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Chapter 5

Chapter 5.  Oxygenation Assessments. Oxygen Transport Review. Oxygen is carried in the blood in two ways: As dissolved oxygen in the blood plasma Bound to the hemoglobin (Hb) Most oxygen is carried to the tissue cell bound to the hemoglobin. Oxygen Dissolved in the Blood Plasma.

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Chapter 5

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  1. Chapter 5  Oxygenation Assessments

  2. Oxygen Transport Review Oxygen is carried in the blood in two ways: • As dissolved oxygen in the blood plasma • Bound to the hemoglobin (Hb) • Most oxygen is carried to the tissue cell bound to the hemoglobin.

  3. Oxygen Dissolved in the Blood Plasma • At normal body temperature, about 0.003 mLof oxygen (O2) will dissolve in each 100 mL of blood for every 1 mm Hg of PO2. • Thus when the PaO2 is 100 mm Hg, about0.3 mL of dissolved O2 exists in every 100 mLof plasma: 0.003 × 100 mm Hg = 0.3 mL • Clinically, written as 0.3 volume percent (vol%) • 0.3 vol% O2

  4. Oxygen Bound to Hemoglobin • Hb value in normal man: 14 to 16 g/100 mL • Hb value in normal woman: 12 to 15 g/100 mL • Clinically, the weight measurement of hemoglobin, in reference to 100 mL of blood, is known as the grams percent hemoglobin (g% Hb)

  5. Oxygen Bound to Hemoglobin (Cont’d) • Each g% Hb can carry 1.34 mL of O2 • Thus if the Hb level is 12 g% and if the Hb is fully saturated, about 15.72 vol% of O2 will be bound to the Hb: O2 bound to Hb = 1.34 mL O2 × 12 g% Hb = 15.72 vol% O2

  6. Oxygen Bound to Hemoglobin (Cont’d) • At a normal PaO2 of 100 mm Hg, however, the Hb saturation (SaO2) is only about 97% because of these normal physiologic shunts: • Thebesian venous drainage into the left atrium • Bronchial venous drainage into the pulmonary veins

  7. Oxygen Bound to Hemoglobin (Cont’d) Thus the amount of arterial oxygen in the calculation must be adjusted to 97%: 15.72 vol% O2 × .97 15.24 vol% O2

  8. Total Oxygen Content • To calculate the total amount of oxygen in 100 mL of blood, the following must be added together: • Dissolved oxygen • Oxygen bound to hemoglobin

  9. Case Example A 44-year-old woman with a long history of asthma arrives in the emergency room in severe respiratory distress. Her vital signsare respiratory rate 36 breaths/min, heart rate 130 bpm, and blood pressure 160/95 mm Hg. Her hemoglobin concentration is 10 g%, and her PaO2 is 55 mm Hg (SaO2 85%). Based on these data, the patient’s total oxygen content is determined on the next slide:

  10. 1. Dissolved O2 55 PaO2 × 0.003 (dissolved O2 factor) 0.165 vol% O2

  11. 2. Oxygen Bound to Hemoglobin 10 g% Hb × 1.34 (O2 bound to Hb factor) 13.4 vol% O2 (at SaO2 of 100%) Above answer is then followed by the SaO2 factor: 13.4 vol% O2 × .85 SaO2 11.39 vol% O2(at SaO2 of 85%)

  12. 3. Total Oxygen Content. 11.39 vol% O2(bound to hemoglobin) + 0.165 vol% O2(dissolved O2) 11.555 vol% O2(total amount of O2/100 mL of blood)

  13. Total Oxygen Content (Cont’d) The total oxygen content can be calculated in the patient’s: • Arterial blood (CaO2) • Venous blood (CvO2) • Pulmonary capillary blood (CcO2)

  14. Total Oxygen Content (Cont’d) CaO2 = Oxygen content of arterial blood (Hb × 1.34 × SaO2) + (PaO2 × 0.003) CvO2 = Oxygen content of mixed venous blood (Hb × 1.34 × SvO2) + (PvO2 × 0.003) CcO2 = Oxygen content of pulmonary capillary blood (Hb × 1.34) + (PAO2 × 0.003)

  15. Oxygenation Indices • Oxygen tension–based indices • Oxygen saturation and content indices

  16. Oxygen Tension–Based Indices • Arterial oxygen tension (PaO2) • Alveolar-arterial oxygen tension difference (P[A-a]O2)

  17. Arterial Oxygen Tension (PaO2) • Good indicator of the patient’s oxygenation status • The PaO2, however, may be misleading in these clinical situations: • Low Hb • Decreased cardiac output • Peripheral shunting • Carbon monoxide and cyanide exposure

  18. Alveolar-Arterial Oxygen Tension Difference (P[A-a]O2) • The P(A-a)O2 is the oxygen tension difference between the alveoli and arterial blood.

  19. Alveolar-Arterial Oxygen Tension Difference (P[A-a]O2) (Cont’d) PAO2 = FIO2 (PB − PH2O) − PaCO2 (1.25)

  20. Case Study Example If a patient is receiving an FIO2 of 0.30 on a day when the barometric pressure is 750 mm Hg, and if the patient’s PaCO2 is 70 mm Hg and PaO2 is 60 mm Hg, the P(A-a)O2 can be calculated as shown on the next slide:

  21. Case Study Example (Cont’d) PAO2 = FIO2 (PB − PH2O) − PaCO2 (1.25) = 0.30 (750 − 47) − 70 (1.25) = (703) 0.30 − 87.5 = (210.9) − 87.5 = 123.4 mm Hg

  22. Case Study Example (Cont’d) Using the PaO2 obtained from the ABG: 123.4 (PAO2) − 60.0 (PaO2) 63.4 mm Hg [P(A-a)O2] The normal P(A-a)O2 ranges from 7 to 15 mm Hg and should not exceed 30 mm Hg.

