
• Target SpO2 88-92% in all causes of acute hypercapnic respiratory failure, including COPD 1, 2
• Maximize NIV time in first 24 hours depending on patient tolerance 1
• Obtain repeat ABG within 1-2 hours to assess response 1

Oxygen Therapy in At-Risk Patients

For COPD or patients with known CO2 retention, use controlled oxygen titration 2:

• Start with low-flow oxygen at 1 L/min 2
• Titrate up in 1 L/min increments until SpO2 > 90% 2
• Confirm adequacy with repeat ABG after each titration 2, 5
• Monitor for respiratory acidosis (PaCO2 rise > 1 kPa or 7.5 mmHg) indicating need for NIV 2

Criteria for Intubation

Escalate to invasive mechanical ventilation if 1:

• Worsening ABG/pH after 1-2 hours on NIV
• Lack of improvement after 4 hours of NIV
• Respiratory rate > 35 breaths/min despite NIV
• PaCO2 rises > 1 kPa (7.5 mmHg) despite NIV

Special Population Considerations

Respiratory Muscle Weakness

In chronic muscle weakness, ABG abnormalities appear late and indicate severe impairment 6:

• Daytime hypercapnia unlikely unless respiratory muscle strength < 40% predicted and VC < 50% predicted 6
• Mild weakness typically shows PaCO2 less than normal (alveolar hyperventilation) 6
• Hypercapnia predicts mortality: In Duchenne muscular dystrophy, elevated PaCO2 correlates with shorter survival 6
• Daytime ABG underestimates severity: Nocturnal measurements are more sensitive for detecting abnormal gas exchange 6

Hepatopulmonary Syndrome

Use age-adjusted diagnostic criteria 1:

• PaO2 < 80 mmHg or P(A-a)O2 ≥ 15 mmHg for patients < 65 years
• P(A-a)O2 ≥ 20 mmHg cutoff for patients ≥ 65 years
• PaO2 < 60 mmHg warrants liver transplant evaluation

Technical Considerations

Follow these procedural standards 2, 5:

• Perform Allen's test before radial artery puncture to confirm dual blood supply from radial and ulnar arteries 2, 5
• Use local anesthesia for all ABG specimens except emergencies 2, 5
• Obtain consent with discussion of risks including arterial injury, thrombosis, hematoma, and reflex sympathetic dystrophy 5, 7

Critical Pitfalls to Avoid

These common errors lead to management failures 2, 5:

• Normal SpO2 does not exclude acid-base disturbances or hypercapnia, especially in patients on supplemental oxygen 2, 5
• Pulse oximetry cannot detect abnormal acid-base status, ventilation problems, or low oxygen content from anemia 2
• Failing to repeat ABG after oxygen changes in patients at risk for CO2 retention is a critical management error 1, 2
• Delaying NIV initiation while waiting for chest radiography in severe acidosis worsens outcomes 1
• Severe acidosis alone does not preclude NIV trial if performed in appropriate setting with ready access to intubation 1
• Injudicious sedatives or uncontrolled oxygen can precipitate acute-on-chronic respiratory failure in patients with established or incipient hypercapnia 6

Venous Blood Gas as Alternative

In hypotensive patients, venous blood gas (VBG) shows strong correlation with ABG for pH, pCO2, HCO3, lactate, and electrolytes, but cannot assess oxygenation (pO2) 7. VBG is less painful and easier to obtain but should not replace ABG when oxygenation assessment is critical 7.