STUDENT MANUAL. Core Competencies -- Core Topic ACID-BASE DISORDERS

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1 June 2008 STUDENT MANUAL Core Competencies -- Core Topic ACID-BASE DISORDERS Developed by: Susan Bannon, M.D., Internal Medicine College of Human Medicine Michigan State University Block III

2 Page 2 SOLVING ACID-BASE PROBLEMS THE EASY WAY Block III Core Competency Lecture OBJECTIVES: 1. Learn to recognize acid-base problems in patients 2. Learn an 8-step approach to acid-base probems 3. Apply the 8-step approach to clinical problems 4. Feel comfortable answering double and triple acid-base questions in under 2 minutes 5. Apply the formulas for compensation for acid-base problems 6. Be able to calculate and apply the anion gap, delta gap, urine anion gap and osmolar gap 7. Know the differential for metabolic acidosis, metabolic alkalosis, respiratory acidosis and respiratory alkalosis 8. Understand that an acid base disorder can be present even if the ph is normal MOTIVATION: In the usual test situation, you are given approximately 2 minutes to answer a question. This involves reading a detailed scenario and choosing the correct answer. In critically ill patients, there may be multiple acid-base problems happening simultaneously. In addition to managing hemodynamics, ventilator, infections, nutrition, etc, you need to be able to quickly and accurately assess their acid-base problems. Acidbase problems are encountered on a daily basis in both inpatient and outpatient settings. You need to be familiar with how to recognize and treat them. OVERVIEW: 1. Is the patient acidemic or alkalemic? 2. What is the minimum primary diagnosis? 3. Calculate the anion gap. 4. Calculate the delta gap. 5. Calculate the starting bicarbonate. 6. Calculate compensation. 7. List all of the acid-base orders that exist 8. Clinical correlation PREREQUISITES: 1. Recognize that the bicarbonate concentration on the ABG is calculated (from the Henderson- Hasselbach equation), not measured and should not be used to solve anion gap or validity problems (use the bicarbonate from the BMP or electrolyte panel obtained simultaneously). 2. Know the definitions of acidemia and alkalemia. 3. Know that HCO 3 - functions as a base and that CO 2 functions as an acid. 4. Know the differential diagnosis for metabolic and respiratory acidosis and alkalosis. 5. Recognize that there is no need to calculate compensation for mixed (respiratory and metabolic disorder both exist) acid-base disorders. 6. Understand that compensation (toward normal) of primary acid-base problems is a normal and expected physiologic response, not a primary acid base disorder. The term secondary is confusing and should be avoided. 7. Understand that lack of appropriate compensation (either over or under ) defines a primary acidbase disorder.

3 Page 3 8. Understand that the measured sodium should be used to calculate the anion gap, i.e. do not correct the sodium for hyperglycemia to do the calculations. 9. Always calculate the anion gap, even if the ph is normal or high. 10. The maximum number of acid-base disorders that will exist is 3. You will not have 2 simultaneous respiratory disorders. 11. Low or even negative anion gap disorders do exist (hypoalbuminemia/multiple myeloma, rise in unmeasured cations (K, Ca, Mg, Li), bromide, marked hyperlipidemia, hypernatremia >170) 12. There are formulas to determine if your data is accurate. This will not be covered in this workbook. These formulas are helpful to ensure there was no error in measuring or recording the ABG or chem. panel. AGENDA: 1. Pretest 2. Overview of the 8 steps to answer acid-base questions 3. Watch the instructor solve an acid-base problem using these 8 steps 4. Practice solving various acid-base problems 5. Post test

