Causes of Hypoxemia Flashcards
P-F ratio
PaO2/FIO2
What does a large A-a ratio indicate?
pathology of gas exchange in the lun
causes of low PaO2
ambient hypoxia
diffusion impairment
hypoventilation
low V/Q
shunt
ambient hypoxia
low Pb (altitude)
low FIO2
What are the responsibe hypoxemias?
ambient hypoxia
diffusion impairment
hypoventilation
ventilation/perfusion inequality
refractory hypoxemia
arterial hypoxemia that does not significantly improve when FIO2 is increased
ambient hypoxia
results from low barometric pressure (high altitiude) or breathing low concentrations of oxygen in inspired gas
lower inspired PO2, alveolar PO2, and arterial PO2
diffusion problem
not a problem in healthy lung
thickened “respiratory membrane” through fibrosis
increased diffusion distance - emphysema
generally responsive to an increase in FIO2
alveolar hypoventilation
with normal gas exchange, hypoventilation can decrease alveolar and therefore arterial PO2
hallmark is elevated PaCO2 and hypercapnia
inspired oxygen readily reverses the hypoxemia but has no direct effect on the hypercapnia
situations that cause hypoventilation
low lung compliance
depression of the brainstem respiratory oscillator
inadequate ventilator settings
muscle paralysis or fatigue including neurological disorders (myasthenia gravis)
high airway resistance (asthma)
causes of anatomic shunts
bronchial and thesbian circulations - a very small amount
ventricular or atrial septal defects
patent ductus arteriosus
capillary shunting
zero V/Q (unventilated alveoli or atelectasis)
refractory to increased FIO2
What is the normal Qs/Qt ratio?
<7%
low ventilation-perfusion ratio
high V/Q states don’t compensate for regions of low V/Q because of the non-linear hemoglobin dissociation curve
the level of PaO2 will be lower and PaCO2 will be higher in alveoli with less ventilation-perfusion ratios
effecto of V/Q ratio on Pc’O2 and Pc’CO2
How does V/Q change from the top of the lung to the bottom of the lung?
decreases
caused by gravity and regional lung differences in ventilation and perfusion
does gravity have more of an effect on ventilation or perfusion?
it has more of an effect on perfusion
as a result, the V/Q ratio progressively declines as one moves from the top to the base of the lungs
Describe the time/volume relationship during inspiration and expiration for a normal alveolus, reduced compliance, and increased resistance
a is the situation in a normal alveolus
b is decreased compliance
c is increased resistance
compensation fo V/Q maldistribution
local hypoxia and hypercapnia cause vasoconstriction, limiting perfusion to that alveolar unit
local hypocapnia causes bronchoconstriction, reducing ventilation of that alveolar unit
What happens when V/Q is at 0? at infinity?
if V/Q = 0, it is an effective shunt
if V/Q = infinity, it is alveolar dead space
conditions which have elevated A-aPO2 gradients
diffusion impairment
V/Q maldistribution
shunt
conditions where the A-a gradient is not elevated
ambient hypoxia
hypoventilation
PEEP
positive end expiratory pressure
can be used to prevent alveolar collapse
absorption atelectasis
in alveolar units that become plugged after being vilated wth 100% oxygen, there is a continualgradient for the transfer of oxygen from the alveolus to the pulmonary capillary bed
since the alveolus has been blocked off form the major pathways, nothing comes it replace the oxygen moving into the blood and the alveolus will soon collapse
