As a DNB instructor, prepare a one‑hour PowerPoint presentation on basic arterial blood‑gas (ABG) interpretation, incorporating difficult‑level multiple‑choice questions, engaging elements, and a concise take‑home message.

Basic Interpretation of Arterial Blood Gas (ABG)

Systematic Three-Step Approach to ABG Interpretation

Use a systematic three-step method: 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, 2, 3

Step 1: Assess pH Status

• Acidemia is defined by pH < 7.35; alkalemia by pH > 7.45 1
• Normal pH range is 7.35–7.45 1
• A normal pH may represent true normality, full compensation of an underlying disorder, or a mixed disorder 1

Step 2: Evaluate the Respiratory Component (PaCO₂)

• Normal PaCO₂ range: 35–45 mmHg 1
• Respiratory acidosis: PaCO₂ > 45 mmHg together with low pH 1, 2
• Respiratory alkalosis: PaCO₂ < 35 mmHg together with high pH 1, 2
• In chronic hypercapnic conditions (e.g., COPD), look for metabolic compensation with elevated HCO₃⁻ 1

Step 3: Evaluate the Metabolic Component (HCO₃⁻/Base Excess)

• Normal HCO₃⁻ range: 22–26 mmol/L 1
• Metabolic acidosis: base excess < –2 mmol/L or HCO₃⁻ < 22 mmol/L 1, 2
• Metabolic alkalosis: base excess > +2 mmol/L or HCO₃⁻ > 26 mmol/L 1, 2

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Understanding Compensation Patterns

Compensation occurs when the body attempts to normalize pH by adjusting the opposing system (respiratory or metabolic). 1

Types of Compensation

• Uncompensated: Only one system (respiratory or metabolic) is abnormal 1
• Partially compensated: Abnormal pH with both PaCO₂ and HCO₃⁻ abnormal, moving in opposite directions to correct pH 1
• Fully compensated: Normal pH with both PaCO₂ and HCO₃⁻ abnormal 1

Clinical Example: Compensated Respiratory Acidosis

• High PaCO₂ with high bicarbonate and normal pH is known as 'compensated respiratory acidosis' 4
• Common in patients with chronic severe but stable COPD 4
• During acute exacerbations, patients may develop 'acute on chronic' respiratory acidosis because the bicarbonate level was equilibrated with the previous CO₂ level and is insufficient to buffer the sudden further increase 4
• Approximately 20% of patients with acute exacerbation of COPD requiring hospital admission have respiratory acidosis 4

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Assessment of Oxygenation

Normal arterial PaO₂ on room air at sea level is > 90 mmHg; severe hypoxemia is PaO₂ < 60 mmHg and requires urgent intervention. 1, 3

Key Oxygenation Parameters

• Normal peripheral oxygen saturation (SpO₂): > 94% 1, 3
• Normal arterial oxygen saturation (SaO₂) in healthy adults at sea level: 95–98% 4
• Alveolar-arterial O₂ gradient (P(A-a)O₂) is considered normal when < 15 mmHg (or < 20 mmHg in patients ≥ 65 years) 1, 3

Clinical Significance of Hypoxemia

• Sudden exposure to SaO₂ levels below 80% can cause impaired mental functioning even in healthy participants 4
• The brain is the most sensitive organ to adverse effects of hypoxia 4
• Most experts emphasize keeping SaO₂ above 90% for the majority of acutely ill patients 4
• Target saturation range of 94–98% for most patients 4

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Clinical Indications for ABG Testing

All critically ill patients require ABG testing to assess oxygenation, ventilation, and acid-base status. 1, 3

Specific Indications

• Patients in shock or with systolic blood pressure < 90 mmHg (arterial sample required) 1
• Unexpected drop of SpO₂ < 94% on room air or supplemental oxygen 1
• A ≥ 3% fall in SpO₂ or worsening dyspnoea in previously stable chronic hypoxaemia 1
• Patients at risk of hypercapnic respiratory failure who develop acute dyspnoea, falling SpO₂, drowsiness, or other signs of CO₂ retention 1
• Dyspnoea where a metabolic cause is suspected (e.g., diabetic ketoacidosis, renal-failure-related metabolic acidosis) 1

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Common Acid-Base Disorders

Respiratory Acidosis

• CO₂ + H₂O → H₂CO₃ → H⁺ + HCO₃⁻ 4
• Acute respiratory acidosis occurs if pH falls below 7.35 in the presence of raised CO₂ level 4
• If present for more than a few hours, the kidney retains bicarbonate to buffer the acidity 4

Metabolic Acidosis

• Caused by failure to excrete acid (e.g., renal failure) or increased acid production (e.g., diabetic ketoacidosis) 4
• May result from direct loss of bicarbonate from kidney or gut (e.g., chronic diarrhoea) 4
• Common cause: lactic acidosis from tissue hypoxia due to decreased oxygen delivery (hypoxaemia, low cardiac output) or conditions like sepsis 4
• In all forms, there is low blood bicarbonate level, either due to loss or buffering of excess acid 4

