SM_158a: Hypoxemia Flashcards
The alveolar to arterial PO2 difference (A-aPO2) indicates ________
The alveolar to arterial PO2 difference (A-aPO2) indicates gas exchange problem while breathing room air
The P-F ratio (PaO2/FIO2) indicates _______
The P-F ratio (PaO2/FIO2) indicates gas exchange problem while breathing elevated O2
What are the causes of low PaO2?
- Ambient hypoxia
- Diffusion impairment
- Hypoventilation
- Shunt
- V·/Q· inequality
What causes ambient hypoxia?
- Low PB (high altitude)
- Low FIO2 (enclosed space with another gas displacing O2)
What casues diffusion impairment?
- Thickened respiratory membrane (idiopathic pulmonary fibrosis)
- Increased diffusion distance (emphysema, minor contribution to hypoxemia)
In which condition is a thickened respiratory membrane the cause for diffusion impairment?
Idiopathic pulmonary fibrosis
In which condition is increased diffusion distance responsible for diffusion impairment?
Emphysema
When a thickened respiratory membrane causes diffusion impairment, increasing FIO2 _______
When a thickened respiratory membrane causes diffusion impairment, increasing FIO2 increases driving force for O2 diffusion, which corrects hypoxemia
Diffusion impairment due to increased diffusion distance is ______ to an increase FIO2
Diffusion impairment due to increased diffusion distance is responsive to an increase FIO2
Alveolar hypoventilation causes a(n) _______ in PaCO2
Alveolar hypoventilation causes an increase in PaCO2
What can cause alveolar hypoventilation?
- CNS depression (brain injury, disease, drug abuse)
- Neuromuscular disorders (ALS, myasthenia gravis)
- Obstructive pulmonary disease (COPD, obesity-hypoventilation syndrome)
- Restrictive pulmonary disease (kyphoscoliosis)
All increase PaCO2 (hallmark of hypoventilation)
Hypoventilation is defined by _______
Hypoventilation is defined by elevated PaCO2
What are the types of shunt?
- Anatomic shunt
- Capillary shunt
What are the causes of anatomic shunting?
- Bronchial and thebesian veins (very small amounts)
- VSD or ASD
- PDA
What characterizes capillary shunting?
Zero V·/Q· lung regions
(Blood flow through capillaries past collapsed alveoli (atelectasis) or unventilated alveoli)
Why does capillary shunting involve V·/Q· lung regions?
Blood flow through capillaries past collapsed alveoli (atelectasis) or unventilated alveoli
Shunting is relatively ______ to increased FIO2
Shunting is relatively refractory to increased FIO2
By adding mixed venous blood to the arterial circulation, a shunt tends to ______ PaO2 and ______ PaCO2
By adding mixed venous blood to the arterial circulation, a shunt tends to reduce PaO2 and increase PaCO2
Increasing alveolar ventilation will often resolve ______ but not ______ resulting from a shunt
Increasing alveolar ventilation will often resolve hypercapnia but not the hypoxemia resulting from a shunt
Why is it difficult to correct the hypoxemia due to a shunt by elevating the FIO2?
The HbO2 curve flattens at PO2 > 60 mmHg
- Increasing V·A or FIO2 will not appreciably increase O2 content in ventilated portions of the lung
- The lack of an increase in O2 content in ventilated portions of the lung means that addition of venous blood reduces PaO2
Increasing V·A or FIO2 will _______ O2 content in ventilated portions of the lung, so addition of venous blood _______ PaO2
Increasing V·A or FIO2 will not appreciably increase O2 content in ventilated portions of the lung, so addition of venous blood reduces PaO2
Compared to O2, why is it less likely that a shunt will elevate PaCO2?
