4.Mechanisms of Hypoxemia 422
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Transcript of 4.Mechanisms of Hypoxemia 422
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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.