Causes of Hypoventilation

Hypoventilation — 

The lung alveolus is a space in which gas makes up 100 percent of the contents. This means that once the partial pressure of one gas rises, the other must decrease. Both arterial (PaCO₂) and alveolar (PACO₂) carbon dioxide tension increase during hypoventilation, which causes the alveolar oxygen tension (PAO₂) to decrease. As a result, diffusion of oxygen from the alveolus to the pulmonary capillary declines with a net effect of hypoxemia and hypercapnia. Because the respiratory quotient (Defined as CO_{2 eliminated}/O_{2 consumed}) is assumed to be 0.8, hypoventilation affects PaCO₂more than O₂.

Hypoxemia due to pure hypoventilation (ie, in the absence of an elevated A-a gradient) can be identified by two characteristics. First, it readily corrects with a small increase in the fraction of inspired oxygen (FiO₂). Second, the paCO₂ is elevated. An exception exists when the hypoventilation is prolonged because atelectasis can occur, which will increase the A-a gradient . Abnormalities that cause pure hypoventilation include:

●CNS depression, such as drug overdose, structural CNS lesions, or ischemic CNS lesions that impact the respiratory center

●Obesity hypoventilation (Pickwickian) syndrome

●Impaired neural conduction, such as amyotrophic lateral sclerosis, Guillain-Barré syndrome, high cervical spine injury, phrenic nerve paralysis, or aminoglycoside blockade

●Muscular weakness, such as myasthenia gravis, idiopathic diaphragmatic paralysis, polymyositis, muscular dystrophy, or severe hypothyroidism

●Poor chest wall elasticity, such as a flail chest or kyphoscoliosis

Bhopalwala. H