Easiest way to solve Acid-Base questions on Medicine Board Exam

Recently I took the board exam. I got good score. I posted my experience here. I got a few Acid Base questions and I found those questions interesting as I could solve them easily with the following steps.

You will be given ABG values and BMP values in acid base questions.

Step 1: Write down pH, PCO2 from ABG on the first line. If pH is low, Primary disorder is Acidosis. If pH is higher than 7.40 then Primary disorder is Alkalosis.

Step 2: Write down Na, HCO3, and Cl- from BMP on the second line.

If HCO3 is less than 24, it’s Metabolic Acidosis. If HCO3 is more than 24, it’s Metabolic Alkalosis.

Step 3: Find Expected PCO2 from HCO3 in BMP which is equal to HCO3 +1.5 X8 [+/_ 2]. In the exam simply add 15 to HCO3 to quickly get Expected PCO2 level.

If PCO2 in ABG is lower/higher/same than this expected PCO2,

then patient has Respiratory Alkalosis/Respiratory Acidosis/Neither.

Step 4: If HCO3 is lower than normal i.e. if Metabolic Acidosis is present, Find Anion Gap which is equal to Na-[HCO3 + Cl].

If Anion Gap is normal, then patient has NAGMA [Non Anion Gap Metabolic Acidosis].

If this Anion Gap is more than normal, then patient has HAGMA [High Anion Gap Metabolic Acidosis].

If patient has HAGMA, proceed to the next step.

Step 5: Find Delta HCO3 and Delta AG.

If amount of decrease in HCO3 is more than the amount of increase in AG, patient has Metabolic Acidosis.

If amount of decrease in HCO3 is less than the amount of increase in AG, patient has Metabolic Alkalosis.

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Practice Question 1:

Step 1: ABG: pH= 7.30, PCO2=50 ==> Patient has Acidosis based on pH.

Step 2: BMP: Na=135, Cl =82, HCO3=24

Step 3: Expected PCO2=24 +15=39, But patient here has PCO2 of 50 in ABG which means he has RESPIRATORY ACIDOSIS.

Step 4: Anion Gap [AG]=135-[24+82]=29 which means patient has HAGMA [High Anion Gap METABOLIC ACIDOSIS]. Patient here has Metabolic Acidosis because with PCO2 of 50, patient’s HCO3 should have compensated more than 24 but it’s only 24 which means patient has some Metabolic acidosis simultaneously. See all the way at the bottom for Acid-Base compensation.

Step 5: Delta HCO3=24-24=0, Delta Anion Gap=29-12=17

Since Decrease in Delta HCO3 [0] is less than Increase in AG [17], patient has METABOLIC ALKALOSIS.

So, based on steps 3,4 and 5, patient has Respiratory acidosis, Metabolic Acidosis and Metabolic alkalosis.

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Practice Question 2:

Step 1: ABG: pH= 7.10, PCO2=50 ==> Patient has Acidosis based on pH.

Step 2: BMP: Na=145, Cl =100, HCO3=15

Step 3: Expected PCO2=15 +15=30, But patient here has PCO2 of 50 in ABG which means he has RESPIRATORY ACIDOSIS.

Step 4: Anion Gap [AG]=145-[15+100]=30 which means patient has HAGMA [High Anion Gap METABOLIC ACIDOSIS]

Step 5: Delta HCO3=24-15=9, Delta Anion Gap=30-12=18

Since Decrease in Delta HCO3 [9] is less than Increase in AG [18], patient has METABOLIC ALKALOSIS.

So, based on steps 3,4 and 5, patient has Respiratory acidosis, Metabolic Acidosis and Metabolic alkalosis.

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Acid-Base compensation:

+1 –>For every 10 units rise in PCO2, HCO3 goes up by 1 unit acutely.

-2 –> For every 10 units drop in PC02, HCO3 drops by 2 units acutely.

+3-4 –> For every 10 units rise in PCO2, HCO3 goes up by 3-4 units chronically, pH drops by 0.03.

-5 –> For every 10 units drop in PCO2, HCO3 drops by 5 units chronically.

Please try it on a few questions and lemme know feedback. Thanks.

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