Metabolic and respiratory acidosis: Clinical

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Metabolic and respiratory acidosis: Clinical

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 A 14-year-old boy comes to the emergency department because of worsening respiratory distress. This patient was seen 3 days ago with a 2-day history of shortness of breath and fever; he was diagnosed with pneumonia and discharged with amoxicillin-clavulanate, azithromycin, and instruction to follow up with his primary care physician. In the past day he has developed additional symptoms of abdominal pain, nausea, non-bilious emesis, and polyuria. His temperature is 37.9°C (100.3°F), pulse is 106/min, respirations are 26/min, and blood pressure is 110/74 mm Hg. Physical examination shows a well-developed, ill-appearing patient in respiratory distress with Kussmaul respirations and fruity breath odor. Chest x-ray shows consolidation of the lower left lobe. Laboratory studies show:
Na+: 130 mEq/L
K+: 3.4 mEq/L
Cl-: 92 mEq/L
HCO3-: 13 mEq/L
Blood Urea Nitrogen: 30
Creatinine: 1.5
Amylase: 162 U/L

Arterial blood gas shows:
pH: 7.30
PaO2: 95 mm Hg
PCO2: 28 mm Hg

Which of the following is the most appropriate next step in management?


In metabolic acidosis, the blood pH is below 7.35, and it’s due to a bicarbonate or HCO3 concentration in the blood of less than 22 mEq/L.

With metabolic acidosis, the respiratory center is stimulated in order to compensate for the acidosis and the individual hyperventilates, leading to dyspnea.

In addition, associated symptoms are related to the underlying cause, for example, in diabetic ketoacidosis there’s nausea and vomiting.

First thing’s first. Serum chemistries are obtained including serum bicarbonate or HCO3, potassium, sodium and chloride in order to see if there’s any electrolyte imbalance, and BUN and Creatinine are checked to assess renal function.

The diagnosis is usually based on an ABG, and in addition to a pH below 7.35, and HCO3 levels below 22 mEq/L, if there’s respiratory compensation, the pCO2 levels will be under 35 mm Hg.

Generally, for every 1 mEq/L reduction in HCO3 levels, there’s a 1.2 mm Hg fall in pCO2.

Additionally, we can verify if the respiratory compensation is appropriate by using Winter’s formula and comparing the calculated value with the measured pCO2 from the ABG.

It goes like this. Arterial pCO2 equals 1.5 times serum HCO3 plus 8 plus or minus 2. So if our HCO3 is 15, then the calculated arterial pCO2 is: 1.5 times 15 plus 8 plus or minus 2. So 1.5 times 15 is 22.5, and 22.5 plus 8 is 30.5, so it’s 30.5 plus or minus 2, so the range is 28.5 to 32.5.

So if the measured pCO2 is between 28.5 and 32.5, then there’s an appropriate respiratory compensation for the metabolic acidosis.

If the measured pCO2 comes back greater than 32.5, then there’s a metabolic acidosis and an associated respiratory acidosis.

And if the measured pCO2 is lower than 28.5, then there’s a metabolic acidosis and an associated respiratory alkalosis.

Generally, when pH levels are below 7.1, treatment is urgent and IV sodium bicarbonate or Tromethamine or THAM is given.


  1. "The evaluation, diagnosis, and treatment of the adult patient with acute hypercapnic respiratory failure" undefined (Apr 2020)
  2. "Simple and mixed acid-base disorders" undefined (Sep 2020)
  3. "Approach to the adult with metabolic acidosis" undefined (Jun 2020)

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