The ABC’s of ABGs for Grown Ups: Teaching Acid Base and Arterial

Blood Gas Analysis to Adult Learners

Deborah J. DeWaay MDAssistant Professor of MedicineAssociate Vice-Chair of EducationDepartment of Internal MedicineMedical University of South Carolina

Joel A. Gordon, MDProfessor of Medicine Department of Internal MedicineCarver College of Medicine University of Iowa

ObjectivesLearn a step by step method to teach ABGs

that is: ReliableEvidence BasedThe learner can come back long after the

lecture and use the teaching materials.Practical for patient care.

Key MessagesLearners 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 topicA noon conference will be given during the

emergency lecture series on how to use these materials.

Step 1: Gather the necessary dataMake 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 HgHCO3

- = 24 (22-26) meq/L

Points to emphasize with Step 1If 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 | pCO2 | HCO3-

Look at one at a timeLook 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?

Examples7.27/58/28pH = acidemiapCO2 is consistent with an acidosisHCO3- is consistent with an alkalosisRespiratory Acidosis7.58/53/46pH = alkalemiapCO2 is consistent with an acidosisHCO3- is consistent with an alkalosisMetabolic Alkalosis

Other points regarding Step 1-3The 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 chronicRespiratory 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.

Examples7.27/58/28pH= Acidemia CO2= Acidosis HCO3

-

= AlkalosisPrimary etiology = Respiratory AcidosisIf 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[Na+] – ([HCO3-] + [Cl-]) = ________ .

Normal is 8-12 mEq/LCalculate 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: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?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

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 alkalosisIf <22 = a non-anion gap metabolic acidosis

Example7.19/35/9 Albumin = 4.0 Anion Gap = 18 pH = Acidemia CO2= Alkalosis HCO

3- = AcidosisPrimary Etiology: Metabolic AcidosisIf respiratory disturbance is it acute or chronic? N/AAnion 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?Calculate the expected pCO2.

The expected pCO2 = ∆ pC02 + 40Metabolic 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 acidosisIf actual pCO2< expected pCO2

concomitant respiratory alkalosis

Example7.19/35/9 Albumin = 4.0 Anion Gap = 18

2. pH = Acidemia CO2= Base HCO3- = Acid

3. Primary Etiology: Metabolic Acidosis4. If respiratory disturbance is it acute or chronic? N/A5. 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

Example7.54/80/54 Albumin = 4.0 Anion Gap = 12

pH = Alkalemia CO2= Acid HCO3- = Base

Primary Etiology: Metabolic AlkalosisIf respiratory disturbance is it acute or chronic? N/AAnion 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?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

Example7.25/46/20 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?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

Example7.6/20/22 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.

Step 7: solving the problemAnion Gap

Metabolic AcidosisNon-Gap Metabolic

AcidosisAcute Respiratory

AcidosisMetabolic Alkalosis Respiratory Alkalosis

“GOLD MARK”Glycols (Ethylene &

Propylene)Oxoproline

L-LactateD-LactateMethanol

AspirinRenal Failure

Ketoacidosis (EtOH, Starvation, DKA)

“ACCRUED”Acid load

Carbonic AnhydraseInhibitors

Chronic Kidney Disease (Renal

Failure)Renal Tubular

Acidosis Ureteroenterostomy(Volume) Expansion

Diarrhea

Anything that causeshypoventilationCNS depression

Airway obstructionPneumonia

Pulmonary edemaHemo/pneumothorax

Myopathy(Chronic respiratory

acidosisCaused by COPD and

restrictive lung disease)

“CLEVER PD”Contraction

LicoriceEndo (Conn’s,

Cushing’s, Bartter’s)Vomiting

Excess AlkaliRefeeding AlkalosisPost-hypercapnea

Diuretics

“CHAMPS”Anything that causes

hyperventilationCNS disease

HypoxiaAnxiety

Mechanical ventilators

Progesterone: Pregnancy and Liver

DiseaseSalicylates/Sepsis

ReferencesMehtma

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