Acid-Base Response to Chronic Hypercapnia in Man

To define "steady-state" chronic hypercapnia in man, we studied 20 patients with stable carbon dioxide tensions from 34 to 103 mm of mercury and with no apparent complicating disorders. The whole-body titration curve in chronic uncomplicated hypercapnia is characterized by a linear increase in estimated hydrogen ion activity as carbon dioxide tension increases. Over a range of carbon dioxide tensions from 34 to 103 mm of mercury the arterial-blood hydrogen ion activity increased by 0.24 nM per liter per millimeter of mercury increase in carbon dioxide tension. Thus, the whole-body defense of arterial-blood pH appears equally effective over the range of carbon dioxide tensions observed in this study. The physiologic response to chronic hypercapnia in man is defined by a zone that is approximately 11 nM per liter (0.10 pH unit) wide for hydrogen ion and 10 mEq wide for bicarbonate. These significance bands may be used to separate mixed acid-base disorders in patients with chronic hypercapnia.