CO2 curve does not flatten
- Increasing ventilation will decrease PaCO2 in ventilated portions of the lung
- This offsets the effect of the shunt in raising PaCO2
Increasing ventilation will _______ PaCO2 in ventilated portions of the lung, which ______ the effect of the shunt in ______ PaCO2
Increasing ventilation will decrease PaCO2 in ventilated portions of the lung, which offsets the effect of the shunt in raising PaCO2
A shunt tends to ______ PaO2, while the tendency for a shunt to ______ PaCO2 can be compensated for by a(n) ______ in ventilation
A shunt tends to decrease PaO2, while the tendency for a shunt to increase PaCO2 can be compensated for by a(n) increase in ventilation
_______ is the most common cause of hypoxemia clinically
V·/Q· inequality is the most common cause of hypoxemia clinically
Describe what is happening with Goat X
- Hypoxemic while breathing room air
- Hypoxemia corrected but with difficulty to elevated FIO2
- Elevated A-aPO2 difference breathing room air
- Low P-F ratio
- Normocapnic (normal PaCO2)
Describe the ventilation/perfusion model
- At a constant flow (Q), the faster the dye is added (V), the higher the effluent dye concentration
- At a constant V, the faster the Q, the lower the effluent dye concentration
- Dye concentration is dependent on the V·/Q· ratio
V·/Q· = 1 is _____
V·/Q· = 1 is ideal
(PO2 = 100 mmHg, PCO2 = 100 mmHg)
When V·/Q· = 0, V· = ____, indicative of a ____
The alveolus has ____ PO2, PCO2
When V·/Q· = 0, V· = 0, indicative of a shunt
The alveolus has mixed venous PO2, PCO2
When V·/Q· = infinity, Q· = _____, indicative of _____
The alveolus has ____ PO2, PCO2
When V·/Q· = infinity, Q· = 0, indicative of dead space
The alveolus has inspired PO2, PCO2
V·A/Q· line represents all possible PO2 and PCO2 values for a __________
V·A/Q· line represents all possible PO2 and PCO2 values for a single unit in a healthy lung
Decreased V·/Q· indicates a ______
Decreased V·/Q· indicates a shunt
Increased V·/Q· indicates a _____
Increased V·/Q· indicates a dead space
V·/Q· is ____ at the apex
V·/Q· is high at the apex
____ V·/Q· at the apex means _____ PO2 and _____ PCO2
High V·/Q· at the apex means high PO2 and low PCO2
V·/Q· is ____ at the base
V·/Q· is low at the base
____ V·/Q· at the base means _____ PO2 and _____ PCO2
Low V·/Q· at the base means low PO2 and high PCO2
Lung units with _____ V·/Q· add relatively _____ O2 compared to decrement in O2 caused by units with low V·/Q·
Lung units with high V·/Q· add relatively little O2 compared to decrement in O2 caused by units with low V·/Q·
V·/Q· inequality ______ PaO2
V·/Q· inequality decreases PaO2
Describe what is happening with Goat X
Hypoxemia, elevated A-aPO2 difference, low P-F ratio, normocapnia
- Severe V·/Q· maldistribution
- Capillary shunt (atelectasis)
What is the effect of positive end expiratory pressure?
- Raises pressure in airway -> prevents alveolar collapse
- Reduces cardiac output
What is the effect of periodic hyperinflation/sigh?
Elevated tidal volume
How can the hypoxemia, elevated A-aPO2 difference, low P-F ratio, normocapnia of Goat X be prevented?
- Positive end expiratory pressure: prevents alveolar collapse by raising pressure in airway, reduces cardiac output
- Periodic hyperinflation/sigh: elevates tidal volume
A-aPO2 is _____ in a healthy young adult
A-aPO2 is <10 mmHg in a healthy young adult
A-aPO2 = 2.5 + 0.25(age in years)
- Broncial and thebesian circulations
- V·/Q· mismatching
A-aPO2 is elevated in _______, _______, and _______
A-aPO2 is elevated in diffusion impairment, shunt, V·/Q· maldistribution
A-aPO2 is not elevated in ________ and ________
A-aPO2 is not elevated in ambient hypoxia or hypoventilation
