Ventilation-Perfusion Ratio Scharf

Question 1

6 causes of hypoxia

Answer 1

1. Decreased FIO2, 2. Hypoventilation, 3. Shunt (R-L), 4. Diffusion barrier, 5. V/Q mismatch (most common), 6. Decreased mixed venous PO2 (greater shunt means more influence, also important with lower cardiac output)

Question 2

Alveolar gas equation

Answer 2

PAO2=PIO2 - PACO2/RQ (+ F) . RQ is usually .8 but depend on diet (high carb=1, high fat=.7). Can use PaCO2 instead of PA. Relationship between inspired PO2 and alveolar PCO2 is a function of CO2 production and alveolar flow. Normal PIO2 is 149

Question 3

Which is the only hypoxia that won’t correct with 100% O2 administration?

Answer 3

Shunt

Question 4

What is the V/Q ratio with blocked ventilation?

Answer 4

0. Highest PCO2 and lowest PO2.

Question 5

What is the V/Q ratio with open ventilation and perfusion?

Answer 5

1 (ideal)

Question 6

What is the V/Q ratio with blocked perfusion?

Answer 6

Infinity. Lowest CO2 and highest O2.

Question 7

How does V/Q change from bottom to top of lung?

Answer 7

REgional ventilation decreases from bottom to top. Blood flow decreases with gravity (from bottom to top). Even though both decreasing, don’t go in same proportion and ratio between them goes up. Blood flow decreases faster than ventilation. At top of the lung, units with very high V/Q ratios and at the bottom very low V/Q ratios. Apex has highest PO2 because V/Q is highest.

Question 8

How do higher V/Q areas compensate for lower V/Q areas?

Answer 8

Not well, major cause of hypoxia because can’t make up the difference. O2 uptake is impeded with more inequality. CO2 output is also decreased (but to a lesser degree).

Question 9

What are the different ways to assess V/Q inequality

Answer 9

A-a PO2 (gradient between Alveolar and arterial PO2 using the alveolar gas equation, normal is <12) OR ratio a/A (better measure-.67 is normal)

• Poor man’s a/A ratio: a/F ratio. F is the inspired fractino of O2. Normally >450. If <200: ARDS, 200-300: acute lung injury.

Qs/QT

Physiological Dead Space

Computer Models (measure V/Q with different gases and makes sure blood flow and ventilation cluster at 1).

Radionuclide scanning (give xenon and check image)