Effects of acid-base abnormalities on blood capacity of transporting CO(2): adverse effect of metabolic acidosis.

OBJECTIVE

To investigate the effects of some acid-base abnormalities on blood capacity of transporting CO(2).

DESIGN

Prospective study.

SETTING

General and Cardiosurgical ICUs of a University hospital.

PATIENTS: Six groups of ten patients characterized by: metabolic alkalosis; respiratory alkalosis; absence of acid-base abnormalities; metabolic acidosis; uncompensated respiratory acidosis; and compensated respiratory acidosis.

MEASUREMENTS AND

RESULTS

The CO(2) dissociation curve, Haldane effect, and the ratio Ra-v between Ca-vCO(2) and Pa-vCO(2) were calculated from arterial and mixed-venous blood gas analyses. The CO(2) dissociation curve was shifted upwards by metabolic alkalosis and compensated respiratory acidosis and downwards by metabolic acidosis. The slope of the curve was unaffected, but CO(2) transport not due to Haldane effect was significantly lower in respiratory acidosis since the slope was less steep at higher PCO(2) values. In comparison with controls, patients affected by metabolic acidosis showed lower Haldane effect values (0.18+/-0.15 vs 0.59+/-0.26 ml of CO(2) per ml of arterial-mixed venous O(2) content difference; P <.05) and Ra-v values (0.43+/-0.10 vs 0.84+/-0.17 ml of CO(2) transported by 100 ml of blood per Torr of arterial-mixed venous PCO(2) gradient; P <.05).

CONCLUSIONS

Our findings suggest that acid-base abnormalities, particularly metabolic acidosis, markedly affect blood capacity of transporting CO(2) and may worsen tissue hypercarbia associated with hypoperfusion. However, because of possible errors due to small measurements and the assumptions of the method, in the future definitive clarification will require the construction of original CO(2) dissociation curves for each acid-base abnormality.