How do you interpret arterial blood gas (ABG) results in critically ill adult patients with various medical conditions, such as chronic obstructive pulmonary disease (COPD) and sepsis, to guide treatment decisions?

ABG Interpretation Practice Questions

Question 1: Initial Oxygen Therapy in COPD Exacerbation

A 68-year-old patient with known COPD presents to the ED with increased dyspnea. SpO2 is 84% on room air. What is your initial oxygen therapy and target saturation?

Start with a 28% Venturi mask at 4 L/min (or 24% Venturi mask at 2-3 L/min, or nasal cannulae at 1-2 L/min) targeting SpO2 88-92%, and obtain ABG within 30-60 minutes. 1, 2

• Patients with known or suspected COPD should have controlled low-flow oxygen with a target saturation of 88-92% pending blood gas results 1
• If respiratory rate exceeds 30 breaths/min, increase the flow rate into the Venturi mask by up to 50% above the minimum specified on packaging to compensate for increased inspiratory flow (note: this does not increase FiO2, only ensures adequate flow) 1, 3
• Blood gases must be checked at 30-60 minutes after initiating oxygen to assess for rising PCO2 or falling pH, even if initial saturation improves 1, 2

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Question 2: Interpreting Initial ABG Results

The ABG returns: pH 7.32, PaCO2 58 mmHg, PaO2 62 mmHg, HCO3 30 mmol/L, SpO2 90% on 28% Venturi mask. What does this indicate and what is your next step?

This patient has hypercapnic respiratory failure with respiratory acidosis (pH < 7.35, PCO2 > 6 kPa/45 mmHg); initiate non-invasive ventilation immediately if this persists beyond 30 minutes of optimal medical therapy. 1, 2

• The elevated bicarbonate (>28 mmol/L) with hypercapnia suggests chronic CO2 retention, indicating this patient likely has baseline hypercapnia 1
• Maintain target SpO2 of 88-92% for this patient 1
• If pH remains <7.35 with PCO2 >6.5 kPa (49 mmHg) after 30 minutes of bronchodilators and controlled oxygen, start NIV with CPAP 4-8 cmH2O plus pressure support 10-15 cmH2O 2, 4
• Recheck blood gases 30-60 minutes after any intervention or if clinical deterioration occurs 1, 2

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Question 3: Excessive Oxygen Administration

A COPD patient was given 15 L/min via reservoir mask by EMS. On arrival, SpO2 is 98%, but the patient is drowsy. ABG shows: pH 7.22, PaCO2 72 mmHg, PaO2 110 mmHg. What is your immediate action?

Immediately reduce oxygen to 28% or 24% Venturi mask (or nasal cannulae at 1-2 L/min) targeting SpO2 88-92%—never abruptly discontinue oxygen as this causes life-threatening rebound hypoxaemia. 1, 2

• This represents hypercapnic respiratory failure due to excessive oxygen therapy (PaO2 >10.0 kPa/75 mmHg increases risk of respiratory acidosis) 1
• Sudden cessation of supplementary oxygen causes life-threatening rebound hypoxaemia with rapid fall in saturations below the pre-oxygen baseline 1
• With pH <7.25 and severe hypercapnia, prepare for immediate intubation if NIV fails or patient cannot protect airway 2
• Step down oxygen gradually while monitoring continuous SpO2 and repeat ABG in 30 minutes 1, 2

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Question 4: Septic Shock Presentation

A 45-year-old with suspected sepsis presents with BP 85/50, HR 125, RR 32, SpO2 89% on room air. What is your oxygen strategy and ABG timing?

Give maximum oxygen via reservoir mask at 15 L/min immediately, targeting SpO2 94-98%, and obtain ABG stat as this patient has critical illness requiring high-level supplemental oxygen. 1, 2

• Critical illness (shock, sepsis, major trauma) requires initial reservoir mask at 15 L/min regardless of COPD history until spontaneous circulation is stabilized 1, 2
• Once reliable oximetry is obtained and patient stabilizes, titrate down oxygen while maintaining SpO2 94-98% 1
• ABG must be obtained immediately in critically ill patients with SpO2 <90%, shock, or suspected hypercapnic respiratory failure 2
• If this patient has known COPD, switch to controlled oxygen (target 88-92%) only after blood gas results confirm absence of severe hypoxemia and patient is hemodynamically stable 1

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Question 5: Normal pH with Elevated PCO2

ABG shows: pH 7.38, PaCO2 52 mmHg, PaO2 68 mmHg, HCO3 30 mmol/L, SpO2 91% on 2 L/min nasal cannulae. Patient is comfortable. What is your interpretation and target?