  23. P(A-a)O2 Increases • Oxygen diffusion disorders • Decreased V/Q ratios • Right-to-left cardiac shunting • Age

  24. Note: The P(A-a)O2 Loses sensitivity in patients breathing high FIO2

  25. Oxygen Saturation– and Content–Based Indices

  26. Oxygen Saturation– and Content–Based Indices • CaO2 = (Hb × 1.34 × SaO2) + (PaO2 × 0.003) • CvO2 = (Hb × 1.34 × SvO2) + (PvO2 × 0.003) • CcO2 = (Hb × 1.34) + (PAO2 × 0.003)

  27. Most Common Oxygen Saturation–and Content–Based Indices • Total oxygen delivery • Arterial-venous oxygen content difference • Oxygen consumption • Oxygen extraction ratio • Mixed venous oxygen saturation • Pulmonary shunting

  28. Total Oxygen DeliveryDO2 = QT × (CaO2 × 10) The total oxygen delivery is the amount of oxygen delivered to the peripheral tissue cells.

  29. Total Oxygen Delivery DO2 = QT × (CaO2 × 10) (Cont’d) For example, if a patient has a cardiac output of 4 L/min and a CaO2 of 15 vol%, the DO2 is 600 mL of oxygen per minute—as calculated on the next slide:

  30. Total Oxygen Delivery DO2 = QT × (CaO2 × 10) = 4 L/min × (15 vol% × 10) = 600 mL O2 per minute Normally, about 1000 mL/min

  31. Total Oxygen Delivery (Cont’d) Decreases: • Low PaO2 • Low SaO2 • Low Hb • Low cardiac output

  32. Total Oxygen Delivery (Cont’d) Increases: • Increased PaO2 • Increased SaO2 • Increased Hb • Increased cardiac output

  33. Arterial-Venous Oxygen Content DifferenceC(a-v)O2 = CaO2 − CvO2 The arterial-venous oxygen content difference (C[a-v]O2) is the difference between the CaO2 and the CvO2—that is, CaO2 − CvO2.

  34. Arterial-Venous Oxygen Content DifferenceC(a-v)O2 = CaO2 − CvO2 (Cont’d) For example, if a patient’s CaO2 is 15 vol% and the CvO2 is 8 vol%, the C(a-v)O2 is 7 vol%—as calculated on the next slide:

  35. Arterial-Venous Oxygen Content Difference C(a-v)O2 = CaO2 − CvO2 = 15 vol% − 8 vol% = 7 vol% Normally, 5 vol%

  36. Arterial-Venous Oxygen Content Difference (Cont’d) Increases: • Decreased cardiac output • Exercise • Seizures • Hyperthermia

  37. Arterial-Venous Oxygen Content Difference (Cont’d) Decreases: • Increased cardiac output • Skeletal relaxation • Peripheral shunting • Cyanide • Hypothermia

  38. Oxygen ConsumptionVO2 = QT [C(a-v)O2] × 10 Oxygen (VO2) consumption is the amount of oxygen consumed by the peripheral tissue cells during a 1-minute period.

  39. Oxygen ConsumptionVO2 = QT [C(a-v)O2] × 10 (Cont’d) For example, if a patient has a cardiac output of 4 L/min and a C(a-v)O2 of 6 vol%, the total amount of oxygen consumed by the tissue cells in 1 minute would be 240 mL—as calculated on the next slide:

  40. Oxygen Consumption VO2 = QT [C(a-v)O2] × 10 = 4 L/min × 6 vol% × 10 = 240 mL O2/min Normal is 250 mL O2/min

  41. Oxygen Consumption (Cont’d) Increases: • Seizures • Exercise • Hyperthermia • Body size

  42. Oxygen Consumption (Cont’d) Decreases: • Skeletal muscle relaxation • Peripheral shunting • Certain poisons (e.g., cyanide) • Hypothermia

  43. Oxygen Extraction RatioO2ER = CaO2 − CvO2 CaO2 The O2ER is the amount of oxygen consumed by the tissue cells divided by the total amount of oxygen delivered.

  44. Oxygen Extraction RatioO2ER = CaO2 − CvO2 CaO2 (Cont’d) For example, if a patient’s CaO2 is 15 vol% and the CvO2 is 10 vol%, the O2ER would be 33%—as calculated on the next slide:

  45. Oxygen Extraction Ratio O2ER = CaO2 − CvO2 CaO2 = 15 vol% − 10 vol% 15 vol% = 5 vol% 15 vol% = 0.33 Normal is 0.25%

  46. Oxygen Extraction Ratio (Cont’d) Increases: • Decreased cardiac output • Periods of increased O2 consumption • Exercise, seizures, hyperthermia • Anemia • Decreased arterial oxygenation • ↓ Hb, ↓ PaO2

  47. Oxygen Extraction Ratio (Cont’d) Decreases: • Increased cardiac output • Skeletal muscle relaxation • Peripheral shunting • Certain poisons (e.g., cyanide) • Hypothermia • Increased arterial oxygenation • ↑ Hb, ↑ PaO2

  48. Mixed Venous Oxygen Saturation SvO2 Signals changes in the: • C(a-v)O2 • VO2 • O2ER Normally about 75%

  49. Mixed Venous Oxygen Saturation SvO2 (Cont’d) Decreases: • Decreased cardiac output • Exercise • Seizures • Hyperthermia

  50. Mixed Venous Oxygen Saturation SvO2 (Cont’d) Increases: • Increased cardiac output • Skeletal muscle relaxation • Peripheral shunting • Certain poisons (e.g., cyanide) • Hypothermia

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