4 Page 4 8 STEPS TO SOLVING ACID-BASE PROBLEMS 1. Is the patient acidemic (i.e. ph<7.36) or alkalemic (i.e. ph>7.44)? Acidosis and alkalosis are processes; -emia refers to a blood reading. One, two, or three processes can occur simultaneously. 2. What is the minimum primary diagnosis? That is, based on the PaCO 2, and HCO 3 -, what is the one diagnosis that, at minimum, must be present to explain the ph. (Note: At times, perturbations in both PaCO 2 and HCO 3 - may explain the ph. You may list either as the minimum primary diagnosis or you may list both. If you pick up a mixed problem, i.e. metabolic and respiratory-there will be no need to calculate compensation in step 6!) ph - HCO 3 Metabolic Acidosis Acidemia PaCO 2 Respiratory Acidosis - HCO 3 Metabolic Alkalosis Alkalemia PaCO 2 Respiratory Alkalosis 3. Calculate the anion gap. Anion gap = [Na + ] [HCO 3 - ] [Cl - ] A normal gap is 12 +/- 4 (i.e. 8 to 16). The anion gap is the difference between measured cations and measured anions in serum. This difference does not reflect a true disparity between positive and negative charges, given that serum actually is electrically neutral when all serum cations and anions are measured. Rather, the anion gap is a measurement artifact resulting from the fact that only certain cations and anions are routinely measured. Anion gap metabolic acidosis is secondary to the addition of endogenous or exogenous acid. ***If the anion gap is normal, skip steps 4 and 5.*** 4. Calculate the delta gap. gap = anion gap - 12 (the calculated anion gap minus normal anion gap) This is where you start to pick up a coexistent metabolic alkalosis or non-gap acidosis! 5. Calculate the starting bicarbonate. gap + (HCO 3 - ) = starting bicarbonate The purpose of this calculation is to assess the body s ability to change HCO 3 - in response to a metabolic acid. In cases with a pure anion gap metabolic acidosis, the rise in anion gap from 12 should equal the fall in HCO 3 - from 24. (A bicarb was lost for each additional acid) If there is significant discrepancy, then another metabolic disorder is present. If the "starting bicarbonate" is too high, there is a coexistent metabolic alkalosis. If the "starting bicarbonate" is too low, there is a coexistent nonanion gap metabolic acidosis (hyperchloremic metabolic acidosis). Hyperchloremic metabolic acidosis is associated with impairment of bicarbonate buffering. When bicarbonate is lost in urine or stool, there is a compensatory increase in chloride, so that the anion gap remains normal. This is the principle behind the concept of "gap-gap" (a ratio of change in anion gap to the change in HCO 3 - ) and the

5 Page 5 delta-delta. (Other approaches to acid-base) This workbook will only use the starting bicarbonate method. Additional clarification: delta gap is the difference between the patient s aniong gap and a normal anion gap. In simple anion-gap metabolic acidosis the delta gap will be the same as the difference between the patient s bicarbonate and a normal serum {HCO 3 - }. If the delta gap and the change in bicarb are not the same you have at least 2 primary acid-base disturbances. 6. Calculate compensation? Metabolic Acidosis: compensated PaCO 2 = 1.5(HCO 3 - ) + 8 ± 2 Winter s Formula Alternative: PaO mmhg per 1 meq HCO 3 - This is the formula to determine respiratory compensation for a metabolic acidosis. Compare to the measured PaCO 2. They should be within 2 of each other. If higher, there is also a respiratory acidosis. If lower, there is also a respiratory alkalosis. Metabolic Alkalosis: ΔPaCO 2 =0.6(ΔHCO 3 - ) Alternative: : PaO mmhg per 1 meq HCO 3 - This is the formula to determine respiratory compensation for a metabolic alkalosis. A greater increase in PaCO 2 means there is also a respiratory acidosis. A smaller change in PaCO 2 means there is also a respiratory alkalosis. Respiratory Acidosis Acute ( HCO - 3 )=0.10 PaCO 2 [<12-24 hours] Chronic ( HCO - 3 )=0.35 PaCO 2 [3-5 days] Alternative: HCO (acute) 3 (chronic) per 10 mm Hg PaCO 2 Respiratory Alkalosis Acute ( HCO - 3 )=0.2 PaCO 2 [<12 hours] Chronic ( HCO - 3 )=0.5 PaCO 2 [1-2 days] Alternative: : HCO (acute) 5 (chronic) per 10 mm Hg PaCO 2 There are more sophisticated formulas to determine how much of the disorder is acute and how much is chronic. For most problems, this exactness is not necessary and if the clinic scenario could be acute on chronic, you can calculate both and if they fall somewhere between the numbers you can assume they are accurate. You would need to be given baseline ABGs to know how to answer the more sophisticated numbers and that is beyond the scope of this workbook. 7. List all acid-base disorders that exist 8. Clinical correlation

6 Watch the instructor solve a case! Acid-Base Workbook Page 6 A 99-year-old man is wheeled in from a nursing home in shock and hypothermic. Labs: Na + =143 meq/l K + =3.9 meq/l Cl - =98 meq/l HCO - 3 =22 meq/l Glu=126 g/dl BUN=9 g/dl ph=7.61 PaCO 2 =23 mmhg PO 2 =95 mmhg

