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# Phys2 Vent-perfusion

How do ventilation and BF vary within the lung? Both are greater at the base than the apex. **BF increases more than ventilation from apex to base (steeper slope).
4 processes that determine arterial PO2 1.Ventilation(determines alveolar PO2). 2.Diffusion (not totally complete 100:96). 3.Shunt. 4.Ventilation/perfusion inequality (efficiency of gas transfer in the lung).
4 causes of hypoxemia 1.Hypoventilation(problem w/ ventilation). 2.Diffusion impairment (problem w/ diffusion). 3.shut inequality (problem w/ shunt). 4.Ventilation/perfusion inequality.
What is hypoxemia low BL PO2
Alveolar PO2 is dependent on what? Ventilation/perfusion (V/Q) ratio. the rates of addition of O2 to the alveoli by ventilation and the removal of O2 from the alveoli by perfusion.
V/Q ratio >1 Alveolar PO2 will be higher than 100mmHg. (closer it will be to inspired PO2) **Adding more O2 than your taking out.
V/Q ratio <1 Alveolar PO2 will be lower than 100mmHg **taking more O2 out than bringing in
Ideal Alveolar PO2 equation PO2(alveoli) = PO2(inspired) - [PCO2(arterial)/R] + F. **PiO2: 149 mmHg as sea level. **R: respiratory quotient (0.8). **F:correction fraction.
How would Alveolar PO2 change: HYPOventilation DECREASE due to increase PCO2 (in alveolar and arterial). **Alveolar PO2 is INVERSELY proportional to PCO2.
How would Alveolar PO2 change: HYPERventilation INCREASE due to decrease PCO2 (in alveolar and arterial). **Alveolar PO2 is INVERSELY proportional to PCO2.
Alveolar-arterial (A-a) difference The difference between Alveolar PO2 and arterial PO2. **difference will be greater in a system where there are high & low PO2 lung units, end up w/ lower than expected PaO2.
Causes of hypoxemia using the A-a difference A-a difference will not be increased in HYPOventilation. Both Shunts and V/Q will have INCREASED A-a difference. **can usually rule out diffusion as a cuase due to lungs large reserve capacity for gas exchange.
Why doesn't the A-a difference increase with hypoventilation? b/c the alveolar PO2 decreases as much as the arterial PO2.
Hyopventilation's affect on: Alveolar PO2, arterial PO2, Alveolar PCO2, arterial PCO2, and A-a difference. 1.Alveolar PO2: DEC. 2.arterial PO2: DEC. 3.Alveolar PCO2: INC. 4.Alveolar PCO2: INC. 5.A-a difference: NO CHANGE
When does "ideal" gas transfer occur in the lungs? when ventilation is matched to perfusion. **V/Q=1 (CO 5L/min, V 5L/min).
What would a shunt do to the V/Q ratio? 0 b/c it will stop ventilation to that area. **perfusion but no ventilation
How does dead space affect the V/Q ratio? ventilation is normal but perfusion is 0. V/Q rate is infinite.
2 normal "Right-to-Left" shunts (perfusion w/o ventilation)? 1.Bronchial artery BL goes into pulmonary veins. 2.Coronary venous BL into Left atria
How do "Right-to-Left" shunts cause hypoxemia? They allow BL to bypass ventilation, which adds deoxygenated BL to oxygenated BL, DECREASING the areterial PO2. **Due to O2 dissociation plateau, small O2 content reduction = LARGE arterial PO2 reduction.
will pure O2 reverse the hypoxemia if induced by pathologic "Right-to-Left" shunts? NOOO!!! perfusion w/o ventilation!!
Alveolar gas composition with V/Q ratio = 0 R-to-L shunt. No new O2 to the alveoli b/c its perfusion w/o ventilation, CO2 is added to alveoli by pulmonary arterial BL. PO2:40mmHg. PCO2:46mmHg.
Alveolar gas compistion with V/Q ratio = infinity Dead space. there is no removal of O2 b/c it is ventilation with NO perfusion!! Also no CO2 can be added. PO2:149mmHg. PCO2:0mmHg.
V/Q ratio in an upright lung INCREASES from base to apex b/c BF decreases MORE than ventilation. Base: Less than 1 (b/c perfusion is greater than ventilation). Apex: Greater than 1 (b/c vent is greater than perfusion).
Where is the V/Q ratio = 1? Middle of the lung.
What happens to PO2 and PCO2 as you move from apex, to middle, to base of lungs? PO2 goes from high to low and PCO2 goes from low to high due to V/Q. Apex: PO2:132 (high), PCO2:28 (low). Middle: PO2:100, PCO2:40. Base: PO2:89 (low), PCO2:42 (high). **higher BV coming from base of lungs = larger influence on arterial PO2.
2 reasons V/Q inequality depresses arterial PO2 in a healthy patient 1.Larger BV comes from base of the lungs (Alveolar PO2 89mmHg). 2.Due to the O2 dissociation curve, a small reduction in overall O2 content (from base of lungs, low V/Q) cuases a large reduction in PO2 of the mixed BL.
Created by: WeeG

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