Analyse this ABG at 60% Fio2, ph 7.48, pco2 22.8, hco3 16.6, po2 123
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ABG arterial blood gas interpretation respiratory alkalosis metabolic acidosis compensation
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oxygenation assessment PaO2 FiO2 ratio hypoxemia supplemental oxygen
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Here is the full ABG analysis:
ABG Analysis — FiO₂ 60%
| Parameter | Value | Reference Range |
|---|---|---|
| pH | 7.48 | 7.35–7.45 |
| PaCO₂ | 22.8 mmHg | 35–45 mmHg |
| HCO₃⁻ | 16.6 mmol/L | 22–26 mmol/L |
| PaO₂ | 123 mmHg | 80–100 mmHg (on room air) |
| FiO₂ | 60% | — |
Step-by-Step Interpretation
Step 1 — pH
7.48 → Alkalosis
Step 2 — Primary Disorder
- PaCO₂ 22.8 mmHg (↓) → Primary Respiratory Alkalosis
- HCO₃⁻ 16.6 mmol/L (↓) → points toward metabolic acidosis or compensation
Step 3 — Compensation (Expected HCO₃⁻)
For chronic respiratory alkalosis, the expected metabolic compensation is:
ΔHCO₃⁻ = 0.5 × ΔPCO₂
- ΔPCO₂ = 40 − 22.8 = 17.2 mmHg
- Expected ΔHCO₃⁻ = 0.5 × 17.2 = 8.6
- Expected HCO₃⁻ = 24 − 8.6 = 15.4 mmol/L
The actual HCO₃⁻ is 16.6, which is very close to the expected value of 15.4.
➡ This is appropriate metabolic compensation — consistent with chronic respiratory alkalosis, with no evidence of a concurrent primary metabolic acidosis.
(For acute respiratory alkalosis, the compensation would only be 0.2 × ΔPCO₂, giving an expected HCO₃⁻ of ~20.6 — far higher than the actual value — making the chronic pattern the better fit here.)
Step 4 — Oxygenation: P/F Ratio
PaO₂/FiO₂ ratio = 123 / 0.60 = 205 mmHg
| P/F Ratio | Classification |
|---|---|
| >300 | Normal |
| 200–300 | Mild hypoxaemia / Mild ARDS |
| 100–200 | Moderate ARDS |
| <100 | Severe ARDS |
➡ P/F ratio of 205 = Mild hypoxaemia / Mild ARDS threshold — this patient requires 60% FiO₂ to maintain a PaO₂ of 123 mmHg, indicating significant underlying oxygenation impairment.
Step 5 — A-a Gradient
A-a gradient = [FiO₂ × (713) − PaCO₂/0.8] − PaO₂
- PAO₂ = 0.60 × 713 − (22.8 / 0.8) = 427.8 − 28.5 = 399.3 mmHg
- A-a gradient = 399.3 − 123 = 276.3 mmHg (markedly elevated; normal on 60% FiO₂ should be ~30–50 mmHg)
➡ Markedly raised A-a gradient confirms significant V/Q mismatch or shunt as the cause of hypoxaemia.
Summary & Clinical Interpretation
| Finding | |
|---|---|
| Primary disorder | Respiratory alkalosis |
| Compensation | Appropriate chronic metabolic compensation |
| Mixed disorder? | No — compensation is adequate |
| Oxygenation | Impaired (P/F = 205; elevated A-a gradient) |
Final diagnosis: Chronic respiratory alkalosis with appropriate metabolic compensation, on a background of significant hypoxaemic respiratory failure.
Differential Diagnosis for This ABG Pattern
The combination of hyperventilation + hypoxaemia strongly suggests the hyperventilation is driven by hypoxia or an underlying pulmonary process. Key differentials include:
- Pulmonary embolism — classic cause of hypoxia-driven hyperventilation with elevated A-a gradient
- Pneumonia / ARDS — V/Q mismatch, P/F <300 fits
- Pulmonary fibrosis / ILD — chronic hypoxaemia driving chronic hyperventilation
- Pulmonary oedema — cardiogenic or non-cardiogenic
- Sepsis — peripheral hyperventilation plus lung involvement
The chronicity of the compensation (full renal response) suggests this has been ongoing for ≥2–3 days rather than an acute event.
(Harrison's Principles of Internal Medicine, 21st Ed., p. 1462)
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