AM Report Pearls: Six Step Approach to the Challenging ABG
Step 1: Acidemic (pH < 7.35) or alkalemic (pH > 7.45) on ABG?
Step 2: Is the primary disturbance respiratory or metabolic?
*Hint: Whichever side of 7.4 the pH is on, the process that caused it to shift to that side is the primary abnormality!*
Step 3: For metabolic acidosis, is there an increased anion gap?
*Hints: Remember to correct for albumin, and remember NS as cause of non-gap metabolic acidosis*
Step 4: For primary respiratory causes, is it acute or chronic?
Step 5: Is there appropriate respiratory compensation for a metabolic acidosis?
*Hint: Winter’s formula gives you expected PCO2 = 1.5 x HCO3 + 8 +/- 2*
Step 6: Are there other superimposed disturbances?
*Hint: Calculate the delta gap = calculated AG – 12 (normal AG) and add this delta gap to the HCO3 to get corrected HCO3*
For more detailed information, check out the classic UCSF Dr. Haber paper on acid-base attached and a helpful handout from UC-Denver here: http://www.ucdenver.edu/academics/colleges/medicalschool/departments/medicine/intmed/imrp/CURRICULUM/Documents/Evaluation%20of%20Acid%20Base%20Disorders.pdf
Here is also a newer NEJM paper on Acid-Base disturbances that’s pretty helpful as well http://www.nejm.org/doi/full/10.1056/NEJMra1003327