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Understanding Oxygen Transport in the Blood for Clinical Assessments

This chapter reviews how oxygen is carried in the blood, both dissolved in plasma and bound to hemoglobin. Learn about oxygen content calculations, case examples, oxygenation indices, and clinical implications for assessing oxygenation status.

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Understanding Oxygen Transport in the Blood for Clinical Assessments

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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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