This patient has compensated chronic hypercapnia (pH ≥7.35 with elevated PCO2 and HCO3 >28 mmol/L); maintain target SpO2 88-92% and avoid increasing oxygen unnecessarily. 1

• The normal pH with elevated PCO2 and high bicarbonate indicates long-standing hypercapnia with metabolic compensation 1
• These patients should maintain target range of 88-92% to avoid worsening hypercapnia 1
• Repeat blood gases at 30-60 minutes to ensure PCO2 is not rising and pH is not falling 1
• Document this as the patient's baseline for future admissions and consider patient-specific target range if on long-term home oxygen 1

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Question 6: Carbon Monoxide Poisoning

A patient from a house fire has SpO2 reading of 98% on room air but appears confused. You suspect CO poisoning. What is your oxygen approach and why is oximetry unreliable?

Give maximum oxygen via reservoir mask at 15 L/min immediately, disregarding the normal oximetry reading, because pulse oximetry cannot differentiate carboxyhaemoglobin from oxyhaemoglobin due to similar absorbances. 1, 4

• A normal or high SpO2 reading must be disregarded in suspected CO poisoning 1, 4
• The blood gas PaO2 will also appear normal despite tissue hypoxia 1
• Check carboxyhaemoglobin levels on blood gas analysis 1
• Continue maximum oxygen until carboxyhaemoglobin levels normalize and consider hyperbaric oxygen therapy 1, 4

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Question 7: Post-Intubation ABG Interpretation

A mechanically ventilated septic patient has ABG: pH 7.28, PaCO2 38 mmHg, PaO2 85 mmHg, HCO3 17 mmol/L, lactate 4.2 mmol/L. What acid-base disorder is present?

This is metabolic acidosis (pH <7.35 with normal/low PaCO2 and low HCO3) with elevated anion gap, likely from lactic acidosis due to septic shock; treat the underlying sepsis while maintaining SpO2 94-98%. 4

• The low pH with low bicarbonate and normal PCO2 indicates primary metabolic acidosis 4, 5
• Calculate anion gap: Na - (Cl + HCO3) to determine if gap is elevated (>12 suggests lactic acidosis, ketoacidosis, or toxins) 5, 6
• The elevated lactate confirms tissue hypoperfusion from septic shock 6
• Target SpO2 94-98% in this critically ill patient without COPD 1, 4
• Address the underlying cause (source control, antibiotics, fluid resuscitation) rather than attempting to correct acidosis with bicarbonate unless pH <7.15 6

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Question 8: When to Escalate to Intubation

A COPD patient on NIV for 2 hours has worsening mental status. Repeat ABG: pH 7.18, PaCO2 78 mmHg, PaO2 58 mmHg on NIV with FiO2 0.35. What is your threshold for intubation?

Intubate immediately—pH <7.25 with hypercapnia despite NIV is an absolute indication for mechanical ventilation. 2

• pH <7.25 with hypercapnia represents severe respiratory acidosis requiring immediate intubation 2
• Worsening mental status indicates inability to protect airway and NIV failure 2, 4
• Alternative threshold: pH <7.35 with PaCO2 >6.0 kPa (45 mmHg) and progressive respiratory failure with hypoxemia despite NIV 2
• Do not delay intubation in deteriorating patients—early intubation is safer than emergent intubation 2

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Question 9: VBG vs ABG Utility

Can you use a central venous blood gas instead of ABG in a hemodynamically stable ICU patient to assess ventilation?

Yes, in hemodynamically stable patients, central VBG pH and PCO2 correlate well with ABG (mean difference 0.03-0.05 pH units and 4-6.5 mmHg PCO2), but VBG cannot assess oxygenation adequacy. 7, 8

• Convert central VBG to estimated ABG: arterial pH = venous pH + 0.05 units; arterial PCO2 = venous PCO2 - 5 mmHg 8
• In circulatory failure or shock, the difference between central venous and arterial values is 4-fold greater, making VBG unreliable 8
• VBG cannot replace ABG for oxygenation assessment—must use pulse oximetry in conjunction with VBG 2, 7
• VBG is useful as a screening tool for hypercapnia and can reduce need for arterial punctures in stable patients 7, 8

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Question 10: Monitoring After Oxygen Initiation

You start a COPD patient on 24% Venturi mask. When should you recheck ABG and what are you monitoring for?

Recheck ABG at 30-60 minutes after starting oxygen therapy, monitoring specifically for rising PCO2 or falling pH indicating worsening hypercapnic respiratory failure. 1, 2

• First repeat ABG should be 30-60 minutes after initiating oxygen in at-risk patients 2, 3
• Second repeat ABG should be 1 hour after achieving intended therapeutic flow rate 2
• Even if initial PCO2 was normal, recheck blood gases as hypercapnia can develop during hospital admission 1
• Also recheck immediately if any clinical deterioration occurs (increased drowsiness, rising respiratory rate, falling SpO2) 1, 2
• If pH and PCO2 normalize, may adjust target to 94-98% unless history of previous hypercapnic respiratory failure requiring NIV 1