Skip to Main Content

We have a new app!

Take the Access library with you wherever you go—easy access to books, videos, images, podcasts, personalized features, and more.

Download the Access App here: iOS and Android. Learn more here!

MECHANICAL VENTILATION AND PULMONARY ASSESSMENT: BLOOD GAS INTERPRETATION

Step 1: Acidemia or Alkalemia?

  • Normal pH 7.35–7.45

    • ✓ Alkalemia: pH above normal range

    • ✓ Acidemia: pH below normal range

Step 2: Metabolic or Respiratory?

  • Primary respiratory alkalosis: pH above normal range and paCO2 <40

  • Primary metabolic alkalosis: pH above normal range and paCO2 >40

  • Primary respiratory acidosis: pH below normal range and paCO2 >40

  • Primary metabolic acidosis: pH below normal range and paCO2 <40

Step 3: Is the problem acute or chronic?

  • For respiratory acidosis/alkalosis, a 10 mmHg change in paCO2 causes a 0.08 change in pH in the acute setting or a 0.03 change in the chronic setting.

  • For metabolic acidosis/alkalosis, a 10 mEq/L change in HCO3 causes a 0.15 change in pH.

  • For acute respiratory acidosis, expect an increase in HCO3 of 1 mEq/L for every increase in PaCO2 of 10 mmHg.

  • For acute respiratory alkalosis, expect a decrease in HCO3 of 1–3 mEq/L for every decrease in PaCO2 of 10 mmHg.

  • For chronic respiratory acidosis, expect an increase in HCO3 of 4 mEq/L for every increase in PaCO2 of 10 mmHg.

  • For chronic respiratory alkalosis, expect a decrease in HCO3 of 2–5 mEq/L for every decrease in PaCO2 of 10 mmHg.

Step 4: Is there a second primary problem?

  • In other words, are changes in pH greater than expected from the primary disorder alone?

    • Example: An infant with bronchopulmonary dysplasia is receiving diuretics and is hypochloremic, with pH 7.42, paCO2 75, and HCO3 34. The blood gas values reflect not only the patient’s chronic respiratory acidosis but also a second primary problem (i.e., hypochloremic metabolic alkalosis).

    • ✓ A compensatory process alone never restores pH completely back to normal.

    • ✓ For metabolic acidosis, expect a decrease in paCO2 of 1–1.5 mmHg for every decrease in HCO3 of 1 mEq/L.

    • ✓ For metabolic alkalosis, expect an increase in paCO2 of 0.5–1 mmHg for every increase in HCO3 of 1 mEq/L.

Step 5: In metabolic acidosis, calculate the anion gap.

Anion Gap=Na(Cl+HCO3)

  • The normal anion gap is less than 12 mEq/L.

  • In normal gap metabolic acidosis, hyperchloremic acidosis results from the loss of HCO3 in the gut or kidneys.

  • Anion-gap acidosis results from the addition of nontitratable acid to the system. Causes include MUDPILES (methanol, uremia, diabetic ketoacidosis, paraldehyde/propylene glycol, isoniazid/iron/infections, lactic acidosis, ethanol, salicylates).

Step 6: If there is an anion-gap metabolic acidosis, consider the possibility of a second metabolic abnormality.

  • This is done by calculating the delta–delta gap (where AG = anion gap):

    Δ...

Pop-up div Successfully Displayed

This div only appears when the trigger link is hovered over. Otherwise it is hidden from view.