Pathophysiology

A portion of each tidal volume remains in the non-gas-exchange area of the respiratory system—termed the "dead space"—and is usually fixed by the anatomic size of the conducting airways (trachea and bronchi). The portion of the tidal volume that reaches the alveoli is that which remains after the dead space volume is subtracted: Ta = TV - Td. Alveolar ventilation per minute is the volume multiplied by the rate: A = Ta * R = (TV - Td) * R. Alveolar hypoventilation can result from a decrease in respiratory rate, a decrease in tidal volume, or an increase in dead space.

Since the medullary chemoreceptors stimulate both respiratory rate and tidal volume in response to increased CO 2, alveolar ventilation is finely controlled to maintain Paco2 within a narrow range under most circumstances. In essence, alveolar ventilation is balanced relative to the production of CO 2 to maintain the Paco2 within a narrow range. Decreased respiratory drive is associated with CNS lesions and toxic depression ( Tabl§„„5.8.-.2). Thoracic cage and neuromuscular disorders produce hypoventilation by allowing a slower respiratory rate and/or a decreased tidal volume relative to the production of CO 2. Intrinsic lung diseases, such as COPD, produce alveolar hypoventilation due to an increase in dead space.

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