Blood Gas Analysis

1. Blood Gas Analysis

2. MOTIVATION FOR LEARNING ABOUT BLOOD GAS INTERPRETATION

3. MOTIVATION In a survey conducted at a university teaching hospital, 70% of the participating physicians claimed that they were well versed in the diagnosis of acid-base disorders and that they needed no assistance in the interpretation of arterial blood gases (ABGs). These same physicians were then given a series of ABG measurements to interpret, and they correctly interpreted only 40% of the test samples. Hingston DM. A computerized interpretation of arterial pH and blood gas data: do physicians need it? Respir Care 1982;27:809-815. From: THE ICU BOOK - 2nd Ed. (1998)

4. MOTIVATION A survey at another teaching hospital revealed that incorrect acid-base interpretations led to errors in patient management in one-third of the ABG samples analyzed. Broughton JO, Kennedy TC. Interpretation of arterial blood gases by computer. Chest 1984;85:148-149. From: THE ICU BOOK - 2nd Ed. (1998)

5. MOTIVATION These surveys reveal serious deficiencies in an area that tends to be ignored. This can cause trouble in the ICU, where 9 of every 10 patients may have an acid-base disorder. Gilfix BM, Bique M, Magder S. A physical chemical approach to the analysis of acid-base balance in the clinical setting. J Crit Care 1993;8:187-197. From: THE ICU BOOK - 2nd Ed. (1998)

6. Getting an arterial blood gas sample

8. Ulnar Artery Radial Artery

11. What Is An ABG? pH [H+] PCO2 Partial pressure PO2 Partial pressure HCO3 Bicarbonate BE Base excess SaO2 Oxygen Saturation

12. Normal Ranges

13. Acid-base Balance Henderson-Hasselbalch Equation [HCO3-] pH = pK + log ---------------- .03 [PaCO2] For teaching purposes, the H-H equation can be shortened to its basic relationships: HCO3- pH ~ --------- PaCO2

14. Primary Acid-base Disorders:Respiratory Alkalosis • Respiratory alkalosis - A primary disorder where the first change is a lowering of PaCO2, resulting in an elevated pH. Compensation (bringing the pH back down toward normal) is a secondary lowering of bicarbonate (HCO3) by the kidneys; this reduction in HCO3- is not metabolic acidosis, since it is not a primary process. Primary Event Compensatory Event HCO3-↓HCO3- ↑pH ~ ------- ↑ pH ~ -------- ↓PaCO2 ↓PaCO2

15. Primary Acid-base Disorders:Respiratory Acidosis • Respiratory acidosis - A primary disorder where the first change is an elevation of PaCO2, resulting in decreased pH. Compensation (bringing pH back up toward normal) is a secondary retention of bicarbonate by the kidneys; this elevation of HCO3- is not metabolic alkalosis since it is not a primary process. Primary Event Compensatory Event HCO3- ↑HCO3- ↓ pH ~ --------- ↓ pH ~ --------- ↑PaCO2 ↑PaCO2

16. Primary Acid-base Disorders: Metabolic Acidosis • Metabolic acidosis - A primary acid-base disorder where the first change is a lowering of HCO3-, resulting in decreased pH. Compensation (bringing pH back up toward normal) is a secondary hyperventilation; this lowering of PaCO2 is not respiratory alkalosis since it is not a primary process. Primary Event Compensatory Event ↓ HCO3-↓HCO3- ↓ pH ~ ------------ ↓ pH ~ ------------ PaCO2 ↓PaCO2

17. Primary Acid-base Disorders: Metabolic Alkalosis • Metabolic alkalosis - A primary acid-base disorder where the first change is an elevation of HCO3-, resulting in increased pH. Compensation is a secondary hypoventilation (increased PaCO2), which is not respiratory acidosis since it is not a primary process. Compensation for metabolic alkalosis (attempting to bring pH back down toward normal) is less predictable than for the other three acid-base disorders. Primary Event Compensatory Event ↑HCO3-↑HCO3- ↑ pH ~ ------------ ↑ pH ~ --------- PaCO2 ↑PaCO2

18. Steps to an Arterial Blood Gas Interpretation • Acid-base evaluation requires a focus on three of the reported components: • pH • PaCO2 • HCO3

20. Case 1 ABGs obtained in the ICU pH 7.18 PCO2 20 mmHg HCO3 7 mEq/L

21. Case 2 Arterial blood gas sample is taken, revealing the following: pH : 7.30 PCO2 : 65 mm Hg

22. Case 2 What is the acid-base disorder?

23. Example 1 • Jane Doe is a 45-year-old female admitted to the nursing unit with a severe asthma attack. She has been experiencing increasing shortness of breath since admission three hours ago. Her arterial blood gas result is as follows:

