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The ABC’s of ABGs for Grown Ups:  Teaching Acid Base and Arterial Blood Gas Analysis to Adult Learners. Deborah J. DeWaay MD Assistant Professor of Medicine Associate Vice-Chair of Education Department of Internal Medicine Medical University of South Carolina Joel A. Gordon, MD

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The ABC’s of ABGs for Grown Ups:  Teaching Acid Base and Arterial Blood Gas Analysis to Adult Learners

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Deborah J. DeWaay MD

Assistant Professor of Medicine

Associate Vice-Chair of Education

Department of Internal Medicine

Medical University of South Carolina

Joel A. Gordon, MD

Professor of Medicine

Department of Internal Medicine

Carver College of Medicine University of Iowa

objectives
Objectives
  • Learn a step by step method to teach ABGs that is:
    • Reliable
    • Evidence Based
    • The learner can come back long after the lecture and use the teaching materials.
    • Practical for patient care.
key messages
Key Messages
  • Learners can be taught to read ABGs in a systematic way that is not confusing.
  • Make sure the ABG results are interpretable. The measured HCO3- from the BMP and the calculated ABG need to be within 2 meq/L.
  • The pH rarely if ever fully compensates to a normal pH of 7.40. If the CO2 and HCO3- are both abnormal with a pH of 7.4 there are 2 problems.
  • If both explain the pH  the patient has two disorders.
  • Anion gap goes ↓2.5 meq/L for every ↓ in albumin of 1 gm/dL. The learner shouldn’t miss an AG metabolic acidosis.
introduction to the topic
Introduction to the topic
  • A noon conference will be given during the emergency lecture series on how to use these materials.
step 1 gather the necessary data
Step 1: Gather the necessary data
  • Make sure the ABG results are interpretable. The measured HCO3- from the BMP and the calculated ABG need to be within 2 meq/L.
  • H20 + CO2 H2CO3  [H+] + [HCO3-]
    • “Normal”
      • pH = 7.4(7.38-7.42)
      • pCO2 = 40 (38-42) mm Hg
      • HCO3- = 24 (22-26) meq/L
points to emphasize with step 1
Points to emphasize with Step 1
  • If the student memorizes:
    • H20 + CO2 H2CO3  [H+] + [HCO3-]
    • Then they can talk themselves through what the consequences of a low or high value from the ABG.
  • Although there is a “range” of normal, have we them pick one number to do the calculations. It is easier for them to keep track of the calculations.
  • Remind the students that ABGs are tests, and like any other test, the interpretation of the test helps create a differential, but that differential must always be applied back to the patient.
steps 2 3 ph pco 2 hco 3
Steps 2 & 3: pH | pCO2 | HCO3-
  • Look at one at a time
  • Look at pH
    • If pH >7.4, then patient is alkalemic (BASE)
    • If pH <7.4, then patient is acidemic (ACID)
  • Look at the pCO2:
      • Is it consistent with an acidosis, >40?
      • Is it consistent with an alkalosis, <40?
  • Look at the HCO3-:
      • Is it consistent with an acidosis, <24?
      • Is it consistent with an alkalosis, >24?
  • Does the pCO2 or the HCO3- explain the pH? Therefore, is there a primary respiratory or metabolic acidosis/alkalosis?
examples
Examples
  • 7.27/58/28
  • pH = acidemia
  • pCO2 is consistent with an acidosis
  • HCO3- is consistent with an alkalosis
  • Respiratory Acidosis
  • 7.58/53/46
  • pH = alkalemia
  • pCO2 is consistent with an acidosis
  • HCO3- is consistent with an alkalosis
  • Metabolic Alkalosis
other points regarding step 1 3
Other points regarding Step 1-3
  • The pH rarely if ever fully compensates to a normal pH of 7.40.
  • If both explain the pH  the patient has two disorders.
step 4 if primary respiratory disorder determine whether acute or chronic
Step 4: If primary respiratory disorder, determine whether acute or chronic
  • Respiratory acidosis:
    • Acute: pH decreases by 0.008 for every 1 mmHg pCO2 is above 40 mmHg.
    • Chronic: pH decreases by 0.003 for every 1 mmHg pCO2 is above 40 mmHg.
  • Respiratory alkalosis:
    • Acute: pH increases by 0.008 for every 1 mmHg pCO2 is below 40 mmHg.
    • Chronic: pH increases by 0.003 for every 1 mmHg pCO2 is below 40 mmHg.
examples1
Examples
  • 7.27/58/28
  • pH= Acidemia CO2= Acidosis HCO3-= Alkalosis
    • Primary etiology = Respiratory Acidosis
    • If respiratory disturbance is it acute or chronic?
      • CO2 has increased by 18
      • If chronic the pH will decrease 0.054 (0.003 x 18 = 0.054)  pH would be 7.35 (7.346)
      • If acute the pH will decrease 0.144 (0.008 x 18 = 0.144) pH would be 7.26 (7.256)
  • This is an acute respiratory acidosis
step 5 calculate the anion gap
Step 5: Calculate the anion gap
  • [Na+] – ([HCO3-] + [Cl-]) = ________ . Normal is 8-12 mEq/L
  • Calculate the excess anion gap, also called the ∆∆ gap
  • Excess/∆∆ gap = actual anion gap (corrected for albumin) – 10 [normal AG]
  • Anion gap goes ↓2.5 meq/L for every ↓ in albumin of 1 gm/dL
let s review where we are
Let’s review where we are:
  • At this point the students should understand how to do the following:
    • Identify the primary disorder
    • If it is a respiratory disorder, identify if the disorder is acute or chronic.
    • Identify if there is an anion gap.
step 6 is there another disorder
Step 6: Is there another disorder?
  • At this point the student should have a primary disorder identified.
  • Find the primary disorder they have identified under Step 6 and follow the directions.
step 6 anion gap metabolic acidosis
Step 6: Anion Gap Metabolic Acidosis
  • If the patient has a PRIMARY anion gap metabolic acidosis:
    • Calculate the corrected or potential HCO3-. This tells you what the HCO3- would be if the anion gap is corrected for.
    • The corrected or potential HCO3- = Excess [∆∆ gap] + measured serum HCO3-
      • If >26 = a metabolic alkalosis
      • If <22 = a non-anion gap metabolic acidosis
example
Example

