Arterial Blood Gas Analysis: Example Set 3

Case A

A 40 year-old mountain climber ascends to an elevation of 15,000 feet. She remains there for a period of 3 weeks as part of a research project. At the end of that time, she has an arterial blood gas drawn which show

• pH 7.44
• PCO₂ 24
• PO₂ 55
• HCO₃- 16.

The serum anion gap was normal (11).

Step 1:   The pH is high (alkalemia)

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

Step 3:   The serum anion gap normal.

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

Summary

If you had measured the ABG immediately upon arrival, the bicarbonate would have been close to normal and the pH would have been much higher. This is because it takes several days for metabolic compensation to occur.

Case B

A 65 year-old man with very severe COPD (FEV₁ = 25% predicted) has an arterial blood gas done as part of routine pulmonary function testing to help determine if he requires home oxygen therapy. The sample shows

• pH 7.36
• PCO₂ 60 PO₂ 60 HCO₃- 36. The anion gap is 8.

Step 1:   The pH is low (acidemia)

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

Step 3:   The serum anion gap is normal at 8.

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

Summary

This patient likely has chronic carbon dioxide retention due to his very severe COPD. Because this is a long-standing process, he has had adequate time for metabolic compensation to occur.

Case C

A 40 year-old woman develops a severe case of diarrhea with multiple loose bowel movements over the course of a one-day period. When she presents to the ER, an arterial blood gas is obtained and shows

• pH 7.35
• PCO₂ 32
• PO₂ 75
• HCO₃- 18

The anion gap is 10.

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 10. The metabolic acidosis from Step 2, therefore, is a non-gap acidosis.

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

Summary

The patient’s metabolic acidosis is likely due to her diarrhea as diarrhea leads to bicarbonate loss through the lower gastrointestinal tract. Even though her symptoms have been present for only one day, her pH has come back up towards normal already. That is because respiratory compensation for metabolic processes occurs almost immediately. In severe acidosis, this may not be enough to bring the pH back close to normal but at a minimum respiratory compensation starts quickly.

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