The influence of graded degrees of chronic hypercapnia on the acute carbon dioxide titration curve

Abstract

Studies were carried out to determine the influence of the chronic level of arterial carbon dioxide tension upon the buffering response to acute changes in arterial carbon dioxide tension. After chronic adaptation to six levels of arterial CO₂ tension, ranging between 35 and 110 mm Hg, unanesthetized dogs underwent acute whole body CO₂ titrations. In each instance a linear relationship was observed between the plasma hydrogen ion concentration and the arterial carbon dioxide tension. Because of this linear relationship, it has been convenient to compare the acute buffering responses among dogs in terms of the slope, dH⁺/dPaco₂. With increasing chronic hypercapnia there was a decrease in this slope, i.e. an improvement in buffer capacity, which is expressed by the equation dH⁺/dPaco₂=-0.005 (Paco₂)_chronic + 0.95. In effect, the ability to defend pH during acute titration virtually doubled as chronic Paco₂ increased from 35 to 110 mm Hg. The change in slope, dH⁺/dPaco₂, was the consequence of the following two factors: the rise in plasma bicarbonate concentration which occurs with chronic hypercapnia of increasing severity, and the greater change in bicarbonate concentration which occurred during the acute CO₂ titration in the animals with more severe chronic hypercapnia. These findings demonstrate the importance of the acid-base status before acute titration in determining the character of the carbon dioxide titration curve. They also suggest that a quantitative definition of the interplay between acute and chronic hypercapnia in man should assist in the rational analysis of acid-base disorders in chronic pulmonary insufficiency.