Hypoventilation

Alveolar ventilation is the gas exchange within the alveoli within a certain time, often 1 minute. The normal value of tidal volume is 500 mL. Of these 500 mL approx. 350 mL reach the alveoli while 150 mL fill the dead space. When we take deep breaths the ratio between alveolar ventilation and dead space ventilation increases. When we take shallow breaths the ratio decreases.

When the alveolar ventilation is insufficient compared to the requirements of the body, it's called alveolar hypoventilation (often just hypoventilation). This causes hypercapnic respiratory failure, which is when the arterial pCO2 is increased above the normal 44 mmHg.

Etiology

Physiological ventilation requires a normal respiratory drive from the CNS, normal conduction of nerve impulses from the CNS to the respiratory muscles, normal function of the chest wall and respiratory muscles, normal conduction of air through the upper airways, and normal functioning of the lungs.

As such, hypoventilation may occur due to problems in several different organ systems. Problems with the central nervous system can impair the normal drive to ventilate, problems with the peripheral nervous system, respiratory muscles, chest wall, or upper airways may make the patient unable to breathe despite the respiratory drive, and problems with the lung can impair gas exchange to such a degree that any amount of ventilation is insufficient for gas exchange.

• Problems with the CNS
  • Medications which decrease respiratory drive (benzodiazepines, alcohol, opioids)
  • Stroke
  • Encephalitis
• Impaired neuromuscular transmission
  • Injury to the cervical spine between C3 and C5
  • Neuromuscular disorders (ALS, GBS, MG)
  • Tetanus
  • Botulism
  • Electrolyte disorders
• Impaired function of chest wall
  • Severe kyphosis or scoliosis
  • Pectus excavatum
• Impaired airways (rarely causes hypoventilation by itself)
  • Acute exacerbation of COPD or asthma
  • Foreign body aspiration
  • External airway obstruction (goitre, tumour)

Consequences

Hypoventilation leads to hypercapnic respiratory failure, where there is both hypoxaemia and hypercapnia. This can be life-threatening. Respiratory failure is covered in its own topic. ‎