2. ARTERIAL BLOOD GAS Section of Pediatric Pulmonology UPCM-Philippine General Hospital

3. Clinical Application of Blood Gases 5 th Edition Shapiro, et. Al.

4. ABG measures respiratory function 1. Oxygenation status 2. Acid-base balance

5. Points to Remember Body always tries to maintain normal pH CO 2 – respiratory HCO 3 – metabolic Lungs compensate rapidly Kidneys compensate slowly There is no overcompensation except in chronic ventilatory failure Consider the underlying disease

6. Normal ABG Values pH 7.35-7.45 7.3-7.5 clinically acceptable PO 2 80-100 60-80 for newborn, preterm age 60 PCO 2 35-45 30-50 clinically acceptable

7. Oxygenation Status At room air, sea level: At room air, sea level: PaO 2 80-100 normal or acceptable PaO 2 < 80 mild hypoxemia PaO 2 < 60 moderate hypoxemia PaO 2 < 40 severe hypoxemia On oxygen support: On oxygen support: PaO 2 80-100 corrected hypoxemia PaO 2 > N overcorrected hypoxemia PaO 2 < N uncorrected hypoxemia

8. Nomenclature for pH & PaCO2 Outside of normal range pH > 7.45Alkalemia pH < 7.35Acidemia PaCO2 > 45 mmHgRespiratory Acidosis (hypercapnia) PaCO2 < 35 mmHgRespiratory Alkalosis (hypocapnia)

9. FiO2 vs PaO2 The minimally acceptable PaO2 increases by approximately 50 mmHg for every 10% increment of inspired oxygen concentration.

10. FiO 2 vs PaO 2 PAO 2 = [ FiO 2 (P B – P H 2 0 ) ] – PaCO 2 0.8 PaO 2 = FiO 2 x 5

11. What is the expected PAO 2 if you are breathing normally at room air? = [.21 (760 – 47) ] – 40 / 0.8 = 150 – 50 = 100 PaO 2 = FiO 2 x 5 =.21 x 5 = 105 PAO 2 = [ FiO 2 (P B – P H 2 0 ) ] – PaCO 2 0.8

12. In normal individuals the PAO2 is more or less equal to the PaO2 with a normal shunt of about 5 % PAO2 vs PaO2

13. FiO 2 vs PaO 2 P(A-a)O 2 = PAO 2 - PaO 2 Normal: < 30 mm Hg at room air < 50 mm Hg at FiO 2 1.0 > 450 mmHg is indicative of severe respiratory failure

14. PaCO2 – pH RELATIONSHIP 1.For every 10 mm Hg increase in the PaCO2, the pH will decrease by 0.05 unit. 2.For every 10 mm Hg decrease in the PaCO2, the pH will increase by 0.10 unit.

15. Respiratory pH (pH R ) PaCO 2 pH R 40 7.4 60 7.3 80 7.2 30 7.5 20 7.6

16. Respiratory pH (pH R ) PaCO 2 pH R 40 7.4 60 7.3 80 7.2 For every  in pCO 2 of 10, pH  by 0.05

17. Computing for pH R Normal pCO 2 = 40 and normal pH = 7.4 pH R = (40 – Actual pCO 2 ) x 0.05 + 7.4 10 If actual pCO 2 > 40:

18. Respiratory pH (pH R ) PaCO 2 pH R 40 7.4 30 7.5 20 7.6 For every  in pCO 2 of 10,pH  by 0.1

19. Computing for pH R Normal pCO 2 = 40 and normal pH = 7.4 pH R = (40 – Actual pCO 2 ) x 0.05 + 7.4 10 If actual pCO 2 > 40: If actual pCO 2 < 40: pH R = (40 – Actual pCO 2 ) x 0.1 + 7.4 10

20. What is the expected pH when the pCO 2 is 45? If actual pCO 2 > 40: pH R = (40 – Actual pCO 2 ) x 0.05 + 7.4 10 pH R = (40 – 45) x 0.05 + 7.4 10 = 7.375

21. What is the expected pH when the pCO 2 is 35? If actual pCO 2 < 40: pH R = (40 – Actual pCO 2 ) x 0.1 + 7.4 10 pH R = (40 – 35) x 0.1 + 7.4 10 = 7.45

22. pH R vs pH If pH R compared to actual pH is: < 0.03  purely respiratory > 0.03  compensated

23. pH R vs pH If actual pH > pH R partially compensated If actual pH < pH R mixed If actual pH = pH R purely respiratory

24. PaCO2 – Plasma BICARBONATE RELATIONSHIP 1.An acute PaCO2 increase of 10 mmHg will increase the plasma bicarbonate by 1 mmol/L 2.An acute PaCO2 decrease of 10 mmHg will decrease the plasma bicarbonate by 2 mmol/L The difference between the calculated respiratory plasma bicarbonate value and the actual plasma bicarbonate value provides a rapid and easy assessment of the metabolic component.

25. Approximate PaCO2-pH Relationship PaCO2pHHCO3 (mmHg)(mmol/L) 307.5022 207.6020 807.2028 607.3026 407.4024

26. Minute Ventilation vs PaCO 2 MV PaCO 2 Range N40 35-45 2N30 25-35 4N20 15-25 The existence of a significant minute MV to PaCO2 disparity should alert the clinician to the possibility that a deadspace-producing pathologic condition may be present.

27. FiO 2 -PaO 2 RELATIONSHIP Inspired Oxygen to PaO2 Relationship in Normal lungs. FiO2 Inspired O2(%)PaO2 0.3030>150 0.4040>200 0.5050>250 0.8080>400 1.00 100>500

28. DETERMINING BASE EXCESS/DEFICIT Under normal circumstances, a 10 mmol/L variance from the normal buffer base represents a pH change of approximately 0.15 unit. If we move the pH decimal point two places to the right, we have a 10 to 15 relationship, which can be expressed as a 2/3 realtionship.

