Approach to ABG Interpretation

1. Approach to ABG Interpretation Srikanth G

3. At the end of this talk • Systematic approach (The classic 5-step) to acid-base disorders • Calculate the compensations • Identify Complex Acid-base problems

4. Beware!!! • Incremental learning • Cannot move to the next step without thorough understanding of the previous step

5. The 5-Step approach • What is the pH • Identify the primary disorder Metabolic/ Respiratory? • Is the disturbance Acute/ Chronic? Calculate the compensations. • For Metabolic Acidosis- What is the anion gap? • For HIgh anion gap Metabolic acidosis- What is the delta gap?

6. The Primary Acid-base Disorders • Metabolic Acidosis- • High Anion Gap • Normal Anion Gap • Metabolic Alkalosis • Respiratory Acidosis • Acute • Chronic • Respiratory Alkalosis • Acute • Chronic

7. Metabolic Acidosis High Anion Gap Normal Anion Gap Diarrhea Renal Tubular Acidosis Renal failure- Early Hyperkalemia Ureteral Diversion External Pancreatic Drainage • Lactic Acidosis • Sepsis/ Shock • Limb Ischemia • Keto Acidosis • Uncontrolled Diabetes • Uremia • Renal failure • Exogenous

8. Metabolic Alkalosis • Vomiting/ Nasogastric suction • Diuretics • Bartter’s/ Gitelman’s • Mineralocorticoid syndromes • Aldosterone excess/ Cushing’s/ Liddle’s/ SAME • Contraction Alkalosis

9. Respiratory Acidosis Alkalosis Pulmonary embolism Pulmonary edema Pregnancy/ cirrhosis (Progesterone) Drugs- Aspirin, theophylline, nicotine • Hypoventilation- Drugs- Alcohol, Gabapentin, BDZ, Obesity- hypoventilation, Neuromuscular- Transverse myelitis, spinal trauma, Myasthenia, OP poisoning, Malnutrition, Hypothyroidism Kyphoscoliosis • ILD, Pulmonary edema

10. The 5 step Approach

11. Normal Values

12. 1. What is the pH?

13. Step 1- What is the pH? Identify the difference between Acidemia and Acidosis

14. 2. The Primary Acid base disorder

15. The Primary Disorder

16. Step 2:- Identify the primary disorder as Metabolic or Respiratory

17. Example 1 55 year old male with septic shock 1. What is the pH Acidemia 2. Identify the primary disorder Metabolic/ Respiratory? Metabolic Acidosis pH 7.22 Hc03- 12 PCo2- 26 138 116 33 182 4.3 12 1.6

18. Example 2 58 year old male with Cardio renal syndrome 1. What is the pH Alkalemia 2. Identify the primary disorder Metabolic/ Respiratory? Metabolic Alkalosis pH 7.48 Hc03- 34 PCo2- 48 138 92 33 182 4.3 34 1.6

19. 3. Compensations

20. Step 3 Compensations Why to evaluate?? • By predicting the normal compensatory response, mixed acid base disorders can be identified. • The value if above or below the expected, suggests an additional problem • Respiratory compensation for a primary metabolic disorder • Metabolic Compensation for a primary respiratory disorder • Acute/ Chronic

21. STEP 3

22. Easy to remember

23. Example 3 55 year old gentleman with Vomiting and Lower respiratory tract infection 1. What is the pH Alkalemia 2. Identify the primary disorder Metabolic/ Respiratory? Metabolic Alkalemia 3. Compensation?! 40 + (3/4 X 20) = 55 Actual value = 35 Additional Respiratory alkalosis pH 7.71 Hc03- 45 PCo2- 35.9 128 116 33 182 2.6 45 1.6

24. Example 4 26 year old lady Acute onset Breathlesness 1. What is the pH Alkalemia 2. Identify the primary disorder Metabolic/ Respiratory? Respiratory Alkalemia 3. Compensation?! 24- (2 x 2) = 20 Actual value = 20 Appropriate pH 7.45 Hc03-20 PCo2- 20 140 104 33 182 4.6 20 1.6

25. Example 5 50 year male with with cardiac arrest 1. What is the pH Acidemia 2. Identify the primary disorder Metabolic/ Respiratory? Metabolic Acidemia 3. Compensation?! 40- (5/4 x 18) = 22.5 Actual value 29 Respiratory Acidosis pH 7.0 Hc03-6.0 PCo2- 29 144 104 33 182 5.0 6 1.6

26. 4. The Anion Gap

27. Anion Gap- Concept of electroneutrality

28. The electroneutrality Cations Anions

29. The Anion Gap

31. Anion gap

32. Example 5 1. What is the pH Acidemia 2. Identify the primary disorder Metabolic/ Respiratory? Metabolic Acidemia 3. Compensation 40- (5/4 X 10) = 27.5 Actual value = 28 Appropriate 4. Anion Gap Na – (Cl- + HCO3-) 10 mEq/L Normal Anion Gap Metabolic Acidosis 55 year old male with diarrhea pH 7.32 Hc03- 14 PCo2- 28 138 114 26 122 3.6 14 1.0

