[The mechanism and clinical significance of decreased anion gap in patients with COPD and acid-base imbalance].

The blood-gas and electrolytes of 427 blood samples from patients with COPD and acid-base imbalance were analysed. The results showed that 14 types of acid-base imbalance were present in patients with COPD and the anion gap decreased in 8 types of acid-base imbalance, especially in respiratory acidosis, metabolic alkalosis and coexistence of both. The anion gap in respiratory acidosis and metabolic alkalosis were 0.8 +/- 6.6mmol/L and 2.1 +/- 7.3mmol/L, respectively. The anion gap in respiratory acidosis with metabolic alkalosis was negative. In all patients the HCO3- increased, and the total anion and total cation diminished. The anion gap decreased owing to the diminution of unmeasured anion. Clinically, the criterion for judging metabolic acidosis with increased anion gap is a value of anion gap over 16 mmol/L. But in case respiratory acidosis (or metabolic alkalosis) and metabolic acidosis coexist in a patient, the anion gap for judging metabolic acidosis should be decreased appropriately.