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Management of Acute Hypercapnic Respiratory Failure

Initiate non-invasive ventilation (NIV) when pH < 7.35 and PaCO₂ > 6.5 kPa (≈ 49 mmHg) despite optimal medical therapy. 1, 2, 3

Oxygen Therapy Targets

• Target SpO₂ of 88–92% for patients with COPD and for all causes of acute hypercapnic failure 1, 2, 3
• Begin supplemental oxygen at 1 L/min and increase in 1 L/min increments until SpO₂ exceeds 90% 1, 3
• Repeat ABG 30–60 minutes after starting or changing oxygen therapy in patients at risk of CO₂ retention 1, 3

NIV Monitoring and Escalation

• Obtain ABG both before and after starting NIV 2
• Maximize time on NIV in the first 24 hours depending on patient tolerance 2
• A rise in PaCO₂ > 1 kPa (≈ 7.5 mmHg) during titration signals clinically unstable disease and warrants reassessment 1, 3

Criteria for Intubation

• No improvement in ABG/pH after 4 hours of NIV 2
• Worsening ABG/pH within 1–2 hours on NIV 2
• Respiratory rate > 35 breaths/min 2
• Severe acidosis alone does not preclude a trial of NIV if immediate access to intubation is available 2

Discontinuation of NIV

• Discontinue NIV when pH and pCO₂ normalize with general improvement in patient condition 2
• Taper daytime NIV use over 2–3 days depending on pCO₂ self-ventilating before discontinuing overnight 2

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Technical Considerations for ABG Sampling

Perform an Allen test before radial arterial sampling to verify dual arterial supply to the hand. 1, 3

Sampling Protocol

• Obtain informed consent after discussing potential risks 1, 3
• Use local anaesthesia for all arterial samples unless the situation is emergent 1

Alternative Sampling Methods

• Capillary blood gas (CBG): Can replace ABG for re-measuring PaCO₂ and pH during oxygen titration 1, 3
• Transcutaneous capnography: Can monitor PaCO₂ trends but does not provide pH or HCO₃⁻ 1
• Arterialised earlobe blood: For non-critical patients needing pH and PaCO₂; PO₂ less reliable (under-estimates by ~0.5–1 kPa) 1

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Critical Pitfalls to Avoid

Normal SpO₂ does not rule out significant acid-base disturbance or hypercapnia; patients may have normal PO₂ with abnormal pH or PaCO₂. 1, 3

Common Management Errors

• Failing to repeat ABG after a change in oxygen therapy in patients at risk of CO₂ retention is a critical management error 1, 2, 3
• In chronic respiratory disorders, base excess shifts as part of metabolic compensation, whereas in acute disorders it remains initially normal 1
• Do not delay initiation of NIV for severe acidosis while awaiting a chest radiograph 1, 2
• NIV use should not delay escalation to invasive mechanical ventilation when more appropriate 2

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Multiple-Choice Questions (Difficult Level)

Question 1

A 68-year-old male with known COPD presents with worsening dyspnoea. ABG on room air shows: pH 7.38, PaCO₂ 58 mmHg, HCO₃⁻ 33 mmol/L, PaO₂ 55 mmHg. What is the most accurate interpretation?

A) Acute respiratory acidosis
B) Compensated respiratory acidosis
C) Mixed respiratory and metabolic acidosis
D) Acute on chronic respiratory acidosis

Correct Answer: B

Explanation: The pH is normal (7.38), PaCO₂ is elevated (58 mmHg), and HCO₃⁻ is elevated (33 mmol/L), indicating fully compensated respiratory acidosis. This is common in chronic severe but stable COPD where the kidney has retained bicarbonate over time to buffer the chronically elevated CO₂. 4, 1

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Question 2

A 45-year-old diabetic patient presents with Kussmaul breathing. ABG shows: pH 7.18, PaCO₂ 22 mmHg, HCO₃⁻ 8 mmol/L, PaO₂ 98 mmHg. What is the primary disorder?

A) Respiratory alkalosis
B) Metabolic acidosis with respiratory compensation
C) Mixed metabolic and respiratory acidosis
D) Compensated metabolic acidosis

Correct Answer: B

Explanation: The pH is low (7.18), indicating acidemia. The HCO₃⁻ is markedly low (8 mmol/L), indicating the primary problem is metabolic acidosis. The PaCO₂ is low (22 mmHg), representing respiratory compensation (hyperventilation) attempting to normalize pH. This is partially compensated metabolic acidosis, typical of diabetic ketoacidosis. 4, 1

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Question 3

A patient on NIV for acute hypercapnic respiratory failure has an ABG repeated after 2 hours showing: pH 7.28 (previously 7.30), PaCO₂ 68 mmHg (previously 65 mmHg), respiratory rate 38/min. What is the most appropriate next step?

