Which statement about acute vs chronic respiratory acidosis is true?

Which statement about acute vs chronic respiratory acidosis is true?

• A. Acute pH drop per 10 mmHg CO2 is 0.08; Chronic is 0.03
• B. Acute pH drop per 10 mmHg CO2 is 0.03; Chronic is 0.08
• C. Both have equal pH drops per 10 mmHg CO2
• D. Chronic has no pH change

Answer: (A)

The key idea is how the body compensates for elevated CO2 differently in acute versus chronic respiratory acidosis. When CO2 rises, hydrogen ions increase and pH falls. In an acute setting there isn’t time for the kidneys to compensate, so the pH drop for each 10 mmHg rise in CO2 is relatively large. Over time, renal compensation kicks in, increasing bicarbonate and buffering the acidosis, so the same rise in CO2 produces a smaller pH decrease. Thus, the magnitude is about 0.08 pH units drop per 10 mmHg CO2 in the acute phase, versus about 0.03 pH units per 10 mmHg CO2 after chronic compensation. This makes the described distinction true. The other statements conflict with the physiology: chronic acidosis does involve pH change (just a smaller amount), the drops are not equal, and chronic does not produce no pH change.

The key idea is how the body compensates for elevated CO2 differently in acute versus chronic respiratory acidosis. When CO2 rises, hydrogen ions increase and pH falls. In an acute setting there isn’t time for the kidneys to compensate, so the pH drop for each 10 mmHg rise in CO2 is relatively large. Over time, renal compensation kicks in, increasing bicarbonate and buffering the acidosis, so the same rise in CO2 produces a smaller pH decrease.

Thus, the magnitude is about 0.08 pH units drop per 10 mmHg CO2 in the acute phase, versus about 0.03 pH units per 10 mmHg CO2 after chronic compensation. This makes the described distinction true. The other statements conflict with the physiology: chronic acidosis does involve pH change (just a smaller amount), the drops are not equal, and chronic does not produce no pH change.