1. The respiratory response to chronic metabolic alkalosis was defined on the basis of 182 determinations of the arterial blood pH, Pco2, HCO3− concentration and Po2 obtained in thirty uraemic patients undergoing chronic haemodialysis. Plasma bicarbonate concentrations, [HCO3−], ranged from 20 to 38 mmol/l before dialysis, and were elevated by increasing the acetate content of the dialysate.
2. The calculated arterial carbon dioxide tension (Pa,co2) was linearly related to HCO3−, Pa,co2 reaching 50 mmHg at a [HCO3−] of 37.5 mmol/l. This response was virtually identical with that previously reported in chronic metabolic acidosis. The 95% confidence limits of the respiratory adaptation to chronic metabolic acid-base disorders were calculated for [HCO3−] ranging from 11 to 38 mmol/l.
3. Analysis of the individual response lines of thirteen patients revealed no difference among their slopes but significant differences among their elevations. Each subject appears thus to have his own threshold to acid-base stimuli.
4. Hypoxaemia developed with increasing [HCO3−], Pa,o2 falling in a curvilinear fashion to reach 70 mmHg at a [HCO3−] of 37.5 mmol/l.