Phys - Ventilation/Perfusion Relationships Flashcards
How does pulmonary vascular resistance compare to systemic vascular resistance?
Lower
Pulmonary artery pressure v. aortic pressure
14 mmHg v. 100 mmHg
Right ventricular output =
Left ventricular output = CO
Amount of blood entering pulmonary artery in 1 min
5 L (= CO since LV output = RV output)
So pressure and resistance are both lower in the pulmonary vasculature compared to systemic vasculature but the flow is the same. How is this possible?
Q=P/R; pressure and resistance in pulmonary vasculature decrease proportionally, so the ratio, hence the flow, is the same in both vascular beds
PBF =
(P_pulm artery - P_LV)/R
High/low at apex: Palv Pa Q R
Palv- high
Pa- low
Q- low
R- high
High/low at base: Palv Pa Q R
Palv- low
Pa- high
Q- high
R- low
Which zone?
Pa>PA>Pv
Zone 2
Which zone?
PA>Pa>Pv
Zone 1
Which zone?
Pa>Pv>PA
Zone 3
Besides blood flow, why is keeping pulmonary arterial pressure low important?
Keeps lungs dry
Promote fluid movement into or out of capillary: interstitial fluid pressure
Out
Promote fluid movement into or out of capillary: oncotic pressure of plasma proteins
In
Promote fluid movement into or out of capillary: Capillary hydrostatic pressure
Out
Promote fluid movement into or out of capillary: interstitial oncotic pressure
Out
3 things causing hypoxic vasoconstriction of pulmonary vasculature
- Hypoxia
- Hypercapnia (increased CO2)
- Acidosis
(remember CO2 and H+ right shift O2-Hb curve)
What is the purpose of hypoxic vasoconstriction?
Reduce PBF to alveoli that are not well ventilated (hence hypoxic) to not waste blood flow
Mechanism of hypoxic vasoconstriction
O2 diffuses from alveoli to SMC of arteriole; if SMC senses PO2<70, depolarizes; Ca2+ entry; constriction
Vasoconstriction (increases, decreases) resistance, thus (increases, decreases) flow
Increases Decreases (Q=P/R)
Name 2 conditions under which hypoxic vasoconstriction would be global
- High altitude
2. Fetal circulation (doesn’t breathe, so PO2<70)
Average V/Q
0.8
80% of blood is ventilated
O2 v. CO2 content: high V/Q
More O2
O2 v. CO2 content: low V/Q
More CO2
High or low V/Q: Apex
High; perfusion is low at apex
High or low V/Q: Base
Low; perfusion is high at base
O2 v. CO2: Hypoperfusion
More O2
O2 v. CO2: Hypoventilation
More CO2
Positive A-a gradient means:
Problem with perfusion (a)
Negative A-a gradient means:
Problem with ventilation (A)
Supplemental O2 will not fully correct:
Shunt
Which affects PACO2 more: V/Q mismatch or shunt?
NEITHER!!!!
Only thing that affects PACO2
Production of CO2 relative to ventilation of CO2
remember alveolar gas equation: PaCO2 = VCO2/Valv
How to treat if PaO2 is low and PaCO2 is low
Supp O2
How to treat if PaO2 is low and PaCO2 is high
Hyperventilate