24. Step One • Assess the pH to determine if the blood is within normal range, alkalotic or acidotic. • above 7.45 : alkalotic • below 7.35 : acidotic

25. Example 1 Acidosis

26. Step Two • Determine if it is caused primarily by a : respiratory or metabolic problem. • To do this, assess the PaCO2 level. • Remember that with a respiratory problem: • pH decreases below 7.35, the PaCO2 rise. • pH rises above 7.45, the PaCO2 fall. • If pH and PaCO2 are indeed moving in opposite directions, then the problem is : respiratory

27. Example 1 Acidosis Respiratory

28. Step Three • Assess the HCO3 value. • Remember that with with a metabolic problem: • pH increases, the HCO3 also increase. • pH decreases, the HCO3 also decreases • If pH and HCO3 are moving in the same direction, then the problem is metabolic.

29. Step Four • Look for Compensation When a patient develops an acid-base imbalance, the body attempts to compensate. • If PaCO2 rise → HCO3 will rise too → pH return to normal • If PaCO2 fall → HCO3 will fall too → pH return to normal • If HCO3 rise → PaCO2 will rise too → pH return to normal • If HCO3 fall → PaCO2 will fall too → pH return to normal

30. Uncompensated, partially compensated, or fully compensated • In uncompensated or partially compensated: pH remains outside the normal range. • In fully compensated states : pH has returned to within the normal range. • If pH in normal range but > 7,4 → Alkalosis fully compensated • If pH in normal range but < 7,4 → Acidosis fully compensated

31. Example 1 Acidosis Respiratory uncompensated

32. Step Five • Assess the PaO2. • A value below 80 mm Hg can indicate hypoxemia, depending on the age of the patient. • If PaO2 < 80 mmHg : Hypoxemia • If PaO2 > 100 mmHg : Hyperoxemia

33. Example 1 Acidosis Respiratory uncompensated with hypoxemia

34. PaO2 [oxygen tension]SaO2 [oxygen saturation] a = arterial

35. Pulse Oximeter Measures SaO2

37. Example 2 Jane Doe is a 54-year-old female admitted for an ileus. She had been experiencing nausea and vomiting. An NG tube has been in place for the last 24 hours. Here are the last ABG results: alkalosis fully compensated With hyperoxemia metabolic

38. Example 3 John Doe is a trauma patient with an altered mental status. His initial arterial blood gas result is as follows: acidosis partially compensated respiratory With hypoxemia

39. Example 4 John Doe is admitted to the hospital. He is a kidney dialysis patient who has missed his last two appointments at the dialysis center. His arterial blood gas values are reported as follows: acidosis partially compensated metabolic With hypoxemia

40. Exercise 1 Interpretation? Acidosis respiratory partially compensated with hypoxemia

41. Exercise 2 Interpretation? Alkalosis metabolic uncompensated with hyperoxemia

42. Exercise 3 Interpretation? Normal

43. Exercise 4 Interpretation? Acidosis respiratory uncompensated with hypoxemia

44. Exercise 5 Interpretation? Alkalosis respiratory partially compensated with hyperoxemia

45. Exercise 6 Interpretation? Alkalosis respiratory uncompensated

46. Exercise 7 Interpretation? Acidosis metabolic uncompensated with hypoxemia

47. Exercise 8 Interpretation? Alkalosis respiratory fully compensated

48. Practice ABG’s • PaO2 90 SaO2 95 pH 7.48 PaCO2 32 HCO3 24 • PaO2 60 SaO2 90 pH 7.32 PaCO2 48 HCO3 25 • PaO2 95 SaO2 100 pH 7.30 PaCO2 40 HCO3 18 • PaO2 87 SaO2 94 pH 7.38 PaCO2 48 HCO3 28 • PaO2 94 SaO2 99 pH 7.49 PaCO2 40 HCO3 30 • 6. PaO2 62 SaO2 91 pH 7.35 PaCO2 48 HCO3 27 • PaO2 93 SaO2 97 pH 7.45 PaCO2 47 HCO3 29 • PaO2 95 SaO2 99 pH 7.31 PaCO2 38 HCO3 15 • PaO2 65 SaO2 89 pH 7.30 PaCO2 50 HCO3 24 • 10. PaO2 110 SaO2 100 pH 7.48 PaCO2 40 HCO3 30

49. Answers to Practice ABG’s • uncompensated Respiratory alkalosis • uncompensated Respiratory acidosis , Hypoxemia • uncompensated Metabolic acidosis • Fully Compensated Respiratory acidosis • uncompensated Metabolic alkalosis • Fully Compensated Respiratory acidosis Hypoxemia • Fully Compensated Metabolic alkalosis • uncompensated Metabolic acidosis • uncompensated Respiratory acidosis Hypoxemia • uncompensated Metabolic alkalosis Hyperoxemia

50. Mixed respiratory andmetabolic acid-base disorder