Interpretation of ABG.pptx

A primer into the interpretation of ABG.

1. Interpretation of ABG Presenter : Dr. Saran A K 26 September 2023

2. It is a diagnostic procedure in which a blood is drawn from an artery directly by an arterial puncture or from an indwelling arterial catheter. Indications : Obtain information about patient’s Ventilation(pCO2) Oxygenation (pO2) Acid-Base balance DR. SARAN A K 2

3. Objectives ABG sampling Interpretation of ABG Oxygenation status Acid base status DR. SARAN A K 3

4. ABG sampling Site: Radial artery Brachial artery Femoral artery Pre-heparinised ABG syringes  syringes should be flushed with 0.5mL 1:1000 Heparin solution and emptied Do not leave excess Heparin in the syringe  Dilutional effect  decreases HCO3 and Pco2 DR. SARAN A K 4

5. Preparatory phase Record the FiO2 the patient is on Explain the procedure to the patient Perform Allen's test Wait at least 20 minutes after initiating, changing, or discontinuing oxygen therapy. DR. SARAN A K 5

6. Technical errors Risk of alteration of results Increased size of syringe / needle -Use < 3mL syringe Decreased volume of sample- Syringes must have > 50% blood Air bubbles (Po2 150mmHg and Pco2 0mmHg) Increase Po2 and decrease Pco2. Seal syringe immediately after sampling Body temperature ABG analyser controlled for normal body temperatures DR. SARAN A K 6

7. Need for cold chain 0.01mL O2 consumed/dL/min Marked increase in high TLC / Platelet counts  decreases Po2 Hence, chilling / immediate analysis. Change / 10 minutes Uniced 37oC Iced 4oC pH 0.01 0.001 Pco2 1 mmHg 0.1 mmHg Po2 0.1% 0.01% DR. SARAN A K 7

8. Complications Arteriospasm Hematoma Hemorrhage Distal ischemia Infection Numbness DR. SARAN A K 8

9. ABG components p H : indicates H+ ion concentration pH = - log[H+] p O2 : O2 that is dissolved in the blood , it reflects the body’s ability to pick up oxygen from the lungs pCO2 : CO2 that is carried by the blood for excretion through the lungs - R espiratory parameter HCO3 : Metabolic parameter . It reflects the kidney’s ability to retain and excrete HCO3 DR. SARAN A K 9

10. Interpretation of ABG Gas Exchange Acid Base Status DR. SARAN A K 10

11. A. Gas Exchange Determination of PO 2 PaO 2 is dependent upon Age, FiO 2 , P atm As age increases, PaO 2 decreases PaO 2 = 109- 0.4 (Age) As FiO 2 increases, the expected PaO 2 increases (Alveolar Gas Equation) PAO 2 = (P B -P H20 ) X FiO 2 – PCO 2 /R DR. SARAN A K 11

12. Hypoxemia Normal PaO 2 = 95-100 mm Hg Mild Hypoxemia : PaO 2 60-80 mm Hg Moderate Hypoxemia : PaO 2 40-60 mm Hg – tachycardia, hypertension, cool extremities Severe Hypoxemia : PaO 2 < 40 mm Hg – severe arrythmias, brain injury, death DR. SARAN A K 12

13. 2. Alveolar- Arterial O2 gradient P(A-a)O 2 is the alveolar- arterial difference in partial pressures of oxygen PAO 2 = 150 – PaCO 2 /RQ Normal Range : 5-25 mm Hg (increases with age) Increase P(A-a)O 2 – lung parenchymal disease DR. SARAN A K 13

14. 3. PaO 2 /FiO 2 Ratio Inspired Air FiO 2 = 21% PiO 2 = 150 mmHg PAO 2 = 100 mm Hg PaO 2 = 90 mm Hg PaO 2 /FiO 2 Ratio Inference 200-300 mmHg Mild ARDS 100-200 mmHg Moderate ARDS <100 mm Hg Severe ARDS Berlin Criteria for ARDS Severity DR. SARAN A K 14

15. 4. Hypercapnia PaCO 2 is directly proportional to CO2 production and inversely proportional to alveolar ventilation. Normal PaCO 2 is 35-45mm Hg DR. SARAN A K 15

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17. B. Acid Base Status pH = -log [H + ] : Sorensen formula DR. SARAN A K 17

18. Henderson Hesselbalch Equation Correlates metabolic and respiratory regulations. DR. SARAN A K 18

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20. Bicarbonate Buffer System DR. SARAN A K 20

21. Respiratory Regulation DR. SARAN A K 21

22. Renal Regulation Kidneys control the acid base balance by excreting a basic or an acidic urine Excretion of HCO 3 - Regeneration of HCO 3 - with excretion of H + DR. SARAN A K 22

23. Response Bicarbonate Buffer System – acts in few seconds Respiratory Regulation – starts within minutes good response by 2 hrs , complete by 12-24 hrs Renal Regulation – starts after few hrs , complete by 5-7hrs DR. SARAN A K 23

