|Respiratory pathway affecting carbon dioxide elimination
Central nervous system↓
Peripheral nervous system↓
Chest wall and pleura↓
|Abnormal gas exchange: "Can't breathe enough"
|Schematic figure representing the respiratory pathway, along which a variety of diseases can affect carbon dioxide elimination and result in hypercapnia. Note that gas exchange abnormalities alone are relatively uncommon causes of hypercapnia, but gas exchange problems in the setting of reduced mechanical capability of the ventilatory pump are very common explanations for acute and chronic hypercapnia.
|Mechanism and etiologies of hypercapnia
|Decreased minute ventilation (global hypoventilation; extra pulmonary causes)
|Decreased central respiratory drive
|Decreased respiratory neuromuscular or thoracic cage function
|Primary spinal cord/lower motor neuron/muscle disorders
|Thoracic cage disorders
|Toxins, poisoning, drugs
|Increased dead space (gas exchange abnormalities; pulmonary parenchymal causes or airway disorders)
|Increased carbon dioxide production
|Upper airway disorder◊
* Hyperthyroidism is also a rare cause of respiratory muscle weakness.
¶ Injury or disease process needs to be between cervical spine level 3 and 5 (C3 to 5) for clinically significant diaphragmatic paresis/paralysis to occur.
Δ Hypermagnesemia, hypokalemia, and hypercalcemia can also cause respiratory muscle weakness and contribute to hypercapnia.◊ Upper airway disorders are rare causes of hypercapnia. They either diminish total ventilation or lead to dynamic hyperinflation and reduced tidal volume while simultaneously causing increased work of breathing and carbon dioxide production.
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