Respiratory failure - acute
Description
- Failure of the respiratory system to achieve adequate gas exchange, developing over minutes to hours
| Type 1 (hypoxaemic) | Type 2 (hypercapnic/ventilatory) | |
|---|---|---|
| Definition | PaO2 <60 mmHg on room air | PaCO2 >45 mmHg with pH <7.35 |
| Problem | Oxygenation - V/Q mismatch, shunt | Ventilation - the pump has failed |
| A-a gradient | Raised | Normal if pure hypoventilation; raised if lung disease too |
| Prototype | Pneumonia, ARDS, pulmonary oedema, PE | COPD exacerbation, opioid overdose, neuromuscular disease |
- Expected A-a gradient = (Age/4) + 4 (on room air)
- *A hypercapnic patient with a raised A-a gradient has both a lung problem and a pump problem*
Acute vs acute-on-chronic - decided by pH and bicarbonate
| pH | HCO3- | Base excess | |
|---|---|---|---|
| Acute | Low | Normal | Normal |
| Chronic (compensated) | Normal | High | High |
| Acute-on-chronic | Low | High | High |
- Rule of thumb: acute CO2 rise increases HCO3- by ~1 mmol/L per 10 mmHg; chronic by ~4 mmol/L per 10 mmHg
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