Blood Gas
Also written as ABG.
| Arterial blood gas (ABG) | Is there hypoxaemia or hypercapnia indicating respiratory failure? | PaO₂ < 60 mmHg or PaCO₂ > 45 mmHg in chronic bronchitis suggests advanced disease; normal ABG in mild acute bronchitis. |
What our notes say
- | Arterial blood gas (ABG) | Is there hypoxaemia or hypercapnia indicating respiratory failure? | PaO₂ < 60 mmHg or PaCO₂ > 45 mmHg in chronic bronchitis suggests advanced disease; normal ABG in mild acute bronchitis. | (Bronchitis (Acute and Chronic))
- Investigations: chest X-ray (consolidation); full blood count and CRP; blood cultures and sputum for Gram stain, culture and GeneXpert or AFB; urea and electrolytes; pulse oximetry or arterial blood gas; HIV test; pleural fluid analysis if there is an effusion. (Internal Medicine Respiratory CAT 1: 30 MCQs and 5 short answer questions)
- Pulse oximetry/arterial blood gas; FBC (raised white cells), CRP, U&E, LFTs. (Pneumonia)
- Arterial blood gas and lactate: show the depth of shock and acidosis. (General Surgery CAT 2 (MBSG 4600): 4 October 2021)
- CRP and procalcitonin, blood glucose, urea and electrolytes, and blood gas. (Integrated case paper: Medicine, Surgery, Gynaecology and Paediatrics (160 marks)
- Arterial blood gas, pulse oximetry and ECG (to exclude cardiac contusion). (Internal Medicine Respiratory CAT 1: 30 MCQs and 5 short answer questions)
Study material on Blood Gas
- Respiratory failure and arterial blood gas interpretation
- Chemical Pathology MCQs: Clinical Chemistry, Endocrine & Renal
- Chemical Pathology MCQs
- Indications for dialysis
- Chemical Path, Long Answer and Short Answer
- Chemical Pathology MCQs
- Clinical Biochemistry — Practice Questions
Images
Practice questions on Blood Gas
A 35-year-old diabetic man presents with confusion and deep rapid breathing. Blood gas: pH 7.1, PaCO₂ 22 mmHg, HCO₃⁻ 10 mmol/L, glucose 550 mg/dL. What is the primary acid-base disorder?
- Metabolic acidosis
- Respiratory acidosis
- Metabolic alkalosis
- Mixed disorder Correct
Answer: A. Low pH (7.1) with low bicarbonate (10) and compensatory hyperventilation (low CO₂) indicates metabolic acidosis from DKA.
A 72-year-old man with COPD presents with confusion. Blood gas: pH 7.28, PaCO₂ 60 mmHg, HCO₃⁻ 28 mmol/L. What is the most likely acid-base disorder?
- Respiratory acidosis
- Respiratory alkalosis
- Metabolic acidosis
- Metabolic alkalosis Correct
Answer: A. Low pH with high CO₂ indicates respiratory acidosis. Elevated bicarbonate suggests chronic compensation.
A 45-year-old man presents with confusion and deep rapid breathing. Blood gas shows: pH 7.15, PaCO₂ 25 mmHg, HCO₃⁻ 10 mmol/L, Na⁺ 140 mmol/L, Cl⁻ 100 mmol/L. What is the anion gap?
- 25
- 30
- 20
- 15 Correct
Answer: B. Anion gap = Na⁺ - (Cl⁻ + HCO₃⁻) = 140 - (100 + 10) = 30. High anion gap suggests organic acidosis.
A 60-year-old diabetic presents with confusion, Kussmaul breathing, and fruity breath odor. Blood gas: pH 7.15, PaCO₂ 22 mmHg, HCO₃⁻ 10 mmol/L. What is the diagnosis?
- Metabolic alkalosis
- Diabetic ketoacidosis
- Respiratory acidosis
- Normal anion gap metabolic acidosis Correct
Answer: B. Classic DKA presentation: acidosis (pH 7.15), compensatory hyperventilation (low CO₂), ketotic breath, and altered mental status.
A 70-year-old COPD patient presents with confusion. Blood gas: pH 7.28, PaCO₂ 60 mmHg, HCO₃⁻ 32 mmol/L. What is the acid-base disorder?
- Metabolic acidosis
- Respiratory acidosis
- Respiratory alkalosis
- Mixed disorder Correct
Answer: B. Low pH with high CO₂ indicates respiratory acidosis. Elevated bicarbonate shows chronic renal compensation.
A young patient with aspirin overdose has the following blood gas: pH 7.45, PaCO₂ 24 mmHg, HCO₃⁻ 18 mmol/L. What is the likely diagnosis?
- Metabolic acidosis
- Mixed respiratory alkalosis & metabolic acidosis
- Metabolic alkalosis
- Respiratory acidosis Correct
Answer: B. Salicylates cause both direct respiratory stimulation (alkalosis) and metabolic acidosis, creating a mixed disorder. ## Cardiovascular Pathology & Biomarkers