Figure 41-6 Changes in acid-based and electrolyte composition in patients with respiratory acidosis. Left to right, the panels depict normal acid-base status; adaptation to an acute rise in the partial pressure of arterial carbon dioxide (PaCO₂ ) to 80 mm Hg; adaptation to a long-term rise in PaCO₂ to 80 mm Hg; superimposition of an acute further increment in PaCO₂ (to a level of 100 mm Hg) in the same patient; and post-hypercapnic alkalosis resulting from an abrupt reduction in PaCO₂ to the level of 40 mm Hg in the same patient. A^- denotes unmeasured plasma anions, and the numbers within the bars give ion concentrations in millimoles per liter. (Adapted from Androgue H, Madias N: Management of life-threatening acid-base disorders. First of two parts. N Engl J Med 338:26–34, 1998.)