Breathlessness with Clear Lungs on Chest X-Ray
| Disorder | Arterial PO2 | Arterial PCO2 | Arterial pH | Suggestive clinical features |
|---|---|---|---|---|
| Acute asthma (Chapter 60) | Normal or low | Low | High | Wheeze with reduced peak flow, diurnal variability, family history, atopy. |
| Exacerbation of COPD (Chapter 61) | Low | May be high | Normal or low | Fever, wheeze and productive cough; smoking history. Exacerbations in winter. |
| Pulmonary embolism (Chapter 57) | Normal or low | Low | High | Risk factors for thromboembolism (Table 56.1). |
| Acute coronary syndrome (Chapters 45 and 46) | Normal (unless pulmonary oedema) | Normal | Normal | Coronary risk factors, previous exertional breathlessness, abnormal ECG or raised troponin. |
| Sepsis (Chapter 35) | Normal or low | Low | Low | Low or high temperature, raised white count and C-reactive protein, raised lactate. |
| Metabolic acidosis (Chapter37) | Normal or high | Low | Normal or low | Hyperventilation, diabetes, or renal failure, drug overdose. |
| Pre-radiological pneumonia† | Low | Low | High | Fever, raised white count and C-reactive protein. |
| Hyperventilation without organic disease | Normal or high | Low | High | Often emotional stress, resolves with rebreathing or breathing exercises. |
COPD, chronic obstructive pulmonary disease.
† Most commonly due to viruses or Pneumocystis jirovecii (Table 34.4).