How do I obtain, interpret, and manage an arterial blood gas (ABG) in an adult patient?

Arterial Blood Gas (ABG): Obtaining, Interpreting, and Managing

When to Obtain an ABG

Blood gases should be checked in all critically ill patients, those with shock or hypotension (systolic BP <90 mmHg), and any patient with SpO₂ <94% on room air or oxygen. 1

Specific Clinical Indications:

• All critically ill patients 1, 2
• Shock or systolic blood pressure <90 mmHg (arterial sample required) 1
• Unexpected fall in SpO₂ below 94% on air or oxygen 1
• Deteriorating oxygen saturation (fall of ≥3%) or increasing breathlessness in patients with previously stable chronic hypoxemia 1
• Any patient with risk factors for hypercapnic respiratory failure (COPD, neuromuscular disease, chest wall deformity, morbid obesity) who develops acute breathlessness, falling SpO₂, or drowsiness 1
• Suspected metabolic conditions: diabetic ketoacidosis or metabolic acidosis from renal failure 1, 2
• Post-cardiopulmonary resuscitation 1, 3
• After any increase in oxygen therapy in patients at risk of CO₂ retention 1, 2

Critical Pitfall:

A normal SpO₂ does NOT rule out significant acid-base disturbances or hypercapnia—patients may have normal PO₂ with abnormal pH or PCO₂, especially if on supplemental oxygen. 1, 2, 4

How to Obtain an ABG

Pre-procedure Steps:

• Perform Allen's test before radial artery puncture to ensure dual blood supply to the hand from both radial and ulnar arteries 1, 4
• Obtain informed consent with discussion of possible risks 1, 4
• Use local anesthesia for all ABG specimens except in emergencies 1, 4, 3

Sample Source Priority:

• For critically ill patients or those with shock/hypotension, obtain arterial samples (not venous or capillary) 1, 2, 4
• Use existing arterial lines when available to minimize complications 3
• For non-critical patients, arterialised earlobe blood gases may be used for pH and PCO₂ (but PO₂ underestimates by 0.5-1 kPa) 1, 4

Alternative Sampling Methods:

• Capillary blood gases (CBG) can be used for re-measuring PCO₂ and pH during oxygen titration in LTOT assessment 1, 4
• Transcutaneous capnography can monitor PCO₂ trends but does NOT provide pH or HCO₃⁻ 1, 4

How to Interpret an ABG

Use a systematic three-step approach: (1) assess pH, (2) examine PaCO₂ for the respiratory component, (3) evaluate HCO₃⁻/base excess for the metabolic component. 2, 4

Step 1: Assess pH

• pH <7.35 = acidemia 2, 4
• pH >7.45 = alkalemia 2, 4
• Normal pH (7.35-7.45) may represent true normality, full compensation, or a mixed disorder 4

Step 2: Identify Respiratory Component

• Respiratory acidosis: PaCO₂ >45 mmHg with low pH 2, 4
• Respiratory alkalosis: PaCO₂ <35 mmHg with high pH 2, 4
• Normal PaCO₂: 35-45 mmHg 4
• In chronic hypercapnic conditions (e.g., COPD), look for metabolic compensation with elevated HCO₃⁻ 4

Step 3: Identify Metabolic Component

• Metabolic acidosis: base excess <-2 mmol/L or HCO₃⁻ <22 mmol/L 2, 4
• Metabolic alkalosis: base excess >+2 mmol/L or HCO₃⁻ >26 mmol/L 2, 4
• Normal HCO₃⁻: 22-26 mmol/L 4

Step 4: Assess Oxygenation

• Normal PaO₂ on room air at sea level: >90 mmHg 4
• Severe hypoxemia requiring urgent intervention: PaO₂ <60 mmHg 4
• Normal alveolar-arterial O₂ gradient: <15 mmHg (<20 mmHg if age ≥65 years) 2, 4

Compensation Patterns:

• Fully compensated: normal pH with both PaCO₂ and HCO₃⁻ abnormal 4
• Partially compensated: abnormal pH with both PaCO₂ and HCO₃⁻ abnormal moving in opposite directions 4
• Uncompensated: only one system abnormal 4

How to Manage 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. 2, 4

NIV Protocol:

• Target SpO₂ 88-92% for all causes of acute hypercapnic respiratory failure including COPD 1, 2, 4
• Obtain ABG before and after starting NIV 2
• Maximize time on NIV in first 24 hours depending on patient tolerance 2
• Repeat ABG 30-60 minutes after starting or changing oxygen therapy 1, 2, 4
• Monitor for worsening pH and respiratory rate as indicators to change management 2

Intubation Criteria:

• Worsening ABG/pH after 1-2 hours on NIV 2
• Lack of improvement after 4 hours of NIV 2
• Respiratory rate >35 breaths/min 2
• Severe acidosis alone does not preclude NIV trial if performed in appropriate setting with ready access to intubation 2

Oxygen Titration Protocol

Start at 1 L/min and increase in 1 L/min increments until SpO₂ >90%, then obtain ABG to confirm PaO₂ ≥8 kPa (60 mmHg) without inducing respiratory acidosis. 1, 4

For Patients at Risk of Hypercapnia:

• Use 24% or 28% Venturi mask or 1-2 L/min nasal oxygen initially 1
• Target SpO₂ 88-92% pending blood gas results 1, 4
• A rise in PaCO₂ >1 kPa (7.5 mmHg) during oxygen titration signals clinically unstable disease requiring reassessment 1, 4
• Increase Venturi mask flow by up to 50% if respiratory rate >30 breaths/min 1

For Patients NOT at Risk of Hypercapnia:

• Target SpO₂ 94-98% 1
• If SpO₂ <85%, start with reservoir mask at 15 L/min 1
• Otherwise use nasal cannulae 2-6 L/min or simple face mask 5-10 L/min 1

Severe Acidosis Management

For severe acidosis (arterial pH <7.1 and base excess <-10), consider sodium bicarbonate 50 mmol (50 ml of 8.4% solution), with further doses guided by repeat ABG. 1

Special Situations for Buffer Use:

• Cardiac arrest associated with hyperkalemia 1
• Tricyclic antidepressant overdose 1
• Diabetic ketoacidosis with severe acidosis 1

Critical Management Errors to Avoid:

• Failing to repeat ABG after oxygen therapy changes in patients at risk of CO₂ retention is a critical error 2, 4
• Do not delay NIV initiation for chest radiography in severe acidosis 2, 4
• Do not assume normal SpO₂ excludes acid-base disturbance, especially if patient is on supplemental oxygen 1, 2, 4
• NIV should not delay intubation when more appropriate 2