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Phys2 Vent-perfusion
Question | Answer |
---|---|
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. |