Alveolar Gas Equation

Oxygenationwww.mecriticalcare.net

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The Key to Blood Gas Interpretation:Four Equations, Three Physiologic Processes

Equation Physiologic Process1) PaCO2 equation Alveolar ventilation2) Alveolar gas equation Oxygenation3) Oxygen content equation Oxygenation4) Henderson-Hasselbalch equation Acid-base balance

These four equations, crucial to understanding and interpreting arterial blood gas data.

Normal Arterial Blood Gas Values*

pH 7.35 - 7.45PaCO2 35 - 45 mm HgPaO2 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/LCaO2 16 - 22 ml O2/dl

• * At sea level, breathing ambient air• ** Age-dependent

Oxygenation and Ventilation

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Oxygen Saturation Monitoring by Pulse Oximetry

O2-Hg Dissociation CurveH

b S

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n)%

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PaO2 (mm Hg)

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Oxygen Saturation Monitoring by Pulse Oximetry

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

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

Nasal Cannula: Variable Flow

Simple Face Mask: Variable Flow

Venturi Mask: Fixed Flow

blue = 24%; yellow = 28%; white = 31%; green = 35%; pink = 40%; orange = 50%

Venturi Effect

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

Venturi Effect

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.

4-15 L/min35-45 L/min

Variable Performance Device: Nonrebreather Mask

100

90

80

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60

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40

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10

05 15 25 35 45 55 65 75 85

5 L.min-1

10 L.min-1

20 L.min-1

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Peak inspiratory flow (liters/minute)

Continuous Airway Pressure: CPAP

Alveolar-arterial Oxygen Gradient

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PAO2= (Patm-PH2O) FiO2- PACO2/0.8 760 47 0.21 40

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

PIO2 is the partial pressure of inspired oxygen in the tracheaFIO2 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%.

Hypoxemia Due to Hypercapnia

↓PAO2 = FIO2 (PB – 47 mm Hg) - 1.2 (↑PaCO2)

Hypercapneic Respiratory Failure

Hypoxemia Due to Decreased FiO2

↓PAO2 = ↓FIO2 (PB – 47 mm Hg) - 1.2 (PaCO2)

Suffocation

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High Altitude Hypoxemia

↓PAO2 = FIO2 (↓PB – 47 mm Hg) - 1.2 (PaCO2)

Mountain climbing

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)

A. FIO2 = 1.00, PaCO2 = 30 mm HgPAO2 = 1.00 (713) - 30 = 683 mm Hg, PaO2= 673

B. FIO2 = .21, PaCO2 = 50 mm HgPAO2 = .21 (713) - 1.2 (50) = 90 mm Hg, PaO2 = 80

C. FIO2 = .40, PaCO2 = 30 mm HgPAO2 = .40 (713) - 1.2 (30) = 249 mm Hg, PaO2 = 239

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)

A. FIO2 = .21, PaCO2 = 40 mm HgB. FIO2 = 1.00, PaCO2 = 40 mm HgC. FIO2 = .21, PaCO2 = 10 mm Hg

A. PAO2 = .21 (253 - 47) - 1.2 (40) = - 5 mm HgB. PAO2 = 1.00 (253 - 47) - 40 = 166 mm HgC. PAO2 = .21 (253 - 47) - 1.2 (10) = 31 mm Hg

Alveolar Arterial O2 Gradient

EpitheliumEndothelium

Po2 Po2

Alveolar Gas Capillary Blood

initial Initial

Thickness

A-a Gradient

Alveolar Arterial O2 Gradient

EpitheliumEndothelium

Alveolar Gas Capillary BloodThickness

FIO2= 21% PAO2= 100 PaO2= 95

5FIO2= 50% PAO2= 331 PaO2= 326

FIO2= 100% PAO2= 663 PaO2= 657

O2 Sat= 99%

O2 Sat= 100%

O2 Sat= 100%

Alveolar Arterial O2 Gradient

EpitheliumEndothelium

Alveolar Gas Capillary BloodThickness

200FIO2= 50% PAO2= 331 PaO2= 131

FIO2= 100% PAO2= 663 PaO2= 463

O2 Sat= 100%

O2 Sat= 100%

Alveolar-arterial Oxygen Gradient

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Physiologic Causes of Low PaO2

NON-RESPIRATORY P(A-a)O2Cardiac right-to-left shunt Increased

Decreased PIO2 Normal

Low mixed venous oxygen content* Increased

* Unlikely to be clinically significant unless there is right-to-left shunting or ventilation-perfusion imbalance

Physiologic Causes of Low PaO2

RESPIRATORY P(A-a)O2Pulmonary right-to-left shunt Increased

Ventilation-perfusion imbalance Increased

Diffusion barrier Increased

Hypoventilation )increased PaCO2) Normal

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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 HgP(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

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 HgP(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

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 HgP(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.

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 HgP(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.

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