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Blood Gases and Related Tests

Blood Gases and Related Tests. RET 2414 Pulmonary Function Testing Module 6.0. Objectives. Describe how pH and PCO2 are used to assess acid-base balance Interpret PO2 and oxygen saturation to assess oxygenation Identify the correct procedure for obtaining an arterial blood gas specimen

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Blood Gases and Related Tests

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  1. Blood Gases and Related Tests RET 2414 Pulmonary Function Testing Module 6.0

  2. Objectives • Describe how pH and PCO2 are used to assess acid-base balance • Interpret PO2 and oxygen saturation to assess oxygenation • Identify the correct procedure for obtaining an arterial blood gas specimen • List situation in which pulse oximetry can be used to evaluate a patient’s oxygenation

  3. Objectives • Describe the use of capnography to assess changes in ventilatory-perfusion patterns of the lung • Describe at least two limitation of pulse oximetry

  4. Reasons for Obtaining an ABG • Assessment of ventilatory status • Assessment of acid-base balance • Assessment of arterial oxygenation

  5. Acid – Base Balance The maintenance of cellular function depends on an exacting environment. One of the most important environmental factors is the hydrogen ion concentration (H+), commonly expressed as pH.

  6. Acid – Base Balance The pH is a function of the relation of HCO3- (base) to PCO2 (acid) in the blood in the following fashion: pH ~ HCO3- (base) ~ metabolic component ~ kidney ~ 20 CO2 (acid) respiratory component lungs 1 7.40 Normal pH

  7. Acid – Base Balance • Acidemia;an acidic condition of the blood pH < 7.35 • Alkalemia; an alkaline condition of the blood pH >7.45

  8. Acid – Base Balance • Respiratory Component (PCO2) Tissues Plasma Red Blood Cell CO2 CO2 dCO2 H2CO3

  9. Acid – Base Balance • Excretion of CO2 is one of the lungs main functions PA CO2 40 mmHg PaCO2 PvCO2 46 mmHg 40 mmHg Pc CO2

  10. Acid – Base Balance • Respiratory Component (PCO2) Ventilation PCO2 pH “Respiratory Acidosis”

  11. Acid – Base Balance • Respiratory Component (PCO2) Ventilation PCO2 pH “Respiratory Alkalosis”

  12. Acid – Base Balance • Metabolic Component (HCO3- and BE) • Bicarbonate (HCO3-) is the primary blood base and is regulated by the kidneys and not the lungs. Normal HCO3- 24 mEq/L

  13. Acid – Base Balance • Metabolic Component (HCO3- and BE) • Base Excess (BE) is a measure of metabolic alkalosis or metabolic acidosis expressed as the mEq of strong acid or strong alkali required to titrate one liter of blood to a pH of 7.40 Normal B.E. -2 to +2 mEq/L

  14. Acid – Base Balance • Metabolic Component (HCO3- and BE) HCO3- or B.E. “Metabolic Acidosis”

  15. Acid – Base Balance • Metabolic Component (HCO3- and BE) HCO3- or B.E. “Metabolic Alkalosis”

  16. Acid – Base Balance • Combined Respiratory / Metabolic Respiratory and metabolic component moving toward the same acid/base status PCO2 HCO3- = Acidosis PCO2 HCO3- = Alkalosis

  17. Acid – Base Balance • Compensation Abnormal pH is returned toward normal by altering the component NOT primarily affected, i.e., if PCO2 is high, HCO3- is retained to compensate PCO2 HCO3- HCO3- PCO2

  18. Acid – Base Balance Normal metabolism produces approximately 12,000 mEq of hydrogen ions per day. Less than 1% is excreted by the kidneys, because the normal metabolite is CO2; which is excreted by the lungs.

  19. Acid – Base Balance Acid-Base imbalance is not life-threatening for several hours to days following renal shutdown but becomes critical within minutes following cessation of breathing. WOW!

  20. Normal Values pH PCO2 (mmHg) HCO3- (mEq/L) 7.40 40 24

  21. Acceptable Ranges (2 SD) pH PCO2 HCO3- Normal 7.35 – 7.45 35 – 45 22 - 26 Acidotic <7.35 >45 <22 Alkalotic >7.45 <35 >26

  22. Arterial Oxygenation • Tissue hypoxemia exists when cellular oxygen tensions are inadequate to meet cellular oxygen demands. • PaO2 has become the primary tool for clinical evaluation of the arterial oxygenation status.

  23. Arterial Oxygenation • Hypoxemia; an arterial oxygen tension (PaO2) below an acceptable range. Arterial Oxygen Tensions for Adult and Child Normal 97 mm Hg Acceptable Range ≥80 mm Hg (range decreases with age) Hypoxemia <80 mm Hg

  24. Systematic Interpretation • Assessment of ventilatory status • Assessment of acid-base balance • Assessment of arterial oxygenation

  25. Exercise 1 Acceptable Range ABG Result 7.35 - 7.45 pH 7.26 35 - 45 PCO2 56 22 - 26 HCO3- 24 -2 - +2 BE -4 >80 PO2 50 Acute ventilatory failure with hypoxemia (Acute respiratory acidosis with hypoxemia)

  26. Exercise 2 Acceptable Range ABG Result 7.35 - 7.45 pH 7.56 35 - 45 PCO2 29 22 - 26 HCO3- 24 -2 - +2 BE +3 >80 PO2 90 Acute alveolar hyperventilation without hypoxemia (Acute respiratory alkalosis without hypoxemia)

