October 15, 2010 Deborah J. DeWaay MD Assistant Professor of Medicine Associate Vice Chair i 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
What are some of your experiences with the following? Teaching ABGs: formal, informal? Students interpreting ABG s in patient care settings? Here are some of ours: Students really struggle to interpret ABGs on the spot. Students really struggle to interpret ABGs on the spot. Students rarely (if they are not comfortable reading ABGs) try to read it ahead of time on rounds. They instantly go to the I dunno mode. They are taught ABGs 2 nd year. We don t know if the teaching is inadequate or it is a skill that can t be learned without patients to practice on. Everyone teaches them differently so it takes a long time to get the rules reinforced: pulmonary vs. nephrology.
Objectives Learn a step by step method to teach AGBs that is: Reliable Evidence Based The learner can come back long after the lecture and use the teaching materials. Practical for patient care. The lectures At MUSC, I give two 1 hour lectures on the topic. The At MUSC, I give two 1 hour lectures on the topic. The first hour we cover steps 1 through 5. The second hour we cover step 6 (because this is the part that students struggle with the most).
Goals for Students At MUSC the course is taught for 3 rd year students. Our goal for them is to be able to: Interpret primary disorders when placed on the spot. Interpret acute vs. chronic respiratory disorders when placed on the spot. Calculate the anion gap on the spot and recognize that the albumin concentrations factors into this calculation. Interpret secondary disorders with a little bit of time to walk through the steps. Step 1: Gather the necessary data Make sure the ABG results are interpretable 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 H2CO3 [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
Points to emphasize with Step 1 If the student memorizes: H20 + CO 2 H2CO3 [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. 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?
Examples 727/58/28 7.27/58/28 ph = acidemia pco2 is consistent with an acidosis HCO3 is consistent with an alkalosis Respiratory Acidosis 758/53/46 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 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 Respiratory acidosis: Acute: ph decreases by 0.08 for every 10 mmhg pco 2 is above 40 mmhg. Chronic: ph decreases by 0.03 for every 10 mmhg pco 2 is above 40 mmhg. Respiratory alkalosis: Acute: ph increases by 0.08 for every 10 mmhg pco 2 is below 40 mmhg. Chronic: ph increases by 0.03 for every 10 mmhg pco 2 is below 40 mmhg. The above assumes: pco 2 not > 55 mmhg; no hypoxia (hypoxic drive trumps ph). If pco2>55 without hypoxia, look for primary respiratory disturbance like respiratory acidosis, usually chronic. 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 (0.03 x 1.8 = 0.054) ph would be 7.35 (7.346) If acute the ph will decrease 0.144 (0.08 x 1.8 = 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/l Calculate the excess anion gap, also called the gap Excess/ gap = actual anion gap (corrected for albumin) 10[normal AG] Anion gap gpgoes 2.5 5 meq/l for every in albumin of 1 gm/dl Let s review where we are: This is where I conclude the first lecture. 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 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
Example 7.19/35/9 Albumin = 4.0 Anion Gap = 18 ph = Acidemia C O2 = Alkalosis HC 3 O = 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 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
Example 7.19/35/9 9359 Albumin = 4.0 Anion Gap = 18 2. ph = Acidemia C O2 = Base HC 3 O = 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 Example 7.54/80/54 754/ /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
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 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 0.048048 (0.0808 x 0.6 = 0048) 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 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 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.
Step 7: solving the problem Anion Gap Non Gap Metabolic Acute Respiratory Metabolic Acidosis Acidosis Acidosis 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) Metabolic Alkalosis CLEVER PD Contraction Licorice Endo (Conn s, Cushing s, Bartter s) Vomiting Excess Alkali Refeeding Alkalosis Post hypercapnea Diuretics Respiratory Alkalosis CHAMPS Anything that causes hyperventilation CNS disease Hypoxia Anxiety Mechanical ventilators Progesterone: Pregnancy and Liver Disease Salicylates/Sepsis Struggles for implementation Finding the time to teach it. Finding the teachers to teach it. Convincing students that it is important. Finding example problems. I have created a problem set for students to use for practice. I would eventually like to have the problems be cased based. If you are interested in writing a few cases, let me know.
Why we believe this works Although complicated at first, it is a step by step method. The worksheet and card allows students to re learn on their own, once the class is done and they are on their own. The concepts of anion gap and buffering can be taught/reviewed while they are learning the mechanics. References Mehtma A, Emmett J. GOLDMARK: An Anion Gap Mehtma A, Emmett J. GOLDMARK: An Anion Gap Pneumonic for the Twenty First Century. Lancet (2008) 33:892 Androgué H et al. Assessing Acid Base Disorders. Kidney International (2009) 76:1239 47
Deborah J. DeWaay MD Medical University of South Carolina dewaay@musc.edu 843 792 1302