24. TYPICAL FORMS OF PATHOLOGY OF THE RESPIRATORY SYSTEM

Typical disorders of respiratory function include:

• Disturbances in alveolar ventilation: alveolar hyperand hypoventilation.

• Disturbances in pulmonary circulation.

• Abnormal gas exchange through alveolar-capillary barrier.

• Improper matching of ventilation and perfusion.

These and other disturbances underlie the development of respiratory insufficiency (fig. 31).

Fig. 31

Alveolar hyperventilation

Alveolar hyperventilation exists when PaCO₂ is below 37 mm Hg. Hypoxemia controls ventilation by stimulating the peripheral chemoreceptors. In pulmonary disorders and congestive heart failure, hyperventilation results from stimulation of afferent vagal receptors in the lungs and airways. Low cardiac output and hypotension stimulate the peripheral chemoreceptors and inhibit the baroreceptors, both of which increase ventilation. Metabolic acidosis that occurs in many conditions activates both the peripheral and central chemoreceptors. Psychogenic states and severe cerebrovascular insufficiency may interfere with the inhibitory influence normally exerted by cortical structures of the brainstem respiratory neurons. Fever and sepsis also cause hyperventilation through the effects on the midbrain and hypothalamus.

The alkalemia associated with hypocapnia may produce dizziness, visual impairment, syncope and seizure which are secondary to cerebral vasoconstriction. Tetany is secondary to decreased free serum calcium and muscle weakness is secondary to hypophosphatemia. Patients with a primary respiratory alkalosis are also prone to periodic breathing and central sleep apnea.

Alveolar hypoventilation

Alveolar hypoventilation is clinically important when PaCO₂ is generally in the range of 50 to 80 mm Hg.