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Alveolar Gas Equation

Alveolar Gas Equation. Oxygenation www.mecriticalcare.net. The Key to Blood Gas Interpretation: Four Equations, Three Physiologic Processes. Equation Physiologic Process 1) PaCO2 equation Alveolar ventilation 2) Alveolar gas equation Oxygenation

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Alveolar Gas Equation

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  1. Alveolar Gas Equation Oxygenation www.mecriticalcare.net

  2. The Key to Blood Gas Interpretation:Four Equations, Three Physiologic Processes EquationPhysiologic Process 1) PaCO2 equation Alveolar ventilation 2) Alveolar gas equation Oxygenation 3) Oxygen content equation Oxygenation 4) Henderson-Hasselbalch equation Acid-base balance These four equations, crucial to understanding and interpreting arterial blood gas data.

  3. Normal Arterial Blood Gas Values* pH 7.35 - 7.45 PaCO2 35 - 45 mm Hg PaO2 70 - 100 mm Hg ** SaO2 93 - 98% HCO3¯ 22 - 26 mEq/L %MetHb < 2.0% %COHb < 3.0% Base excess -2.0 to 2.0 mEq/L CaO2 16 - 22 ml O2/dl • * At sea level, breathing ambient air • ** Age-dependent

  4. Oxygenation and Ventilation

  5. Oxygen Saturation Monitoring by Pulse Oximetry

  6. O2-Hg Dissociation Curve 100% 90% Hb Saturation (%) 60 90 600 PaO2 (mm Hg)

  7. Oxygen Saturation Monitoring by Pulse Oximetry

  8. Patient Environments • Ambient Light • Any external light exposure to capillary bed where sampling is occurring may result in an erroneous reading • Excessive Motion • Always compare the palpable pulse rate with the pulse rate indicated on the pulse oximetry • Fingernail polish and false nails • Most commonly use nails and fingernail polish will not affect pulse oximetry accuracy • Some shades of blue, black and green may affect accuracy (remove with acetone pad) • Skin pigmentation • Apply sensor to the fingertips of darkly pigmented patients

  9. Conditions Affecting Accuracy • Patient conditions • Carboxyhemoglobin • Erroneously high reading may present • Methaemoglobin • Anemia • Values as low as 5 g/dl may result in 100% SpO2 • Hypovolemia/Hypotension: • May not have adequate perfusion to be detected by oximetry • Hypothermia: • peripheral vasoconstriction may prevent oximetry detection

  10. Nasal Cannula: Variable Flow

  11. Simple Face Mask: Variable Flow

  12. Venturi Mask: Fixed Flow blue = 24%; yellow = 28%; white = 31%; green = 35%; pink = 40%; orange = 50%

  13. Venturi Effect The pressure at "1" is higher than at "2" because the fluidspeed at "1" is lower than at "2".

  14. Venturi Effect 4-15 L/min 35-45 L/min A flow of air through a venturi meter, showing the columns connected in a U-shape (a manometer) and partially filled with water. The meter is "read" as a differential pressure head in cm or inches of water.

  15. Variable Performance Device: Nonrebreather Mask

  16. 100 90 5 L.min-1 80 10 L.min-1 70 20 L.min-1 60 30 L.min-1 50 Fractional inspired oxygen concentration % 40 30 20 10 0 5 55 25 45 65 85 75 15 35 Peak inspiratory flow (liters/minute)

  17. Continuous Airway Pressure: CPAP

  18. Alveolar-arterial Oxygen Gradient PAO2= (Patm-PH2O) FiO2- PACO2/0.8 760 47 0.21 40

  19. Alveolar Gas Equation PAO2 = PIO2 - 1.2 (PaCO2)* PAO2 = FIO2 (PB – 47 mm Hg) - 1.2 (PaCO2) A-a Gradient= PAO2-PaO2 =5-25 mm Hg PAO2 is the average alveolar PO2 PIO2is the partial pressure of inspired oxygen in the trachea FIO2is fraction of inspired oxygen PB is the barometric pressure. 47 mm Hg is the water vapor pressure at normal body temperature * Note: This is the “abbreviated version” of the AG equation, suitable for most clinical purposes. In the longer version, the multiplication factor “1.2” declines with increasing FIO2, reaching zero when 100% oxygen is inhaled. In these exercises “1.2” is dropped when FIO2 is above 60%.

