Anion Gap in Respiratory Disease

Expansion

Respiratory acidosis (raised carbon dioxide)

• Acute: bicarbonate rises about 1 mmol/l for each 1.3 kPa (10 mmHg) rise in carbon dioxide, from tissue buffering alone
• Chronic: bicarbonate rises about 4 mmol/l for each 1.3 kPa (10 mmHg), from renal retention of bicarbonate and excretion of acid

Respiratory alkalosis (low carbon dioxide)

• Acute: bicarbonate falls about 2 mmol/l per 1.3 kPa fall
• Chronic: bicarbonate falls about 5 mmol/l per 1.3 kPa fall

Compensation begins within hours but takes 3 to 5 days to complete, and it never fully returns pH to normal.

Two practical uses. A patient with a high carbon dioxide and a markedly raised bicarbonate has a chronic problem; one with a high carbon dioxide and a normal bicarbonate has an acute one. And a pH that is fully normal in the presence of gross abnormalities suggests a mixed disorder rather than complete compensation.