ICU Vivas · Oxygen & gas exchange
Data viva — an arterial blood gas in respiratory failure
A data-interpretation viva testing the alveolar gas equation, the A-a gradient, the P/F ratio and the acid-base status.
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Prompt
An arterial blood gas from a patient breathing FiO2 0.8 — to be interpreted, including the A-a gradient and the mechanism of the hypoxaemia.
Stimulus
You are shown the arterial blood gas of a 50-year-old at sea level breathing FiO2 0.8:
| Variable | Value | Reference |
|---|---|---|
| pH | 7.32 | 7.35–7.45 |
| PaCO2 | 30 mmHg | 35–45 mmHg |
| PaO2 | 55 mmHg | Interpret against age, altitude and FiO2 |
| HCO3 | 16 mmol/L | — |
| Lactate | 3.5 mmol/L | — |
Interpret the gas, calculate the A-a gradient, and discuss the mechanism of the hypoxaemia.[1][2]
References10ShowHide
- [1]Hendrix JM, Burns B Alveolar Gas Equation StatPearls [Internet], 2024.PMID 29489223
- [2]Hantzidiamantis PJ, Amaro E Physiology, Alveolar to Arterial Oxygen Gradient StatPearls [Internet], 2023.PMID 31424737
- [3]Trulock EP III Arterial Blood Gases NCBI Bookshelf / Butterworths, 1990.PMID 21250212
- [4]Krishnaprasadh D, Sharma S Hypocarbia StatPearls [Internet], 2026.PMID 29630219
- [5]Baddam S, Tubben RE Lactic Acidosis StatPearls [Internet], 2026.PMID 29262026
- [6]Marano M Evaluation of the expected ventilatory response to metabolic acidosis in chronic hemodialysis patients Hemodial Int, 2018.PMID 28834137
- [7]Kharsa A, Vashisht R, Rout P, Meseeha M Anion Gap and Non-Anion Gap Metabolic Acidosis StatPearls [Internet], 2026.PMID 28846305
- [8]ARDS Definition Task Force, Ranieri VM, Rubenfeld GD, et al. Acute respiratory distress syndrome: the Berlin Definition JAMA, 2012.PMID 22797452
- [9]Bhutta BS, Alghoula F, Berim I Hypoxia and Hypoxemia StatPearls [Internet], 2024.PMID 29493941
- [10]Morikawa MJ, Ganesh PR Acid-Base Interpretation: A Practical Approach Am Fam Physician, 2025.PMID 39964926