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MECHANICAL VENTILATION AND PULMONARY ASSESSMENT: BLOOD GAS INTERPRETATION
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Step 1: Acidemia or Alkalemia?
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Step 2: Metabolic or Respiratory?
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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
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Step 3: Is the problem acute or chronic?
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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.
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Step 4: Is there a second primary problem?
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Step 5: In metabolic acidosis, calculate the anion gap.
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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).
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Step 6: If there is an anion-gap metabolic acidosis, consider the possibility of a second metabolic abnormality.
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