ABG interpretation examples

Seven arterial blood gases, each read in the order from how to interpret an ABG. The ranges used are pH 7.35 to 7.45, PaCO2 35 to 45 mmHg and HCO3− 22 to 26 mEq/L. [MedlinePlus] The labels are the ROME ones. [RegisteredNurseRN: ROME]

1. Respiratory acidosis, not compensated

pH 7.28, PaCO2 50 mmHg, HCO3− 24 mEq/L

  1. pH 7.28 is below 7.35: acidemia.
  2. PaCO2 50 is above 45: high, the acid direction. It moves opposite to the pH, so the cause is respiratory.
  3. HCO3− 24 is inside 22 to 26: normal, so the kidneys have not moved. Uncompensated.

Answer: Uncompensated respiratory acidosis.

2. Metabolic acidosis, not compensated

pH 7.30, PaCO2 40 mmHg, HCO3− 18 mEq/L

  1. pH 7.30: acidemia.
  2. PaCO2 40 is normal.
  3. HCO3− 18 is below 22: low, and it moves with the pH, so the cause is metabolic. PaCO2 is normal, so the ROME label is uncompensated.
  4. Check with Winter’s formula: 1.5 × 18 + 8 = 35, band 33 to 37. A PaCO2 of 40 is above it, which the Merck Manual reads as a respiratory acidosis on top. [Merck Manual: acid-base disorders] The ROME label does not show that.

Answer: Uncompensated metabolic acidosis.

3. Respiratory acidosis, partly compensated

pH 7.32, PaCO2 50 mmHg, HCO3− 30 mEq/L

  1. pH 7.32: acidemia.
  2. PaCO2 50: high, opposite to the pH, so respiratory acidosis.
  3. HCO3− 30 is above 26: the kidneys have moved in the alkaline direction. The pH is still below 7.35, so the compensation is partial.

Answer: Partially compensated respiratory acidosis.

4. Metabolic alkalosis, partly compensated

pH 7.51, PaCO2 47 mmHg, HCO3− 32 mEq/L

  1. pH 7.51 is above 7.45: alkalemia.
  2. PaCO2 47 is above 45: high.
  3. HCO3− 32 is above 26: high, and it moves with the pH, so metabolic alkalosis. The high PaCO2 is the lungs moving the other way, but the pH is still above 7.45: partial.

Answer: Partially compensated metabolic alkalosis.

5. Respiratory alkalosis, fully compensated

pH 7.42, PaCO2 26 mmHg, HCO3− 18 mEq/L

  1. pH 7.42 is inside 7.35 to 7.45. By the nursing convention it is above 7.40, so it is on the alkaline side.
  2. PaCO2 26 is below 35: low, opposite to the alkaline-side pH, so respiratory alkalosis.
  3. HCO3− 18 is below 22: the kidneys have moved in the acid direction, and the pH is back in range: full compensation, by the ROME labels.

Answer: Fully compensated respiratory alkalosis.

6. Combined acidosis

pH 7.10, PaCO2 55 mmHg, HCO3− 16 mEq/L

  1. pH 7.10: a marked acidemia.
  2. PaCO2 55: high, the acid direction.
  3. HCO3− 16: low, also the acid direction. Both values push the pH down: two primary disturbances, a combined acidosis. [Merck Manual: acid-base disorders]

Answer: Combined (mixed) acidosis.

7. Normal

pH 7.40, PaCO2 40 mmHg, HCO3− 24 mEq/L

  1. pH 7.40, PaCO2 40 and HCO3− 24 are all inside their normal ranges.

Answer: Normal acid–base balance.

About these gases

These are textbook-style values, and our app’s classifier gives the same answer for each of them. Real gases are messier. The ROME labels say where the numbers sit, and the Merck Manual’s caution applies: values in a mixed disorder may be deceptively normal, so compare the compensation with its expected size. [Merck Manual: acid-base disorders] Try your own with the practice cases.

Sources

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