what is SpO2? how is it measured?
oxygen saturation
pulse oximetry
what is PaO2? PAO2?
PaO2 = arterial O2 pressure
PAO2 = alveolar O2 pressure
how are PaO2, PAO2, and PvO2 measured?
blood samples via ABG analysis
what are some causes of hypoxemia with normal A-a gradient
low inspired O2 (PiO2)
hypoventilation
what are some causes of hypoventilation?
can’t breathe (ex- neuromuscular diseases, obesity, COPD, etc)
won’t breathe (ex- stroke, head trauma, resp dep meds)
what is a key feature of hypoventilation
↑↑↑PaCO2 (hypercapnia)
what is the most common mechanism of hypoxemia
V/Q mismatch
what is the MOA of V/Q mismatch
Lung attempts matching ventilation to blood flow using hypoxic vasoconstriction
Attempts to help redirect blood flow to better oxygenated areas to optimize gas exchange
Unique to lungs- other organs vasodilate with hypoxia
typically seen with pneumonia, COPD, PE, etc
how does PE cause V/Q mismatch?
causes dead spaces (ventilation without perfusion- due to clot)- doesn’t typically cause hypoxemia unless massive, due to compensation by other lung units
what are some causes of elevated A-a gradient hypoxemia
VQ mismatch
diffusion impairment
shunt
what is considered an elevated A-a gradient
> 15mmHg
what causes diffusion impairment?
thickening of alveolar membrane = slow O2 crossing especially during exercise when blood flows faster
What type of hypoxemia is not significantly improved with O2
Shunt - deoxygenated blood bypasses gas exchange area
A-a gradient =
PAO2 - PaO2
PAO2 =
FiO2(Patm - PH2O) x (PCO2/R)