  20. Hypoxemia Due to Hypercapnia ↓PAO2 = FIO2 (PB – 47 mm Hg) - 1.2 (↑PaCO2) • Hypercapneic Respiratory Failure

  21. Hypoxemia Due to Decreased FiO2 ↓PAO2 = ↓FIO2(PB – 47 mm Hg) - 1.2 (PaCO2) • Suffocation

  22. High Altitude Hypoxemia ↓PAO2 = FIO2 (↓PB– 47 mm Hg) - 1.2 (PaCO2) Mountain climbing

  23. Alveolar Gas Equation: Test Your Understanding What is the expected PaO2 in a normal lung patient at sea level in the following circumstances? (Barometric pressure = 760 mm Hg) • FIO2 = 1.00, PaCO2 = 30 mm Hg PAO2 = 1.00 (713) - 30 = 683 mm Hg, PaO2= 673 • FIO2 = .21, PaCO2 = 50 mm Hg PAO2 = .21 (713) - 1.2 (50) = 90 mm Hg, PaO2 = 80 • FIO2 = .40, PaCO2 = 30 mm Hg PAO2 = .40 (713) - 1.2 (30) = 249 mm Hg, PaO2 = 239

  24. Alveolar Gas Equation: Test Your Understanding What is the PAO2 on the summit of Mt. Everest in the following circumstances? (Barometric pressure = 253 mm Hg) • FIO2 = .21, PaCO2 = 40 mm Hg • FIO2 = 1.00, PaCO2 = 40 mm Hg • FIO2 = .21, PaCO2 = 10 mm Hg PAO2 = .21 (253 - 47) - 1.2 (40) = - 5 mm Hg PAO2 = 1.00 (253 - 47) - 40 = 166 mm Hg PAO2 = .21 (253 - 47) - 1.2 (10) = 31 mm Hg

  25. Alveolar Arterial O2 Gradient A-a Gradient Po2 Po2 initial Initial Epithelium Endothelium Alveolar Gas Capillary Blood Thickness

  26. Alveolar Arterial O2 Gradient 5 FIO2= 21% PAO2= 100 PaO2= 95 O2 Sat= 99% FIO2= 50% PAO2= 331 PaO2= 326 O2 Sat= 100% FIO2= 100% PAO2= 663 PaO2= 657 O2 Sat= 100% Epithelium Endothelium Alveolar Gas Capillary Blood Thickness

  27. Alveolar Arterial O2 Gradient 200 FIO2= 50% PAO2= 331 PaO2= 131 O2 Sat= 100% FIO2= 100% PAO2= 663 PaO2= 463 O2 Sat= 100% Epithelium Endothelium Alveolar Gas Capillary Blood Thickness

  28. Alveolar-arterial Oxygen Gradient

  29. Physiologic Causes of Low PaO2 • * Unlikely to be clinically significant unless there is right-to-left shunting or ventilation-perfusion imbalance

  30. Physiologic Causes of Low PaO2

  31. A 44-year-old woman with: PaCO2 75 mm Hg, PaO2 95 mm Hg, FIO2 0.28 PAO2 = FIO2 (PB – 47 mm Hg) - 1.2 (PaCO2) PAO2 = .28 (713) - 1.2 (75) PAO2= 200 - 90 =110 mm Hg P(A-a)O2 = 110 - 95 = 15 mm Hg Despite severe hypoventilation, there is no evidence here for lung disease. Hypercapnia is most likely a result of disease elsewhere in the respiratory system, either the central nervous system or chest bellows

  32. A young, anxious man with: PaO2 120 mm Hg, PaCO2 15 mm Hg, FIO2 0.21 PAO2 = FIO2 (PB – 47 mm Hg) - 1.2 (PaCO2) PAO2 = .21 (713) - 1.2 (15) PAO2= 150 - 18 =132 mm Hg P(A-a)O2 = 132 - 120 = 12 mm Hg Hyperventilation can easily raise PaO2 above 100 mm Hg when the lungs are normal, as in this case

  33. A woman in the ICU with: PaO2 350 mm Hg, PaCO2 40 mm Hg, FIO2 0.80 PAO2 = FIO2 (PB – 47 mm Hg) - 1.2 (PaCO2) PAO2 = .80 (713) - (40) PAO2= 570 - 40 = 530 mm Hg P(A-a)O2 = 530 - 350 = 180 mm Hg Note that the factor 1.2 is dropped since FIO2 is above 60% P(A-a)O2 is increased. Despite a very high PaO2, the lungs are not transferring oxygen normally.

  34. A man with: PaO2 80 mm Hg, PaCO2 72 mm Hg, FIO2 0.21 PAO2 = FIO2 (PB – 47 mm Hg) - 1.2 (PaCO2) PAO2 = .21 (713) – 1.2(72) PAO2= 150 - 86 = 64 mm Hg P(A-a)O2 = 64 - 72 = -16 mm Hg A negative P(A-a)O2 is incompatible with life (unless it is a transient unsteady state, such as sudden fall in FIO2 -- not the case here). In this example, negative P(A-a)O2 can be explained by any of the following: incorrect FIO2, incorrect blood gas measurement, or a reporting or transcription error.

  35. Thank You

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