4.Mechanisms of Hypoxemia 422

7/25/2019 4.Mechanisms of Hypoxemia 422

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Important Terminology

Anoxia: absence of O2supply in the presence of perfusion- no oxygen

Asphyxia: absence of O2& accumulation of CO2.

Hypoxia: !O2 in the body, often specified where in the body

e.g. tissue hypoxia, alveolar hypoxia

Hypoxemia: !O2 in the blood. Specifically, hypoxemia is determined

by measuring the PO2of arterial blood (plasma)


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RV

RA LV

LA

160 = PB X FIO2

PO2(mmHg)

dry air / sea level

conducting airways

alveoli

150 = PIO2

102 =PAO2

102 =PcO2

94 = PaO2

40 = PvO2

[P(A-a)O2]


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[P(A-a)O2]

Normal range=10-15 mmHg breathing room air, FIO2=0.21

the normal range "with age (1 mmHg per decade due to a

!PaO2as a result of "ventilation perfusion mismatch

is due to venous admixture (anatomic shunt & ventilation perfusion

mismatch in health)

is due to venous admixture (anatomic shunt, "ventilation perfusion

mismatch & physiologic shuntin disease states)


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Causes are subdivided into those with an increase in the P(A-a)O2

and those where the A-a gradient remains within the normal range.

1. Hypoventilation

2.

Low inspired oxygen

3.

R-L shunt4. V/Q inequality (a.k.a. V/Q mismatch)

5. Diffusion Impairment

. .. .


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"PaCO2(hypercapnia)

P(A-a)O2withinnormal range

"FIO2alleviates the hypoxemia

mechanical ventilation required to eliminate hypercapnia


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1. Depression of CNS by drugs

2. Inflammation, trauma or hemorrhage in the brainstem

3. Abnormal spinal cord pathway4. Disease of the motoneurons of the brain stem/spinal cord

5. Disease of the nerves supplying the respiratory muscles.

6. Disease of the neuromuscular junction

7. Disease of the respiratory muscles

8. Abnormality of the chest wall9. Upper airway obstruction


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Low inspired oxygen (PIO2)

!PIO2= (PB - 47 mmHg) FIO2

P(A-a)O2within normal range

!PaCO2(hypocapnia due to hyperventilation in

response to low arterial PO2)


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Right to Left Shunt

"P(A-a)O2

PaCO2within the normal range


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Anatomic ShuntA portion of Blood bypasses the Lungs through an Anatomic Channel

In all Healthy Individuals

a portion of the bronchial circulations venous blooddrains into the pulmonary vein.

a portion of the coronary circulations venous blood

drains through the thebesian veins into the left ventricle.


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Disease States

(Congenital abnormalities)

intra-cardiac shunts

intrapulmonary fistulas

Anatomic ShuntA portion of Blood bypasses the Lungs through an Anatomic Channel

Tetralogy of Fallot


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Key Clinical Feature of R-L Shunts

the accompanying hypoxemia can not becorrected with supplemental oxygen


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CO2

contentofbloo

d(ml/100ml)

44

48

52

5

6

60

PCO2(mm Hg)

30 35 40 45 50


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"P(A-a)O2 PaCO2 within the normal range

most common cause of hypoxemia in disease states


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Normal V/Q Inequality from the Apex to Base of the Lungs

. .


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. .. .

. . . .. .

. .


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O2

CO2

N O2

N CO2

O2,CO2 N O2, N CO2

O2, CO2

O2 CO2

#.

VE


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O2

CO2

N O2

N CO2

O2,CO2 N O2, N CO2

O2, CO2

O2

CO2

O2,CO2

O2, N CO2

O2 CO2

#.

VE


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Diffusion Impairment

P(A-a)O2normal at rest, "s with exercise

PaCO2 within the normal range

a rare observation in clinical setting


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Summary arterial blood venous blood Does supplemental oxygen (FI O2)increase PaO2 substantially?

PO2 PCO2 PO2 PCO2 P(A-a)O2

Hypoxemia

Hypoventilation

normal

yes

PIO2 normal yes

R-L Shunt normal normal no (depends on magnitude of the

shunt)

Diffusion defect normal normal during

exercise

yes

VA/Q inequality

normal

normal

yes

Tissue hypoxia

Anemic hypoxia normal normal normal normal no

CO poisoning normal normal normal normal possibly

Stagnant hypoxia

normal

normal

normal

normal

no

Histotoxic hypoxia

normal

normal

normal

normal

no

Important note that mixed causes of hypoxemia occur frequently. It is often

impossible to define the extent of the contribution of each mechanism in the

acutely ill patient.

4.Mechanisms of Hypoxemia 422

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