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Alveolar Air Equation
calculates the partial pressure of oxygen (oxygen tension) in the alveoli (Alveolar PO2 or PAO2)
A-aDO2 (A-a Gradient)
measures the difference(D) or gradient between alveolar and arterial PO2 (best done after pt on 100% for 20 minutes)
causes of pulmonary shunting
A pulmonary shunt often occurs when the alveoli fill with fluid, causing parts of the lung to be unventilated although they are still perfused
why is increasing the FIO2 unable to help increase the PaO2 when shunting is present?
Because shunt represents areas where gas exchange does not occur
CaO2= Arterial oxygen content
measures the oxygen carried by the hemoglobin as well as that dissolved in the plasma
PaO2/FIO2 ratio or P/F ratio
used in determination of acute lung injury or acute respiratory distress syndrome
Qs/Qt (shunt equation)
the portion of cardiac output that is shunted (perfusion without ventilation)
(shortcut) for every 100 T (mmHg) of my A-a graident I add %5 shunt after starting with 5% normal physiological shunt. (if A-a gradient is 300 mmHg then Qs/Qt would be (5% x 3) +5% = 20% shunt
estimating SaO2 based on PaO2
if PaO2 between 40 and 60 add 30 to get SaO2 (if PaO2 50 then SaO2 is about 80)
if pt has fever how will blood gas values be affected
lower PaCO2, PaO2 and higher pH (higher pH=patient Has fever)
% of tidal volume doesn't participate in gas exchange. Dead-space to tidal volume ratio-ventilation without perfusion (the life is in the blood-so dead-space represents areas of the lung that are being ventilated but not perfused)
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