V/Q Relationships and Causes of Hypoxemia Flashcards
what number classifies a hypoxemia
- PaO2 < 80 mmHg
tracing the path of PO2 as it successively drops
- atmosphere
- alveoli
- arterial blood
- systemic capillary
- mitochondria
why is there a small degree of normal shunt
- due to bronchial circulation and thebesian veins
how to calculate the expected A-a O2 difference
age/4 + 4
causes of normal A-a difference
- diffusion barrier in the lung
- anatomical shunt (bronchial circulation and thebesian veins)
- V/Q inequality that increases with age
causes of hypoxemia
- low PiO2
- hypoventilation
- diffusion impairment
- shunt
- V/Q mismatch
what happens to your PAO2 as you go up in elevation
- it decreases
if CO2 production is constant and ventilation is halved, what will happen to PACO2 and PaCO2
- it will double
if CO2 production is constant and PaCO2 is not elevated, is the patient hyperventilating
- no
what rectifies hypoxemia due to hypoventilation
- supplemental O2
what is the A-a O2 gradient in hypoventilation
- normal
equilibration reserve is reduced when the diffusion barrier is
- thickened
diffusion impairment due to which conditions
- pumonary edema
- fibrosis
treatment of diffusion impairment
result
- O2 therapy
- increases the driving force
abnormal intrapulmonary shunt
- blood goes through lung without being exposed to alveolar ventilation
abnormal extra pulmonary shunt
- right to left cardiac shunt
what do obstructed airways create
what happens
- physiological shunt
- alveolar capillaries cannot become oxygenated
why would the PCO2 be normal in an anatomic shunt
- ventilation of the good part of the lung twice as much
how can we test for an anatomic shunt
- lack of response to O2 therapy
tiny drop in O2 content in a shunt results in _______ in PaCO2
- big drop
where does the extra oxygen go if we add it in a patient with an anatomical shunt
what happens if the shunt is small
and if the shunt is big
- small amount dissolves into blood
- the dissolved oxygen can resaturate the deoxygenated hemoglobin
- it won’t be able to re-saturate all that deoxygenated hemoglobin
why doesn’t O2 content drop too much during a small anatomic shunt
- the hemoglobin from the well ventilated alveoli is already saturated
most important cause of hypoxemia
- V/Q mismatch
compliance of the upper portion of the lung
- low compliance
ventilation of the upper portion of the lung
blood flow
V/Q ratio
- low ventilation
- but LOWER blood flow
- high V/Q ratio
PAO2 of upper portion of lung
- higher PAO2
compliance of lower portion of the lung
- high compliance
ventilation of lower portion of lung
blood flow of lower portion of the lung
V/Q ratio
- high ventilation
- HIGHER blood flow
- lower V/Q
PAO2 of lower lung
- lower PAO2
what causes the normal A-a gradient that exists in the body
- normal V/Q mismatching
blocked ventilation is also called
- shunt
blocked perfusion is also called
- alveolar dead space
do patients with emphysema have decrease ventilation or perfusion?
why?
- decreased perfusion
- have lost capillaries in their walls
do patients with chronic bronchitis have decreased ventilation or perfusion?
why?
- decreased ventilation
- small airways are blocked
will patients with partially obstructed airways in V/Q mismatch respond to O2 therapy
- yes
changes in V/Q during exercise
- (HIGH V/Q RATIO)
- increased cardiac output
- increased ventilation (but much higher ventilation)
comparison of V/Q in exercises versus V/Q in disease
- in exercise, both V and Q increase
- in disease either/both V or/and Q decrease
V/Q mismatch effect on A-a difference
- increases
disease that cause V/Q mismatch
- chronic bronchitis
- asthma
- cystic fibrosis
physiologic shunt effect on A-a difference
- increase
will hypoventilation respond to O2 therapy
- yes, but O2 therapy may not solve the underlying problem
diffusion impairment effect on A-a difference
- increases
intracardiac right to left shunt effect on A-a difference
- increases
reduced partial O2 content (anemia, CO poisoning) effect on A-a difference
why
- normal
- partial pressures are unaffected
anemia response to O2 therapy
why
- low
- you need to increase hemoglobin
will dead space respond to O2 therapy
- yes