29. The difference between the measured pH & the predicted respiratory pH is the metabolic pH change. BE or BD = (actual pH – pH R ) x 2 3 x 100

30. Base excess or deficit 0.15 pH change  10 mEq/L buffer change BE: actual pH > pH R BD: actual pH < pH R BE or BD = (actual pH – pH R ) x 200 3 Normal BE or BD:  2

31. INTERPRETIVE APPROACH Step 1. Assessment of the PCO2 and pH. a. Classify the CO2 tension. b. Consider the pH and determine classification. c. Consider the base excess/deficit or bicarbonate levels and determine classification. Step 2. Assessment of Arterial Oxygenation a. PaO2 b. SaO2

32. pCO 2 < 35 pCO 2 35-45 pCO 2 > 45 pH < 7.35 pH 7.35 – 7.4pH 7.4 – 7.45pH > 7.45 normal acidosis alkalosis

33. pCO 2 < 35 pCO 2 35-45 pCO 2 > 45 pH < 7.35 pH 7.35 – 7.4pH 7.4 – 7.45pH > 7.45 normal acidosis alkalosis metabolic respiratory metabolic respiratory comp part comp

34. Physiologic Mechanisms of Hypoxemia 1.Alveolar hypoventilation 2.Ventilation-perfusion mismatch 3.Right-to-left shunt 4.Diffusion limitation 5.Decreased ambient oxygen tension

35. Physiologic Mechanisms of Hypoxemia Alveolar hypo- ventilation V/Q mismatch R to L shunt Diffusion limitation Response to O 2 good poor CO 2  variableN- CVSN Clinical response A- neuro- muscular A- CVSPP CXRN- central W- alveolar W- lung N- CVS minimal W W –white N - normal P - poor A - absent

36. Metabolic acidosis 1. Renal failure (RTA) 2. Ketoacidosis (DKA, starvation) 3. Lactic acidosis

37. Anion Gap = Na – (Cl + HCO 3 ) Normal: < 15 mEq/L

38. Anion Gap IncreasedNormal Organic acid accumulation Acute renal failure Inborn error of metabolism Lactic acidosis Late metabolic acidosis Toxins Loss of buffer Renal HCO 3 loss Renal tubular acidosis Acetazolamide Renal dysplasia GI HCO 3 loss Diarrhea Cholestyramine Small bowel drainage Dilutional acidosis Hyperalimentation acidosis

39. Metabolic alkalosis 1. Hypokalemia 2. Hypochloremia 3. Vomiting 4. Massive steroid administration 5. NaHCO 3 administration

40. Respiratory acidosis 1. Hypoventilation a.Inadequate respiratory effort CNS problems Neuromuscular disease Mechanical ventilator settings b.Upper airway not patent c.Decreased lung tissue d.Decreased lung compliance

41. Respiratory acidosis 2. Abnormal ventilation-perfusion ratio a. Obstruction of small airways b. Atelectasis c. Pneumonia d. Pulmonary edema 3. Increased extrapulmonary shunt a.Pulmonary vasoconstriction RDS, severe infection b.Pulmonary hypoplasia c.Cyanotic heart disease

42. Respiratory alkalosis 1.With hypoxemia a. Acute pulmonary disease pneumonia and atelectasis, RDS, acute asthma b.Acute myocardial disease MI, pulmonary edema, heart failure, CP bypass 2. Without hypoxemia a.Anxiety, neurosis, psychosis b.Pain c.CNS disease d.Anemia e.Carbon monoxide poisoning

43. Disorders Expected compensation Metabolic pCO2 = 1.5 x HCO3 + 8 +/- 2 Acidosis Metabolic pCO2 increase by 7 mmHg for each Alkalosis 10 mEq/L increase in HCO3

44. Disorder Expected compensation Respiratory acidosis Acute HCO3 increase by 1 for each 10mmHg increase in pCO2 Chronic HCO3 increase by 3.5 for each 10mmHg increase in pCO2 Respiratory Alkalosis Acute HCO3 decrease by 2 for each 10mmHg decrease in pCO2 Chronic HCO3 decrease by 4 for each 10mmHg decrease in pCO2

45. Exercises Compute for the pH R and interpret the ABG values

46. 1. pH 7.45 pO 2 65 pCO 2 32 FiO 2.21 pH R = (40 - 32) x 0.1 + 7.4 = 7.48 10 respiratory compensated with mild hypoxemia BE/BD = (7.45 – 7.48) x 200 = -2 3 alkalosis HCO3

47. 2. pH 7.3 pO 2 120 pCO 2 30 FiO 2.30 pH R = (40 - 30) x 0.1 + 7.4 = 7.5 10 metabolic partially compensated with overcorrected hypoxemia BE/BD = (7.3 – 7.5) x 200 = -13 3 acidosis HCO3

48. 3. pH 7.25 pO 2 90 pCO 2 55 FiO 2.40 pH R = (40 - 55) x 0.05 + 7.4 = 7.325 10 uncompensated with corrected hypoxemia BE/BD = (7.25 – 7.325) x 200 = - 5 3 acidosis respiratory HCO3 N

49. 4. pH 7.42 pO 2 200 pCO 2 35 FiO 2.35 pH R = (40 - 35) x 0.1 + 7.4 = 7.45 10 with overcorrected hypoxemia BE/BD = (7.42 – 7.45) x 200 = -2 3 normal acid-base balance NN HCO3 N

50. Calculating FiO2 requirement using the ABG FiO2 = (desired PaO2) + PaCO2 PaO2RQ PAO2 Pb – PH20

51. Thank you for listening