7.19/35/9 Albumin = 4.0 Anion Gap = 18

  • pH = Acidemia CO2= Alkalosis HCO3- = Acidosis
  • Primary Etiology: Metabolic Acidosis
  • If respiratory disturbance is it acute or chronic? N/A
  • Anion Gap = 18 (alb normal so no correction necessary)
    • Excess Gap = 18-10 = 8
  • Concomitant Disorders:
    • Potential HCO3- = 8 + 9 = 17 which is <22
    • Non-AG Met Acidosis
step 6 if there is a primary metabolic disorder is there also a respiratory disorder
Step 6: If there is a PRIMARY metabolic disorder, is there also a respiratory disorder?
  • Calculate the expected pCO2.
    • The expected pCO2 = ∆ pC02 + 40
  • Metabolic acidosis: ∆ pC02=1.2 x ∆ HCO3-
    • [the CO2will decrease for every 1.2 the HCO3- decreases]
  • Metabolic alkalosis: ∆ pC02=0.7 x ∆ HCO3-
    • [the CO2 will increase for every 0.7 the HCO3- increases.]
  • If actual pCO2 > expected pCO2 concomitant respiratory acidosis
  • If actual pCO2< expected pCO2 concomitant respiratory alkalosis
example1
Example
  • 7.19/35/9 Albumin = 4.0 Anion Gap = 18
    • 2. pH = Acidemia CO2= Base HCO3- = Acid
    • 3. Primary Etiology: Metabolic Acidosis
    • 4. If respiratory disturbance is it acute or chronic? N/A
    • 5. Anion Gap = 18 + Anion Gap (alb normal so no correction necessary)
      • Excess Gap = 18-10 = 8
    • 6. Concomitant Disorders:
      • Potential HCO3- = 8 + 9 = 17 which is <22 Non-AG Met Acidosis
      • Expected CO2 = 19 – 25: CO2 will decrease by 1.2 (∆HCO3-)  1.2 (24-9)  18. 40 – 18= 22 Actual CO2 is higher than expected Respiratory Acidosis
example2
Example
  • 7.54/80/54 Albumin = 4.0 Anion Gap = 12
    • pH = Alkalemia CO2= Acid HCO3- = Base
    • Primary Etiology: Metabolic Alkalosis
    • If respiratory disturbance is it acute or chronic? N/A
    • Anion Gap = 12 (albumin normal so no correction necessary)
    • Concomitant Disorders:
      • Expected CO2 = 61 CO2 will increase by 0.7 (∆HCO3-)  0.7 (54-24)  21  40 + 21 = 61  Actual CO2 is higher than expected Respiratory Acidosis
step 6 if there is a primary respiratory acidosis is there also a metabolic disorder
Step 6: If there is a PRIMARY respiratory acidosis, is there also a metabolic disorder?
  • Calculate the expected HCO3-.
    • The expected HCO3- = ∆ HCO3- + 24.
    • Respiratory Acidosis:
        • Acute: ΔHC03 = 1 mEq/L↑/10mmHg↑pCO2
        • Chronic: ΔHC03 = 3 mEq/L↑/10mmHg↑pCO2
  • If actual HCO3- < expected HCO3- concomitant metabolic acidosis
  • If actual HCO3- > expected HCO3- concomitant metabolic alkalosis
example3
Example

