Compensatory Responses: Metabolic Acidosis

Metabolic Acidosis is an acid base disturbance characterized by a reduction in bicarbonate ions leading to an elevation in the PCO2/HCO3- ratio and thus an elevation in hydrogen ion concentration according to the following equation:
[H+] = 24 ×(PCO2 / [HCO3-])

In metabolic acidosis, this reduction in bicarbonate ions may result from increased extracellular buffering of an increased acid load or less commonly; loss of bicarbonate ions in the urine.

Remember that, HCO3- is the main buffer of nonvolatile or noncarbonic acids in the body and therefore in the presence of excess acid, its concentration will decrease.

The body responds to metabolic acidosis by trying to restore the PCO2 / [HCO3-] ratio. This is done by reducing the PCO2.

The reduction in PCO2 is accomplished by increasing alveolar ventilation. The drop in arterial pH stimulates both the central and peripheral chemoreceptors controlling respiration, resulting in an increase in alveolar ventilation. The inccrease in ventilation is characterized more by an increase in tidal volume than by an increase in respiratory rate, and may if the acidemia is severe, reach a maximum of 30L/min (nl = 5-6 L/min).

In its most pronounced clinical manifiestation, the increase in ventilation is referred to as Kussmaul Respiration.

In general, respiratory compensation results in a 1.2 mmHg reduction in PCO2  for every 1.0 meq/L reduction in the plasma HCO3- concentration down to a minimum PCO2 of 10 to 15mmHg.
 

For example, if an acid load lowers the plasma HCO3- concentration to 9 meq/L, then:
Degree of HCO3- reduction is  24 (optimal value) – 9 = 15.
Therefore, PCO2 reduction should be  15 × 1.2 =  18.
Then PCO2 measured should be 40 (optimal value) – 18 = 22mmHg.

 

Winter's Formula
To estimate the expected PCO2 range based on respiratory compensation, one can also use the Winter's Formula which predicts: PCO2 = (1.5 × [HCO3-]) + 8 ± 2

Therefore in the above example, the PCO2 according to Winter's should be
(1.5 × 9) + 8 ± 2 = 20-24

Another useful tool in estimating the PCO2 in metabolic acidosis is the recognition that the pCO2 is always approximately equal to the last 2 digits of the pH.

 

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