Compensatory Responses: Metabolic Alkalosis
Metabolic alkalosis is an acid base disorder characterized by an elevation in [HCO3-] above the normal range, which leads to a reduction in the PCO2/[HCO3-] ratio and subsequently a reduction in hydrogen ion concentration according to the following equation:
[H+] = 24 ×(PCO2 / [HCO3-])
This elevation in bicarbonate ions is due to an addition in alkali to the body which then cannot be excreted by the kidney. Metabolic alkalosis is always associated with renal impairment of some kind because the kidney has a vast capacity in excreting excess alkali.
Please note, loss of acid from the body as occurs in vomiting induced metabolic alkalosis is equivalent to adding alkali to the body.
In response to the reduction in [H+] and elevation in pH, the body responds by trying to increase the PCO2 to match the increase in [HCO3-] and thus maintain the PCO2/[HCO3] ratio. Elevation in PCO2 is accomplished by lowering alveolar ventilation.
The development of alkalemia is sensed by central and peripheral chemoreceptors, resulting in a reduction in the rate of ventilation and a reduction in tidal volume and thus an elevation in the pCO2. This happens fairly quickly following the onset of metabolic alkalosis.
On average the pCO2 rises 0.7 mmHg for every 1.0 meq/L increment in the plasma [HCO3-].
For example, if an alkali load raises the the plasma HCO3- concentration to 34 meq/L, then:
Degree of HCO3- elevation is 34 – 24 (optimal value)= 10.
Therefore, PCO2 elevation should be 0.7 × 10 = 7.
Then PCO2 measured should be 40 (optimal value) +7 = 47mmHg.
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