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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

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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.

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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 HCO 3 - 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.

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Introduction to the topic A noon conference will be given during the emergency lecture series on how to use these materials.

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Step 1: Gather the necessary data Make sure the ABG results are interpretable. The measured HCO 3 - from the BMP and the calculated ABG need to be within 2 meq/L. H20 + CO 2 H 2 CO 3 [H + ] + [HCO 3 - ] “Normal” pH = 7.4(7.38-7.42) pCO 2 = 40 (38-42) mm Hg HCO 3 - = 24 (22-26) meq/L

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Points to emphasize with Step 1 If the student memorizes: H20 + CO 2 H 2 CO 3 [H + ] + [HCO 3 - ] 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.

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Steps 2 & 3: pH | pCO 2 | HCO 3 - 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 pCO 2 : Is it consistent with an acidosis, >40? Is it consistent with an alkalosis, <40? Look at the HCO 3 - : Is it consistent with an acidosis, <24? Is it consistent with an alkalosis, >24? Does the pCO 2 or the HCO 3 - explain the pH? Therefore, is there a primary respiratory or metabolic acidosis/alkalosis?

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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

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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.

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Step 4: If primary respiratory disorder, determine whether acute or chronic Respiratory acidosis: Acute: pH decreases by 0.008 for every 1 mmHg pCO 2 is above 40 mmHg. Chronic: pH decreases by 0.003 for every 1 mmHg pCO 2 is above 40 mmHg. Respiratory alkalosis: Acute: pH increases by 0.008 for every 1 mmHg pCO 2 is below 40 mmHg. Chronic: pH increases by 0.003 for every 1 mmHg pCO 2 is below 40 mmHg.

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Examples 7.27/58/28 pH= Acidemia CO2= Acidosis HCO 3 - = Alkalosis Primary etiology = Respiratory Acidosis If respiratory disturbance is it acute or chronic? CO 2 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

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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

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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.

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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.

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Step 6: Anion Gap Metabolic Acidosis If the patient has a PRIMARY anion gap metabolic acidosis: Calculate the corrected or potential HCO 3 -. This tells you what the HCO 3 - would be if the anion gap is corrected for. The corrected or potential HCO 3 - = Excess [∆∆ gap] + measured serum HCO 3 - If >26 = a metabolic alkalosis If <22 = a non-anion gap metabolic acidosis

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Example 7.19/35/9 Albumin = 4.0 Anion Gap = 18 pH = Acidemia C O2 = Alkalosis HC O 3- = 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 HCO 3 - = 8 + 9 = 17 which is <22 Non-AG Met Acidosis

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Step 6: If there is a PRIMARY metabolic disorder, is there also a respiratory disorder? Calculate the expected pCO 2. The expected pCO2 = ∆ pC0 2 + 40 Metabolic acidosis: ∆ pC0 2 =1.2 x ∆ HCO 3 - [the CO 2 will decrease for every 1.2 the HCO 3 - decreases] Metabolic alkalosis: ∆ pC0 2 =0.7 x ∆ HCO 3 - [the CO 2 will increase for every 0.7 the HCO 3 - increases.] If actual pCO 2 > expected pCO 2 concomitant respiratory acidosis If actual pCO 2 < expected pCO 2 concomitant respiratory alkalosis

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Example 7.19/35/9 Albumin = 4.0 Anion Gap = 18 2. pH = Acidemia C O2 = Base HC O 3- = 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 HCO 3 - = 8 + 9 = 17 which is <22 Non-AG Met Acidosis Expected CO 2 = 19 – 25: CO 2 will decrease by 1.2 (∆HCO 3 - ) 1.2 (24-9) 18. 40 – 18= 22 Actual CO 2 is higher than expected Respiratory Acidosis

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Example 7.54/80/54 Albumin = 4.0 Anion Gap = 12 pH = Alkalemia CO 2 = Acid HCO 3 - = 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 CO 2 = 61 CO 2 will increase by 0.7 (∆HCO 3 - ) 0.7 (54-24) 21 40 + 21 = 61 Actual CO 2 is higher than expected Respiratory Acidosis

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Step 6: If there is a PRIMARY respiratory acidosis, is there also a metabolic disorder? Calculate the expected HCO 3 -. The expected HCO 3 - = ∆ HCO 3 - + 24. Respiratory Acidosis: Acute: ΔHC0 3 = 1 mEq/L ↑ /10mmHg ↑ pCO 2 Chronic: ΔHC0 3 = 3 mEq/L ↑ /10mmHg ↑ pCO 2 If actual HCO 3 - < expected HCO 3 - concomitant metabolic acidosis If actual HCO 3 - > expected HCO 3 - concomitant metabolic alkalosis

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Example 7.25/46/20 Albumin = 4.0 Anion Gap = 12 pH = Acidemia CO 2 = Acid HCO 3 - = 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.

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Step 6: If there is a PRIMARY respiratory alkalosis, is there also a metabolic disorder? Calculate the expected HCO 3 -. The expected HCO 3 - = ∆ HCO 3 - + 24. Respiratory Alkalosis: Acute: ΔHC0 3 = 2 mEq/L ↓ /10mmHg ↓ pC0 2 Chronic: ΔHCO 3 = 4 mEq/L ↓ /10mmHg ↓ pCO 2 If actual HCO 3 - < expected HCO 3 - concomitant metabolic acidosis If actual HCO 3 - > expected HCO 3 - concomitant metabolic alkalosis

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Example 7.6/20/22 Albumin = 4.0 Anion Gap = 10 pH = Alkalemia CO 2 = Base HCO 3 - = Acid Primary Etiology: Respiratory Alkalosis If respiratory disturbance is it acute or chronic? Acute CO 2 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 HCO 3 - to go down 2 mEq/L for every 10mmHG the p CO 2 goes down below 40. CO 2 is down by 20. 2 x 2.0 = 4. So HCO 3 - should go down between by 4. It is down by 3 (HCO 3 - = 22) so no concomitant disorder.

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Step 7: solving the problem Anion Gap Metabolic Acidosis Non-Gap Metabolic Acidosis Acute Respiratory Acidosis Metabolic AlkalosisRespiratory Alkalosis “GOLD MARK” Glycols (Ethylene & Propylene) Oxoproline L-Lactate D-Lactate Methanol Aspirin Renal Failure Ketoacidosis (EtOH, Starvation, DKA) “ACCRUED” Acid load Carbonic Anhydrase Inhibitors Chronic Kidney Disease (Renal Failure) Renal Tubular Acidosis Ureteroenterostomy (Volume) Expansion Diarrhea Anything that causes hypoventilation CNS depression Airway obstruction Pneumonia Pulmonary edema Hemo/pneumothorax Myopathy (Chronic respiratory acidosis Caused by COPD and restrictive lung disease) “CLEVER PD” Contraction Licorice Endo (Conn’s, Cushing’s, Bartter’s) Vomiting Excess Alkali Refeeding Alkalosis Post-hypercapnea Diuretics “CHAMPS” Anything that causes hyperventilation CNS disease Hypoxia Anxiety Mechanical ventilators Progesterone: Pregnancy and Liver Disease Salicylates/Sepsis

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References Mehtma A, Emmett J. GOLDMARK: An Anion Gap Pneumonic for the Twenty First Century. Lancet (2008) 372: 892. Mehtma A, Emmett J. GOLDMARK: An Anion Gap Pneumonic for the Twenty First Century. Lancet (2008) Androgué H et al. Assessing Acid-Base Disorders. Kidney International (2009) 76:1239-47 Androgué H et al. Assessing Acid-Base Disorders. Kidney International (2009) 76:1239-47

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