7 Page 7 ACID-BASE PROBLEMS 1.) A 55-year-old man is admitted with weakness, anorexia, weight loss and polyuria. VITALS: BP 122/76 mmhg T 101 o F RR 22/min P 105/min LABS: Na + =135 meq/l K + =5.4 meq/l Cl - =101 meq/l HCO - 3 =12 meq/l ph=7.32 PaCO 2 =24 mmhg PaO 2 =104 mmhg

8 2.) A 91-year-old woman is admitted with a one week history of diarrhea.. VITALS: BP 95/60 mmhg P 100/min lying T o F P 85/min RR 16/min BP 70/40 mmhg P125/min standing LABS: Na + =133 meq/l K + =2.5 meq/l Cl - =118 meq/l HCO - 3 =5 meq/l ph=7.11 PaCO 2 =16 mmhg PaO 2 =94 mmhg Acid-Base Workbook Page 8

9 Page 9 3.) A 72-year-old woman with an 80 pack-year smoking history, chronic NSAID abuse, and decreased urinary output is admitted with pneumonia. LABS: Na + =140 meq/l K + =5 meq/l Cl - =102 meq/l HCO - 3 =15 meq/l BUN=86 mg/dl Cr =9.8 mg/dl ph=7.10 PaCO 2 =50 mmhg PaO 2 =51 mmhg

10 Page 10 4.) A 42-year-old woman was admitted yesterday with a psychiatric disorder. Today she was found apneic and pulseless in bed by the nurse. CPR is begun and a code is called. LABS: Na + =141 meq/l K + =6 meq/l Cl - =105 meq/l HCO - 3 =8 meq/l ph=6.99 PaCO 2 =34 mmhg PaO 2 =60 mmhg

11 Page 11 5.) A 53-year-old woman with lupus and chronic renal failure has had shaking chills and RLQ pain for 2 days. VITALS: BP 180/100 mmhg T o F P 110/min RR 16/min LABS: Na + =136 meq/l K + =5.5 meq/l Cl - =106 meq/l HCO - 3 = 8 meq/l BUN = 124 mg/dl Cr = 6.8 mg/dl ph=7.44 PaCO 2 =12 mmhg PaO 2 =108 mmhg

12 Page 12 6.) A 72-year-old man with a 100 pack-year smoking history has had watery diarrhea for 2 days. LABS: Na + =136 meq/l K + =3.3 meq/l Cl - =105 meq/l HCO - 3 =19 meq/l ph=7.09 PaCO 2 =65 mmhg PaO 2 =48 mmhg

13 Page 13 7.) A 33-year-old woman is brought to the ED 5 hours after ingesting pills in a suicide attempt. On exam she is stuporous. VITALS: BP 115/70 P 116 RR 24 Temp 99.0 o F LABS: Na + =140 meq/l K + =4.1 meq/l Cl - =106 meq/l HCO - 3 =10 meq/l ph=7.54 PaCO 2 =12 mmhg PaO 2 =106 mmhg

14 Page 14 8.) A 23-year-old pregnant alcoholic with diabetes mellitus type 1 is admitted three days after stopping insulin. She is experienced severe nausea and vomiting. VITALS: BP 120/80 mmhg P 124/min supine BP 108/80 mmhg P 160/min upright LABS: Na + =136 meq/l K + =3.6 meq/l Cl - =70 meq/l HCO - 3 =19 meq/l ph=7.58 PaCO 2 =21 mmhg PaO 2 =104 mmhg

15 9.) A 25-year-old man with asthma has been dyspneic for 4 days. LABS: Na + =132 meq/l K + =3.6 meq/l Cl - =105 meq/l HCO - 3 =18 meq/l ph=7.44 PaCO 2 =28 mmhg PaO 2 =69 mmhg Acid-Base Workbook Page 15

16 Page ) A 40-year-old woman with gallstones presents with abdominal pain, vomiting, and hyperamylasemia. LABS: Na + =132 meq/l K + =2.9 meq/l Cl - =86 meq/l HCO - 3 =38 meq/l ph=7.56 PaCO 2 =48 mmhg PaO 2 =85 mmhg

