Expansion
Anything reducing alveolar ventilation, from brain to alveolus
Expansion
Working from the brain outwards:
- Central drive: opioids, benzodiazepines, anaesthetics, brainstem stroke, raised intracranial pressure, central hypoventilation, obesity hypoventilation
- Spinal cord: high cervical injury
- Peripheral nerve: Guillain-Barre syndrome, motor neurone disease, phrenic nerve palsy
- Neuromuscular junction: myasthenia gravis, botulism, neuromuscular blockers, organophosphates
- Muscle: myopathy, severe hypokalaemia or hypophosphataemia, fatigue
- Chest wall and pleura: kyphoscoliosis, flail chest, obesity, large effusion, pneumothorax
- Airway: obstruction, severe asthma, chronic obstructive pulmonary disease
- Lung: severe parenchymal disease with exhaustion
Add increased carbon dioxide production exceeding ventilatory capacity: sepsis, malignant hyperthermia, thyroid storm, excessive carbohydrate feeding.
Distinguishing acute from chronic is the key clinical step, using the bicarbonate: a markedly raised bicarbonate indicates chronic retention with renal compensation, while a normal bicarbonate with high carbon dioxide indicates an acute problem needing urgent ventilatory support.