Which statement best describes obesity hypoventilation syndrome?

Which statement best describes obesity hypoventilation syndrome?

• A. It is characterized by elevated PaCO2 that improves with sleep
• B. It is characterized by decreased PaCO2 during sleep
• C. It is characterized by sustained obesity with normal gas exchange
• D. It is characterized by elevated PaCO2 that worsens during sleep

Answer: (D)

Obesity hypoventilation syndrome involves chronic under-ventilation that leads to elevated CO2 levels, and this tendency to retain CO2 becomes worse during sleep. The obesity adds mechanical load on the chest and abdomen, reducing lung volumes and making breathing more work, while sleep—especially REM—reduces respiratory drive and the use of accessory muscles. So PaCO2 is high and tends to rise further during sleep, matching the statement describing elevated PaCO2 that worsens during sleep. The other ideas don’t fit: one option suggests CO2 improves with sleep, which isn’t true because sleep depresses ventilation; another suggests CO2 drops during sleep, which contradicts the pattern of hypoventilation; and another describes obesity with normal gas exchange, which ignores the hypercapnia that defines this condition.

Obesity hypoventilation syndrome involves chronic under-ventilation that leads to elevated CO2 levels, and this tendency to retain CO2 becomes worse during sleep. The obesity adds mechanical load on the chest and abdomen, reducing lung volumes and making breathing more work, while sleep—especially REM—reduces respiratory drive and the use of accessory muscles. So PaCO2 is high and tends to rise further during sleep, matching the statement describing elevated PaCO2 that worsens during sleep.

The other ideas don’t fit: one option suggests CO2 improves with sleep, which isn’t true because sleep depresses ventilation; another suggests CO2 drops during sleep, which contradicts the pattern of hypoventilation; and another describes obesity with normal gas exchange, which ignores the hypercapnia that defines this condition.