17 Page ) A 45-year-old man presents to the ED unconscious and seizing. VITALS: BP 80/50 P 105 RR 24 Temp 99.4 o F LABS: Na + =145 meq/l K + =3.9 meq/l Cl - =70 meq/l HCO - 3 =23 meq/l Cr=6.1 mg/dl BUN=130 mg/dl ph=7.61 PaCO 2 =23.8 mmhg PaO 2 =77.5 mmhg

18 Page ) A 58-year-old smoker is brought to the ED severely dyspneic. LABS: Na + =132 meq/l K + =3.5 meq/l Cl - =92 meq/l HCO - 3 =30 meq/l ph=7.20 PaCO 2 =70 mmhg PaO 2 =50 mmhg

19 Page 19 8 Steps To Solving Acid-Base Problems 1. Is the patient acidemic or alkalemic? 2. What is the minimum diagnosis? 3. Calculate the anion gap Anion gap = (Na +) ( HCO - 3 ) (Cl - ) 4. Calculate the delta gap gap = anion gap Calculate the "starting bicarbonate" starting bicarbonate = gap + (HCO - 3 ) 6. Calculate compensation 7. List all of acid-base disorders identified 8. Clinical correlation Primary Disorder Expected Compensation Metabolic Acidosis PaCO 2 =1.5(HCO 3 - ) + 8 Metabolic Alkalosis PaCO 2 =0.6( HCO 3 - ) Respiratory Acidosis Acute ( HCO 3 - )=0.10 PaCO 2 Chronic ( HCO 3 - )=0.35 PaCO 2 Respiratory Alkalosis Acute ( HCO 3 - )=0.2 PaCO 2 Chronic ( HCO 3 - )=0.5 PaCO 2 Anion Gap Acidosis M=methanol U=uremia D=DKA P=paraldehyde I=iron,isoniazid/ibuprofen/inhalents/CN/strychnine/toluene L=lactate:shock,sepsis,CHF,hypoxia,anemia,CO,CN,metformin, ischemic bowel/liver fx/seizures/leukemia/inborn errors(g6pd, fructose 1,6 diphosphatase, pyruvate carboxylase or dehydrogenase, malignancy, heat stroke, D-lactic acidosis, mitochondrial myopathies) meds-antiretrovirals-stavudine E=ethanol/ethylene glycol S=salicylate, solvents, starvation ketoses Hyperchloremic Metabolic Acidosis Loss of bicarbonate from kidney or GI (replaced by HCl) Gain of HCl (hyperalimentation-cationic acids from H + ) Renal loss (RTA, obstrx) Positive (+20)Urine Anion Gap (UAG) UAG= (Na + )+(K + )-(Cl - ) normal is Negative GI loss (diarrhea, fistula, ileostomy,cholestyramine)neg UAG (-20) Ingestion of CaCl 2, MgCl 2 Diamox, hypoaldosteronism,hyperparathyroidism, Rapid NS hydration, post-tx DKA (replace HCO 3 - with Cl - )

20 Metabolic Alkalosis U Cl- <15= chloride responsive Vomiting NG suction Volume contraction Diuretics Cystic Fibrosis Posthypercapnea Alkali syndrome U Cl- >15= chloride resistant Hypercortisolism (Cushing or Conn) Hyperaldosteronism (1 o or 2 o ) 2 o :CHF,cirrhosis,ascites,malig HTN Sodium bicarbonate therapy Severe renal artery stenosis Bartter,licorice,renin-tumor,chewing tob Liddles,severe K dep,cong adrenal hyper MISC:refeeding alkalosis, non-parathyroid Ca Acid-Base Workbook Page 20 Respiratory Alkalosis (symptoms:lt- headed,,tetany,sz,arrhythmia) C: CNS disorders (infection/cva/mass), cirrhosis L: Lung disease (hyperventilation to maintain oxygen) A: Anxiety, anemia P: Progesterone, pregnancy, pressors, physicians (iatrogenic) S: Salicylates, sepsis Hyperthyroidism Respiratory Acidosis Any process from cortex, to brainstem, spinal cord, peripheral nerve, diaphragm, metabolic, upper airway, lower airway, etc that interferes with respiratory drive/ability CNS depression/depressed respiratory drive C3-5 spinal cord injury Pickwickian syndrome Neuromuscular (GBS,myasthenia,myopathy,hypokalemia) Airway obstruction Paralysis/fatigue of respiratory muscles Chronic lung disease Extreme kyphoscoliosis