Causes of Hypoxemia Flashcards
What is normal barometric pressure in Chicago?
738 mmHg
What is the water vapor at normal body temp?
52 mm Hg
What is inspired oxygen pressure (PiO2)?
144 mmHg
What is the normal range for arterial PCO2?
35 to 40 mmHg, average 38
What is a normal range for arterial PO2?
85 and 100 mmHg, average 92 mmHg
Are alveolar and arterial PCO2 or PO2 generally equal?
PCO2
Impaired gas exchange will always manifest as a decrease in which value:
- arterial PO2
- Alveolar PO2
- Venous PO2
arterial Po2
Ambient hypoxemia, diffusion impairment, hypoventilation and ventilation/perfusion inequality are all forms of hypoxemia that are responsive or refractory to FiO2?
responsive
What kind of hypoxemia is not sensitive to changes in FiO2?
shunt: cardiopulmonary or functional shunting
ambient hypoxemia will manifest as changes in which values?
inspired PO2
alveolar PO2
arterial PO2
In a normal individual, there is equilibration of alveolar gas and capillary blood, allowing for saturation before the end of the capillary. In which individuals may this be an exception?
elite endurance athletes during peak exertion
What kind of hypoxemia can be caused by fibrosis and edema? is this responsive to increases in FiO2?
diffusion impairment
Yes, responsive hypoxemia
What is the diagnostic feature of diffusion impairment hypoxemia?
Increases in PiO2 reverse hypoxemia by increasing gradient for diffusion
What kind of hypoxemia is caused by the following: Inadequate ventilator settings, low lung compliance, depression of brainstem activity, muscle paralysis or fatigue from diseases such as myasthenia gravis, and high airway resistance
alveolar hypoventilation
How is alveolar hypoventilation hypoxemia defined? is it responsive or refractory?
hypercapnia, where PACO2 is inversely proportional to VA and proportional to VCO2.
A responsive hypoxemia BUT hypercapnia is NOT reversed by increased PO2
What are 3 causes of shunting in the lung?
- bronchial and thebesian circulations
- congenital ASD or VSD
- PDA
i. e. RIGHT to LEFT shunts
Hypoxemia and arterial hypercapnia can be caused by alveolar hypoventilation. What other cause of hypoxemia causes this, but is refractory?
shunting
Why is shunting refractory to inspired O2?
the flat slope of the oxygen-Hb dissociation curve when PO2 is greater than 100 mmHg means that concentration of O2 is dependent on alveolar gas pressure BUT mixing in arteries in a closed system means that pressure has no effect on the volume of O2 in the blood due to laws of mass conservation. Therefore increases in pressure have no effect on O2 volume in arterial blood unless arterial PO2 is less than 100 mmHg
What is the most common cause of hypoxemia in disease?
ventilation perfusion mismatch
what are a normal causes of V/Q mismatch?
Gravitational forces that change the dimensions of the lung
what accounts for some of the normal arterial-alveolar PO2 difference in healthy patients?
gravitational forces that change V/Q ratios
Does ventilation increase or decrease towards the top of the lung?
decrease
Does perfusion pressure increase or decrease near the base of the lung?
increase
Do Va/Q ratios increase or decrease from the top to the base of the lung?
Decrease - this is because perfusion changes are more affected by gravity than ventilation
Where is regional alveolar PO2 ventilation highest? what does this mean for primary TB infection?
top of the lung (relatively high V/Q ratio and hyperventilated)
Common site of TB infection
Locally, what is the compensation for local hypoxia and hypercapnia?
vasoconstriction
Locally, what is a compensation for local hypocapnia?
bronchoconstriction to reduce ventilation (only when Va/Q ratios are VERY high)
A region with perfusion but no ventilation (Va/Q = 0) is defined as a …
shunt
A region with ventilation but on perfusion (Va/Q = infinity) is define as a …
dead space
Can you use the alveolar air equation to calculate PO2 in individual alveoli?
nope
What is the normal value for (A-a)PO2?
less than 10 mmHg
HOw can you prevent alveolar collapse during anesthesia?
regular hyperinflation
