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Metabolic and respiratory alkalosis: Clinical
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In metabolic alkalosis, the blood pH is above 7.45, and it’s due to a bicarbonate or HCO3 concentration in the blood over 27 mEq/L.
Associated symptoms are related to the underlying cause.
The diagnosis is usually based on an ABG, and in addition to a pH above 7.45, and HCO3 levels above 27 mEq/L, if there’s respiratory compensation, the pCO2 is usually above 45 mm Hg.
Generally, for every 1 mEq/L elevation in HCO3 above the normal level of 27 mEq/L, pCO2 increases by about 0.7 mm Hg above the normal level of 45 mm Hg, but pCO2 doesn’t usually rise above 55 mm Hg, regardless of HCO3 levels.
Let’s take an example and say that HCO3 level is 30 mEq/L - so it’s 3 mEq/L above the baseline.
In addition, electrolytes are also done to see if there’s any imbalances, like hypokalemia.
Now, if the cause of metabolic alkalosis isn’t obvious from the history, then a spot urine chloride is measured.
If the urine chloride is below 20 mEq/L, that suggests volume depletion from a variety of causes like vomiting which leads to loss of hydrochloric acid, so the treatment is really aimed at addressing the underlying cause of vomiting.
A related cause is aggressive nasogastric suction, so the treatment is stopping or slowing the removal of gastric secretions.
Another cause is loop or thiazide diuretics which block hydrogen ion and chloride ion reabsorption in the kidney.
Chloride is a negatively charged ion, so loss of chloride leads to increased reabsorption of bicarbonate to compensate for the loss.
The loss of chloride causes the urine chloride to go above 20 mEq/L, and a lot of hydrogen is lost-leading to metabolic alkalosis.
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