24. Normal values DR. SARAN A K 24

25. Simple Acid Base Disorders A single primary process of acidosis or alkalosis with or without compensation. DR. SARAN A K 25

26. Compensation The body always tries to normalize the pH - pCO 2 and HCO 3 - rise and fall together in simple disorders Compensation never overcorrects the pH Lack of comp in an appropriate time defines a 2 nd disorder. Requires normally functioning lungs and kidney DR. SARAN A K 26

27. Characteristics of primary acid base disorders DR. SARAN A K 27

28. Metabolic Acidosis Diabetic Ketoacidosis Diarrhoea Renal failure Shock Aspirin overdose Sepsis Metabolic Alkalosis Loss of gastric secretions Overdose of antacids K+ sparing diuretics Respiratory Acidosis Hypoventilation COPD Airway obstruction Drug overdose Neuromuscular disease Respiratory Alkalosis Hyperventilation Hypoxia Anxiety High altitude Fever DR. SARAN A K 28

29. Mixed Acid Base Disorders Presence of more than one acid base disorder simultaneously Clues to a mixed disorder Normal pH with abnormal HCO 3 - or pCO 2 pCO 2 and HCO 3 - move in the opposite directions pH changes in an opposite direction for a known primary disorder DR. SARAN A K 29

30. Step by Step Analysis of Acid Base Balance DR. SARAN A K 30

31. Clinical History A 24-year-old male, k/c/o T1DM on insulin for past 2 years presented to the ED with complaints of SOB for one day, fever for 3 days. The patient also complained about abdominal pain in epigastric region since the previous day. On examination, sensorium impaired , RR>30/min Chest AEBE, S1 S2 Normal, Abdomen soft ABG was ordered. DR. SARAN A K 31

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33. Step 1 : Acidemia/ Alkalemia? DR. SARAN A K 33

34. Step 2 : What is the primary disorder? DR. SARAN A K 34

35. Step 3 : Calculation of compensation DR. SARAN A K 35

36. Response to metabolic acid-base disorder minute ventilation – mediated by peripheral chemoreceptors Fast response = 30-120 min. Metabolic acidosis - washout Winter’s formula : PCO2 = 1.5 X [HCO3] + 8 2 Metabolic alkalosis - CO2 washout = PCO2 E PCO2 = 0.7 X HCO3 + 20 5   DR. SARAN A K 36

37. Winter’s formula : PCO2 = 1.5 X [HCO3] + 8 2 Expected PCO2 = 1.5 x 3.2 + 8 2 Expected PCO2 = 4.8 + 8 2 Expected PCO2 = 12.8 2 = 10.8- 14.8   DR. SARAN A K 37

38. Metabolic Acidosis Metabolic Alkalosis DR. SARAN A K 38

39. Response to respiratory acid-base disorder Proximal renal tubules adjusts the absorption of HCO3 to produce change in plasma HCO3 Slow response : 2-3 days DR. SARAN A K 39

40. Acute respiratory disorders A/c resp. acidosis : A/c resp. alkalosis : Chronic respiratory disorders ↑PaCO2 = ↑ ↓ PaCO2 = ↓ change in HCO3 = 0.4 X PaCO2   DR. SARAN A K 40

41. Respiratory Acidosis If Serum HCO3 > expected bicarbonate, additional metabolic alkalosis and vice versa. Respiratory Alkalosis If Serum HCO3 < expected bicarbonate, additional metabolic acidosis and vice versa DR. SARAN A K 41

42. Step 4 : Metabolic Acidosis - The Anion gap Difference in the measured cations & the measured anions Estimate unmeasured anions Na + UC = (CL + HCO3) + UA Na – (Cl + HCO3) = UA-UC AG = Na-(Cl + HCO3) Normal reference value = 12 2   DR. SARAN A K 42

43. DR. SARAN A K 43

44. HAGMA NAGMA Diabetic Ketoacidosis Chronic Kidney Disease Lactic Acidosis Alcoholic Keto acidosis Aspirin Poisoning Methanol poisoning Ethylene Glycol Poisoning Starvation Diarrhea Renal Tubular Acidosis Addison's Disease Carbonic Anhydrase Inhibitors DR. SARAN A K 44

45. Influence of albumin Principal determinant of AG Low albumin lowers AG-mask the presence of UA AG c = AG + 2.5 ( 4.5 - [albumin in g/dL]) DR. SARAN A K 45

46. Delta Gap The difference between patients AG and normal AG The coexistence of 2 metabolic acid-base disorders may be apparent Delta gap = Anion Gap -12 Delta gap + HCO3- = 22-26 mEq /l If >26, consider additional metabolic alkalosis If < 22, consider additional non-AG metabolic acidosis DR. SARAN A K 46

47. Summary ABG is a diagnostic tool that helps us to understand the ventilation, oxygenation and acid base status of the patient. Stepwise approach to ABG interpretation can help to elucidate the underlying acid base disorders- metabolic or respiratory and acidosis or alkalosis. DR. SARAN A K 47

48. THANK YOU ! [email protected] DR. SARAN A K 48