Respiratory failure - chronic
Description
PaO2 <60 mmHg and/or PaCO2 >45 mmHg, persisting, with renal compensation (raised HCO3-, near-normal pH).
Acute vs chronic - the bicarbonate and the pH tell you
| Acute | Chronic | Acute-on-chronic | |
|---|---|---|---|
| pH | Low | Normal (7.35-7.45) | Low |
| PaCO2 | High | High | Higher still |
| HCO3- | Normal (no time to compensate) | High (renal retention) | High |
| Base excess | Normal | Positive | Positive |
- Rule of thumb: acute rise in PaCO2 of 10 mmHg -> HCO3- inc by ~1; chronic -> HCO3- inc by ~4
- A raised HCO3- on routine bloods is the commonest incidental clue to unrecognised chronic hypercapnia
Type 1 vs type 2
| Type 1 (hypoxaemic) | Type 2 (hypercapnic / ventilatory) | |
|---|---|---|
| Gas | dec PaO2, normal or low PaCO2 | inc PaCO2 +/- dec PaO2 |
| A-a gradient | Raised | Normal if pure hypoventilation; raised if lung disease too |
| Problem | Gas exchange (V/Q mismatch, shunt, diffusion) | The pump (drive, nerve, muscle, chest wall, airway load) |
| Chronic examples | ILD, emphysema, pulmonary vascular disease | COPD, OHS, neuromuscular, kyphoscoliosis |
- A-a gradient = (FiO2 x (Patm - 47) - PaCO2/0.8) - PaO2; normal = age/4 + 4 mmHg on room air
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