Arterial Blood Gas Analysis: Example Set 2

Case A

You are working in the emergency room when the paramedics bring in a 45 year-old man who was found down in Pioneer Square. He is somnolent but arouseable. He has emesis on his shirt. He is hypotensive and tachycardic. Labs are drawn and reveal the following:

• Room air ABG: pH 7.22
• PCO₂ 29 PO₂ 78 HCO₃-   11.
• 
  Chemistry panel: Na⁺ 131
• Cl^- 90
• HCO₃- 12
• 
  Glucose 135

Step 1:   The pH is low (acidemia)

Step 2:   The PCO2 is low (respiratory alkalosis) and the bicarbonate is low (metabolic acidosis). Therefore, the metabolic acidosis is the primary process.

Step 3:   The serum anion gap is elevated at 29. There is, therefore, an elevated anion gap acidosis.

Step 4:   The respiratory alkalosis is the compensatory process for the metabolic acidosis.

Step 5:

• The Delta Gap = Measured SAG – Normal SAG = 29 – 12 = 17
• Calculate the Delta Delta: Delta Gap + measured bicarbonate = 17 + 12 = 29
• Since the Delta Delta is above a normal bicarbonate level, there is a concurrent metabolic alkalosis at work.

Summary

The patient has a primary elevated anion gap acidosis with respiratory compensation (which is not complete) and a concurrent metabolic alkalosis. You would need to sort through the differential diagnosis for an elevated anion gap acidosis to identify the cause of that problem. The metabolic alkalosis is likely due to vomiting.

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Case B

A 60 year-old man was recently in the hospital for treatment of aspiration pneumonia for which he as treated with levofloxacin and clindamycin. One week later, he presents to the ER with severe diarrhea, abdominal pain and hypotension.

He is admitted to the ICU

• Arterial blood gas shows: pH 7.29
• PCO₂ 25
• PO₂ 89
• HCO₃- 10


• Chemistry panel: Na⁺ 29
• Cl^- 99
• HCO₃- 10


• Glucose 135

Step 1:   The pH is low (acidemia)

Step 2:   The PCO₂ is low (respiratory alkalosis) and the bicarbonate is low (metabolic acidosis). Therefore, the metabolic acidosis is the primary process.

Step 3:   The serum anion gap is elevated at 20. There is, therefore, an elevated anion gap acidosis.

Step 4:   The respiratory alkalosis is the compensatory process for the metabolic acidosis.

Step 5:

• The Delta Gap = Measured SAG – Normal SAG = 20 – 12 = 8
• Calculate the Delta Delta: Delta Gap + measured bicarbonate = 8 + 10 = 18
• Since the Delta Delta is below the normal bicarbonate level, there is a concurrent non-gap metabolic acidosis at work.

Summary

The patient has a primary elevated anion gap acidosis with respiratory compensation and a concurrent non-gap metabolic acidosis. He likely has sepsis from C.Difficile Colitis as the cause of his elevated anion gap acidosis. The non-gap acidosis may be from his severe diarrhea (a source of bicarbonate loss).

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Case C

A 56 year-old woman with chronic kidney disease presents to the emergency room with increasing dyspnea. She is noted to be tachypneic, but is afebrile, has a normal lung exam and a normal appearing chest x-ray.

• An arterial blood gas is drawn and reveals: pH 7.28
• PCO₂ 29, PO₂ 85
• HCO₃- 16


• On her chemistry panel, the sodium is 131, chloride 105 and HCO3- 15

Step 1:   The pH is low (acidemia)

Step 2:   The PCO2 is low (respiratory alkalosis) and the bicarbonate is low (metabolic acidosis). Therefore, the metabolic acidosis is the primary process.

Step 3:   The serum anion gap is normal at 11. The patient, therefore, does not have an elevated anion gap acidosis

Step 4:   The respiratory alkalosis is the compensatory process for the metabolic acidosis.

Step 5:

• The Delta Gap = Measured SAG – Normal SAG = 11 – 12 -1
• Calculate the Delta Delta: Delta Gap + measured bicarbonate = -1 + 15 = 14
• The Delta Delta is below the normal bicarbonate level. This tells us there is a non-gap acidosis, but in this case, this acidosis is actually the same acidosis that you identified in Steps 2 and 3 and there are no additional metabolic processes at work.

Summary

The patient has chronic kidney disease. Many such patients develop non-gap acidoses due to renal tubular acidosis. 

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