44. Alveolar hypoventilation. Causes and consequences.

From greek.doctor

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 alveoli ventilation and dead space ventilation increases. When we take shallow breaths the ratio decreases.

Alveolar hypoventilation is when the alveolar ventilation is insufficient compared to the requirements of the body. It’s defined as when the arterial pCO2 is increased above the normal 44 mmHg.

Primary alveolar hypoventilation is a type of alveolar hypoventilation with no known etiology. It is caused by a decreased CO2-sensitivity of the respiratory centre in the CNS. It doesn’t cause any problems by itself, however patients with primary alveolar hypoventilation may have even worse symptoms when they also develop chronic bronchitis or obesity.

Many other factors can cause secondary alveolar hypoventilation:

  • Abnormal respiratory regulation
    • Sleep
    • Metabolic alkalosis
    • Morphine, opioids
    • Encephalitis
    • Tumours
    • Primary alveolar hypoventilation
  • Neuromuscular disorders
    • Guillain-Barré syndrome
    • Poliomyelitis
    • Myasthenia gravis
  • Chest abnormalities
    • Kyphoscoliosis
    • Obesity (Pickwick syndrome)
  • Upper airway obstruction
    • Obstructive sleep apnoea syndrome
    • Extrathoracal obstruction
  • Lung diseases
    • Chronic obstructive pulmonary diseases (COPD)
    • Severe restrictive lung diseases

Pickwick syndrome, or obesity hypoventilation syndrome, is a restrictive lung disease of obese people. For unknown reasons only some obese people develop it. The excess adipose tissue restricts the movement of the respiratory muscles and makes the chest wall less compliant.

Consequences

Because the alveoli are hypoventilated the body won't get enough oxygen and won’t get rid of enough carbon dioxide. The result is global respiratory failure. You can read about the consequences of global respiratory failure in topic 51.