33. Example 6 1. What is the pH Acidemia 2. Identify the primary disorder Metabolic/ Respiratory? Metabolic Acidemia 3. Compensation 40- (5/4 X 12) = 25 Actual value = 26 Appropriate 4. Anion Gap Na – (Cl- + HCO3-) 22 mEq/L High Anion Gap Metabolic Acidosis 48 year old female with septic shock pH 7.30 Hc03- 12 PCo2- 26 129 94 70 122 3.8 13 1.6

34. Example 7 1. What is the pH Alkalemia 2. Identify the primary disorder Metabolic/ Respiratory? Respiratory Alkalosis 3. Compensation 24 – (2 X 2) = 20 Actual value = 14 Additional Met Acidosis 4. Anion Gap Na – (Cl- + HCO3-) 10 mEq/L Respiratory Alkalosis with Normal Anion Gap Metabolic Acidosis 58 year old Gentleman with DM and CKD, now presented with Acute dyspnea of hours pH 7.47 Hc03- 14 PCo2- 20 135 111 24 122 5.6 14 1.9

35. 5. The Delta Gap

36. The Δ Delta gap Lactic Acid Lactate H+ H20 + CO2 HCO3- For every 1 Lactic Acid 1 Lactate anion accumulates- Anion gap 1 Bicarbonate is consumed Δ Anion Gap/ Δ HCO3- = 1 However body has Phosphate and other buffers- Normal Anion gap 1-2

39. Rules of Delta • Check only, if there is HAGMA • Identifies additional Metabolic Alkalosis/ NAGMA

40. Example 8 1. What is the pH Acidemia 2. Identify the primary disorder Metabolic/ Respiratory? Metabolic Acidosis 3. Compensation 40- (5/4 X 10) = 27.5 Actual value = 29 Appropriate compensation 4. Anion Gap Na – (Cl- + HCO3-) 16 mEq/L- HAGMA 5. Delta ratio- 0.6 Additional Normal anion gap Acidosis 75 year old woman with fever and Diarrhea for 2 days, BP 80/40 pH 7.29 Hc03- 14 PCo2- 29 128 98 24 144 3.2 14 1.9

41. Example 9 1. What is the pH Acidemia 2. Identify the primary disorder Metabolic/ Respiratory? Metabolic Acidosis 3. Compensation 40- (5/4 X 12)= 25 Actual value = 27 Appropriate compensation 4. Anion Gap Na – (Cl- + HCO3-) 30 mEq/L- HAGMA 5. Delta ratio- 1.67 High anion gap Metabolic Acidosis 55 year uncontrolled Diabetic with Fever, vomiting and abdominal pain for 3 days pH 7.29 Hc03- 12 PCo2- 27 140 98 20 386 3.8 12 1.9

42. Example 10 1. What is the pH ??!! 2. Identify the primary disorder Metabolic/ Respiratory? Metabolic 3. Compensation 40- (5/4 X 3)= 36. Actual value= 38 Appropriate compensation 4. Anion Gap Na – (Cl- + HCO3-) 30 mEq/L- HAGMA 5. Delta ratio- 6.6 High anion gap Metabolic Acidosis with Metabolic Alkalosis A young woman with Gastroenteritis and Dehydration pH 7.39 Hc03- 21 PCo2- 38 145 94 40 84 3.2 21 2.9

43. Special Situations • NAGMA- Urine anion gap- Use to differentiate- renal vs extrarenal causes • HAGMA- Osmolar Gap- Used in patients with suspected unknown poisoning in the absence of Ketones, Lactic acidosis and Uremia

44. Practice ABG • 1 pH- 7.16, pCO2- 35, HCO3- 12 Na- 140, Cl- 90, HCO3-12 • 2. pH- 7.35, pCO2- 34, pO2- 100, HCO3- 18 Na- 140, Cl- 90, Hco3- 18 • 3. pH- 7.48, pCO2- 47, pO2- 100, HCO3- 34 Na- 140, Cl- 90, HCO3-34

45. Take Home • Use the clinical clues- They give a ‘Preview’ • The 5 steps • The pH • Primary Acid-base disorder • The compensations • 5/4, 3/4, 1-4-2-4 • Anion gap • Delta gap • Check for anion gap in all cases of Metabolic Acidosis • Check for Delta gap in all patients with High Anion gap Metabolic Acidosis • Urinary anion gap in NAGMA helps differentiate renal vs Extrarenal causes • Osmolar gap in patients with suspected poisoning

46. Thank you