1. Oxygenation www.mecriticalcare.net
Alveolar Gas Equation
Oxygenation

2. The Key to Blood Gas Interpretation: Four Equations, Three Physiologic Processes
Equation Physiologic 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
PaCO mm Hg
PaO mm Hg **
SaO %
HCO3¯ mEq/L
%MetHb < 2.0%
%COHb < 3.0%
Base excess to 2.0 mEq/L
CaO 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 (%)
The pulse oximeter is an extremely useful monitor which estimates arterial saturation
the relationship between saturation and PaO2 is described by the oxyhaemoglobin dissociation curve
a saturation ~90% is a critical threshold because below this level a small fall in PaO2 produces a sharp fall in SpO2 .Conversely a rise in arterial PO2 has little effect on saturation and therefore little effect on oxygen delivery to tissues 60 90
600
PaO2 (mm Hg)

7. Oxygen Saturation Monitoring by Pulse Oximetry

9. Patient Environments Ambient Light Excessive Motion
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

10. 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

11. Nasal Cannula: Variable Flow

12. Simple Face Mask: Variable Flow

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

14. Venturi Effect
The pressure at "1" is higher than at "2" because the fluid speed at "1" is lower than at "2".

15. Venturi Effect 35-45 L/min 4-15 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.

16. Variable Performance Device: Nonrebreather Mask

17. 100 90
5 L.min-1 80
10 L.min-1 70
20 L.min-1 60
30 L.min-1
Fractional inspired oxygen concentration % 50 40 30 20 10 5 15 25 35 45 55 65 75 85
Peak inspiratory flow (liters/minute)

18. Continuous Airway Pressure: CPAP

19. Alveolus
Airways
Interstitium
Pleural cavity
Pressure
Inspiration
Expiration
Intrinsic PEEP
Applied CPAP

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

21. PAO2 = FIO2 (PB – 47 mm Hg) - 1.2 (PaCO2)
Alveolar Gas Equation
PAO2 = PIO (PaCO2)*
PAO2 = FIO2 (PB – 47 mm Hg) (PaCO2)
A-a Gradient= PAO2- PaO2 =5-25 mm Hg
PAO2 is the average alveolar PO2
PIO2 is the partial pressure of inspired oxygen in the trachea
FIO2 is 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%.

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

23. Hypoxemia Due to Decreased FiO2
↓PAO2 = ↓FIO2 (PB – 47 mm Hg) (PaCO2)
Suffocation

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

26. 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) (50) = 90 mm Hg, PaO2 = 80
FIO2 = .40, PaCO2 = 30 mm Hg
PAO2 = .40 (713) (30) = 249 mm Hg, PaO2 = 239

27. 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 ( ) (40) = - 5 mm Hg
PAO2 = 1.00 ( ) - 40 = 166 mm Hg
PAO2 = .21 ( ) (10) = 31 mm Hg

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

29. Alveolar Arterial O2 Gradient5
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

30. 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

31. Alveolar-arterial Oxygen Gradient

32. Physiologic Causes of Low PaO2
NON-RESPIRATORY
P(A-a)O2
Cardiac right-to-left shunt
Increased
Decreased PIO2
Normal
Low mixed venous oxygen content*
* Unlikely to be clinically significant unless there is right-to-left shunting or ventilation-perfusion imbalance

33. Physiologic Causes of Low PaO2
RESPIRATORY
P(A-a)O2
Pulmonary right-to-left shunt
Increased
Ventilation-perfusion imbalance
Diffusion barrier
Hypoventilation (increased PaCO2)
Normal

35. Ventilation-perfusion Imbalance
A normal amount of ventilation-perfusion (V-Q) imbalance accounts for the normal P(A-a)O2.
By far the most common cause of low PaO2 is an abnormal degree of ventilation-perfusion imbalance within the hundreds of millions of alveolar-capillary units. Virtually all lung disease lowers PaO2 via V-Q imbalance, e.g., asthma, pneumonia, atelectasis, pulmonary edema, COPD.
Diffusion barrier is seldom a major cause of low PaO2 (it can lead to a low PaO2 during exercise).

36. A 44-year-old woman with: PaCO2 75 mm Hg, PaO2 95 mm Hg, FIO2 0.28
PAO2 = FIO2 (PB – 47 mm Hg) (PaCO2)
PAO2 = .28 (713) (75)
PAO2= =110 mm Hg
P(A-a)O2 = = 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

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

38. A woman in the ICU with: PaO2 350 mm Hg, PaCO2 40 mm Hg, FIO2 0.80
PAO2 = FIO2 (PB – 47 mm Hg) (PaCO2)
PAO2 = .80 (713) - (40)
PAO2= = 530 mm Hg
P(A-a)O2 = = 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.

39. A man with: PaO2 80 mm Hg, PaCO2 72 mm Hg, FIO2 0.21
PAO2 = FIO2 (PB – 47 mm Hg) (PaCO2)
PAO2 = .21 (713) – 1.2(72)
PAO2= = 64 mm Hg
P(A-a)O2 = = -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.

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