A) Continue current NIV settings and recheck in 2 hours
B) Increase oxygen flow rate
C) Prepare for intubation
D) Discontinue NIV and start high-flow nasal cannula

Correct Answer: C

Explanation: The patient shows worsening pH and rising PaCO₂ within 1–2 hours on NIV, plus respiratory rate > 35 breaths/min—all criteria for escalation to invasive mechanical ventilation. NIV failure is indicated by deteriorating parameters despite therapy. 2

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Question 4

A 72-year-old with AECOPD is started on supplemental oxygen. Initial SpO₂ was 84% on room air. After titrating oxygen from 1 L/min to 4 L/min, SpO₂ is now 94%. What is the most important next step?

A) Continue current oxygen and monitor clinically
B) Obtain repeat ABG within 30–60 minutes
C) Increase oxygen to achieve SpO₂ 98%
D) Obtain chest X-ray

Correct Answer: B

Explanation: Patients at risk of CO₂ retention (COPD) must have ABG monitoring after oxygen titration to ensure they are not developing respiratory acidosis. A rise in PaCO₂ > 1 kPa (7.5 mmHg) indicates clinically unstable disease. Failing to repeat ABG is a critical management error. 1, 2, 3

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Question 5

An ABG shows: pH 7.50, PaCO₂ 48 mmHg, HCO₃⁻ 36 mmol/L, PaO₂ 88 mmHg. What is the correct interpretation?

A) Metabolic alkalosis with respiratory compensation
B) Respiratory acidosis with metabolic compensation
C) Mixed metabolic alkalosis and respiratory acidosis
D) Compensated metabolic alkalosis

Correct Answer: C

Explanation: The pH is elevated (7.50), indicating alkalemia. Both the HCO₃⁻ (36 mmol/L, elevated) and PaCO₂ (48 mmHg, elevated) are abnormal but moving in opposite directions. The elevated HCO₃⁻ causes alkalemia (metabolic alkalosis), while the elevated PaCO₂ would cause acidemia (respiratory acidosis). This represents a mixed disorder where both processes coexist. 1

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Question 6

A patient's ABG shows PaO₂ 65 mmHg on room air with P(A-a)O₂ gradient of 35 mmHg. What does this indicate?

A) Normal gas exchange
B) Hypoventilation alone
C) Defect in pulmonary gas exchange (V/Q mismatch, diffusion limitation, or shunt)
D) Metabolic acidosis

Correct Answer: C

Explanation: The P(A-a)O₂ gradient is elevated (normal < 15 mmHg, or < 20 mmHg if age ≥ 65 years). An elevated gradient indicates a defect in pulmonary gas exchange from ventilation-perfusion mismatch, diffusion limitation, or intrapulmonary shunt. Pure hypoventilation would not increase the A-a gradient. 1, 3

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Question 7

A 55-year-old presents with acute dyspnoea. ABG shows: pH 7.32, PaCO₂ 52 mmHg, HCO₃⁻ 26 mmol/L. What is the interpretation?

A) Compensated respiratory acidosis
B) Acute uncompensated respiratory acidosis
C) Mixed respiratory and metabolic acidosis
D) Acute on chronic respiratory acidosis

Correct Answer: B

Explanation: The pH is low (7.32), PaCO₂ is elevated (52 mmHg), and HCO₃⁻ is normal (26 mmol/L). This represents acute uncompensated respiratory acidosis because there has not been sufficient time for renal compensation (bicarbonate retention) to occur. Compensation takes hours to days. 4, 1

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Question 8

For a patient with suspected hepatopulmonary syndrome who is 70 years old, which ABG finding confirms the diagnosis?

A) PaO₂ < 80 mmHg and P(A-a)O₂ ≥ 15 mmHg
B) PaO₂ < 80 mmHg and P(A-a)O₂ ≥ 20 mmHg
C) PaO₂ < 60 mmHg and P(A-a)O₂ ≥ 15 mmHg
D) PaO₂ < 90 mmHg and P(A-a)O₂ ≥ 25 mmHg

Correct Answer: B

Explanation: For hepatopulmonary syndrome diagnosis in patients aged ≥ 65 years, the criteria are PaO₂ < 80 mmHg and P(A-a)O₂ ≥ 20 mmHg (the cutoff increases from 15 mmHg to 20 mmHg for older patients). 2, 3

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Take-Home Messages

1. Always use a systematic three-step approach: pH first, then PaCO₂, then HCO₃⁻/base excess. 1, 2, 3

2. Normal SpO₂ does not exclude significant acid-base disturbances or hypercapnia—always obtain ABG when clinically indicated. 1, 3

3. For acute hypercapnic respiratory failure, initiate NIV when pH < 7.35 and PaCO₂ > 6.5 kPa (49 mmHg), targeting SpO₂ 88–92%. 1, 2, 3

4. Always repeat ABG 30–60 minutes after changing oxygen therapy in patients at risk of CO₂ retention—failure to do so is a critical error. 1, 2, 3

5. Compensated respiratory acidosis (normal pH, high PaCO₂, high HCO₃⁻) is common in stable COPD; acute exacerbations cause 'acute on chronic' acidosis. 4, 1

6. Escalate to intubation if NIV shows no improvement after 4 hours, worsening ABG within 1–2 hours, or respiratory rate > 35/min. 2