  27. Exercise 3 Acceptable Range ABG Result 7.35 - 7.45 pH 7.56 35 - 45 PCO2 44 22 - 26 HCO3- 38 -2 - +2 BE +14 >80 PO2 75 Uncompensated metabolic alkalosis with hypoxemia

  28. Exercise 4 Acceptable Range ABG Result 7.35 - 7.45 pH 7.20 35 - 45 PCO2 38 22 - 26 HCO3- 15 -2 - +2 BE -13 >80 PO2 90 Uncompensated metabolic acidosis without hypoxemia

  29. Exercise 5 Acceptable Range ABG Result 7.35 - 7.45 pH 7.45 35 - 45 PCO2 20 22 - 26 HCO3- 16 -2 - +2 BE -7 >80 PO2 90 Chronic alveolar hyperventilation without hypoxemia (Compensated respiratory alkalosis without hypoxemia)

  30. Exercise 6 Acceptable Range ABG Result 7.35 - 7.45 pH 7.42 35 - 45 PCO2 72 22 - 26 HCO3- 46 -2 - +2 BE +18 >80 PO2 45 Chronic ventilatory failure with hypoxemia (Compensated respiratory acidosis with hypoxemia)

  31. Exercise 6 A 76-year-old man with a long history of symptomatic COPD entered the hospital with basilar pneumonia. He was alert, oriented, and cooperative. Acceptable Range ABG Result 7.35 - 7.45 pH 7.58 35 - 45 PCO2 45 22 - 26 HCO3- 42 -2 - +2 BE +17 >80 PO2 38

  32. Exercise 6 Acceptable Range ABG Result 7.35 - 7.45 pH 7.58 35 - 45 PCO2 45 22 - 26 HCO3- 42 -2 - +2 BE +17 >80 PO2 38 Question: Is this uncompensated metabolic alkalosis with severe hypoxemia?

  33. Exercise 6 Acceptable Range ABG Result 7.35 - 7.45 pH 7.58 35 - 45 PCO2 45 22 - 26 HCO3- 42 -2 - +2 BE +17 >80 PO2 38 Uncompensated metabolic alkalosis with severe hypoxemia?

  34. Exercise 6 Acceptable Range ABG Result 7.35 - 7.45 pH 7.58 35 - 45 PCO2 45 22 - 26 HCO3- 42 -2 - +2 BE +17 >80 PO2 38 A metabolic alkalosis with hypoxemia must be clinically correlated because a disease process causing metabolic alkalosis would not be expected to produce severe hypoxemia.

  35. Exercise 6 Acceptable Range ABG Result 7.35 - 7.45 pH 7.58 35 - 45 PCO2 45 22 - 26 HCO3- 42 -2 - +2 BE +17 >80 PO2 38 Correct Interpretation: Acute alveolar hyperventilation (respiratory alkalosis) superimposed on chronic hypercapnia (chronic ventilatory failure) with severe hypoxemia.

  36. Exercise 7 A 67-year-old man admitted to the Emergency Department with exacerbated COPD. He was alert, oriented, and cooperative. Acceptable Range ABG Result 7.35 - 7.45 pH 7.25 35 - 45 PCO2 90 22 - 26 HCO3- 38 -2 - +2 BE +12 >80 PO2 34

  37. Exercise 7 Acceptable Range ABG Result 7.35 - 7.45 pH 7.25 35 - 45 PCO2 90 22 - 26 HCO3- 38 -2 - +2 BE +12 >80 PO2 34 Acute ventilatory failure superimposed on chronic hypercapnia (chronic ventilatory failure) with severe hypoxemia.

  38. Pulse Oximetry • Pulse oximetry (SpO2) is the noninvasive estimation of SaO2

  39. Pulse Oximetry • SaO2

  40. Pulse Oximetry • SaO2

  41. Pulse Oximetry • SaO2

  42. Pulse Oximetry • SaO2

  43. Pulse Oximetry • Pulse oximetry may be used in any setting in which a noninvasive measure of oxygenation status is sufficient. • O2 therapy • Surgery • Ventilator management • Diagnostic procedures • Bronchoscopy • Sleep studies • Stress testing • Pulmonary Rehabilitation

  44. Pulse Oximetry • Pulse oximetry uses light to work out oxygen saturation. Light is emitted from light sources which goes across the pulse oximeter probe and reaches the light detector.

  45. Pulse Oximetry • If a finger is placed in between the light source and the light detector, the light will now have to pass through the finger to reach the detector. Part of the light will be absorbed by the finger and the part not absorbed reaches the light detector.

  46. Pulse Oximetry • Hemoglobin (Hb) absorbs light. The amount of light absorbed is proportional to the concentration of Hb in the blood vessel. By measuring how much light reaches the light detector, the pulse oximeter knows how much light has been absorbed. The more Hb in the finger , the more light is absorbed.

  47. Pulse Oximetry

  48. Pulse Oximetry • The pulse oximeter uses two lights to analyze hemoglobin, red and infared, to detect the amount of oxyhemoglobin (O2Hb) and deoxyhemoglobin (rHb)

  49. Pulse Oximetry • The pulse oximeter works out the oxygen saturation by comparing how much red light and infra red light is absorbed by the blood. Depending on the amounts of oxy Hb and deoxy Hb present, the ratio of the amount of red light absorbed compared to the amount of infrared light absorbed changes.

  50. Pulse Oximetry

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