Determining whether compensatory mechanisms are as expected

When the primary problem is with serum [HCO3-] the lungs will attempt to counter this by altering pCO2.

Conversely, if the primary problem is with the lungs and the pCO2 level, the kidneys will counter this by altering the serum [HCO3-].

However there may be concurrent problems which interfere with this adaptive response.  Such a mixed acid-base disorder must be identified prior to initiating treatment.

Expected responses:

Metabolic acidosis

  • For every mmol/l fall in serum [HCO3-] from 25, the pCO2 should fall by 1mmHg.

Metabolic alkalosis

  • For every mmol/l rise in serum [HCO3-] from 25, the pCO2 should rise by 0.7 mmHg.  A greater response is limited by hypoxia.

Respiratory acidosis

 

  • Acute

  • For every mmHg rise in pCO2 from 40, the plasma [H+] should rise by 0.8 nmol/l from 40.

  • For every 2-fold rise in pCO2, the serum [HCO3-] should increase by 2.5 mmol/l.

  • Chronic

  • For every mmHg rise in pCO2 from 40, the [H+] should rise by 0.3 nmol/l, or the serum [HCO3-] should rise by 0.3 mmol/l from 25.

Respiratory alkalosis

 

  • Acute

  • For every mmHg fall in pCO2 from 40, the [H+] should fall by 0.8 nmol/l from 40.

  • Chronic

  • For every mmHg fall in pCO2 from 40, the [H+] should fall by 0.2 nmol/l, or the serum [HCO3-] should fall by 0.5 mmol/l from 25.

Note: To convert mmHg to kPa multiply by 0.1333.

Acid-Base Map: area of normal values is labeled N; map actually extends further up than shown (to a pH of 6.6) and further to right than shown (to a pCO2 of 180 mmHg); numbered lines represent isopleths for bicarbonate (in milliequivalents per litre); from Goldberg M et al., Computer-based instruction and diagnosis of acid-base disorders: a systematic approach, JAMA 223:269-275, 1973, p. 270

If the results for your patient fall outside the marked areas, then the compensatory mechanisms are not working as expected and there is likely to be a mixed picture.


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