7.25/46/20 Albumin = 4.0 Anion Gap = 12

  • pH = Acidemia CO2= Acid HCO3- = Acid
  • Primary Etiology: Mixed Respiratory Acidosis with Metabolic Acidosis (would determine based on history which is primary)
  • If respiratory disturbance is it acute or chronic?
    • If chronic the pH will decrease 0.018 (0.03 x 0.6 = 0.018)  pH would be 7.38 (7.382)
    • If acute the pH will decrease 0.048 (0.08 x 0.6 = 0.048) pH would be 7.35 (7.352)
  • Concomitant Disorders: already know there are two disorders so you are done. No anion gap, so there is no concomitant AG metabolic acidosis.
step 6 if there is a primary respiratory alkalosis is there also a metabolic disorder
Step 6: If there is a PRIMARY respiratory alkalosis, is there also a metabolic disorder?
  • Calculate the expected HCO3-.
    • The expected HCO3- = ∆ HCO3- + 24.
        • Respiratory Alkalosis:
          • Acute: ΔHC03 = 2 mEq/L↓/10mmHg↓pC02
          • Chronic: ΔHCO3 = 4 mEq/L↓/10mmHg↓pCO2
  • If actual HCO3- < expected HCO3- concomitant metabolic acidosis
  • If actual HCO3- > expected HCO3- concomitant metabolic alkalosis
example4
Example
  • 7.6/20/22 Albumin = 4.0 Anion Gap = 10
    • pH = Alkalemia CO2= Base HCO3- = Acid
    • Primary Etiology: Respiratory Alkalosis
    • If respiratory disturbance is it acute or chronic? Acute
        • CO2 has dropped by 20.
        • If chronic the pH will increase 0.06 (0.03 x 2.0 = 0.06)  pH would be 7.46
        • If acute the pH will increase 0.16 (0.08 x 2.0 = 0.16) pH would be 7.56
    • Anion Gap = 10 (alb normal so no correction necessary)
    • Concomitant Disorders:
      • Assuming Acute Respiratory Alkalosis we would expect the HCO3- to go down 2 mEq/L for every 10mmHG the p CO2 goes down below 40. CO2 is down by 20. 2 x 2.0 = 4. So HCO3- should go down between by 4. It is down by 3 (HCO3- = 22) so no concomitant disorder.
references
References
  • Mehtma A, Emmett J. GOLDMARK: An Anion Gap Pneumonic for the Twenty First Century. Lancet (2008) 372: 892.
  • Androgué H et al. Assessing Acid-Base Disorders. Kidney International (2009) 76:1239-47