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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 - PowerPoint PPT Presentation
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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 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
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