Ventilation perfusion ratio (The guyton and hall physiology)

Ventilation­ perfusion ratio is :
“The ratio of alveolar ventilation and the amount of blood that perfuse the alveoli”.

FORMULA
It is expressed as VA/Q.
VA is alveolar ventilation
Q is the blood flow (perfusion)
Normal value of ventilation ­perfusion ratio is about
0.8
VA is 4.2 L /min
Q is 5.5 L/min (Same as Cardiac output)
So VA/Q = 4.2/5.5 = 0.8

If VA becomes zero ratio becomes zero
If Q becomes zero ratio becomes infinite.

If ratio becomes zero or infinite then there is no gaseous exchange. So this ratio indicates the efficiency of gaseous exchange in lungs.
In standing or sitting position this ratio is not uniform in all parts of the lungs.
In standing position, in upper parts of lungs there is almost no blood flow so normally in upper parts of lungs the ratio is higher may be near 3.
In lower part of lungs, there is more blood flow so the ratio is decreased may be 0.6.
In certain diseases the VA/Q ratio is higher which means perfusion is inadequate i.e. in some parts of lungs the alveoli are non functional or partially functional. This is seen in cases of pulmonary thrombosis or embolism.

When there is higher VA/Q ratio, PO2 and PCO2 in the alveolar air resembles the values in the inspired air.

When exchange is not occurring because of lack of perfusion, inspired air goes to alveoli, as there is no exchange occurring so the same values of PCO2 and PO2 as in inspired air.

1. VENTILATION PERFUSION RATIO Source: The Guyton and Hall physiology Maryam Fida (o-1827)

2. VENTILATION PERFUSION RATIO Ventilation ­ perfusion ratio is : “The ratio of alveolar ventilation and the amount of blood that perfuse the alveoli”. FORMULA It is expressed as VA/Q. VA is alveolar ventilation Q is the blood flow (perfusion)

3. NORMAL VALUE Normal value of ventilation ­ perfusion ratio is about 0.8

4. CALCULATION

5. VA is 4.2 L /min Q is 5.5 L/min (Same as Cardiac output) So VA/Q = 4.2/5.5 = 0.8 If VA becomes zero ratio becomes zero If Q becomes zero ratio becomes infinite. If ratio becomes zero or infinite then there is no gaseous exchange. So this ratio indicates the efficiency of gaseous exchange in lungs .

6. In standing or sitting position this ratio is not uniform in all parts of the lungs. In standing position, in upper parts of lungs there is almost no blood flow so normally in upper parts of lungs the ratio is higher may be near 3. In lower part of lungs, there is more blood flow so the ratio is decreased may be 0.6. EFFECT OF POSTURE ON VA/Q RATIO

7. In certain diseases the VA/Q ratio is higher which means perfusion is inadequate i.e. in some parts of lungs the alveoli are non functional or partially functional. This is seen in cases of pulmonary thrombosis or embolism. When there is higher VA/Q ratio, PO2 and PCO2 in the alveolar air resembles the values in the inspired air . When exchange is not occurring because of lack of perfusion, inspired air goes to alveoli, as there is no exchange occurring so the same values of PCO2 and PO2 as in inspired air.

8. In emphysema and chronic smokers, there is low VA/Q ratio, there is inadequate ventilation but there is perfusion . When there is inadequate ventilation, there will be maximum attempt for exchange of gases i.e. till the PO2 and PCO2 in the alveolar air becomes near to the values in the pulmonary capillary blood . So there will be shunted blood which means some of the blood goes without exchange and PO2 and PCO2 in the alveolar air resembles the values in the venous blood.

9. PULMONARY BLOOD VESSELS 1. Pulmonary Artery: Pulmonary artery supplies deoxygenated blood pumped from right ventricle to alveoli of lungs. 2. Bronchial Artery: supply oxygenated blood to other structures of lungs

10. Physiologic Shunt It is defined as: “It is a diversion through which the venous blood is mixed with arterial blood ” . Whenever VA/Q is below normal, there is inadequate ventilation to provide the oxygen needed to fully oxygenate the blood flowing through the alveolar capillaries. The fraction of venous blood passing through pulmonary capillaries which doesn’t get oxygenated is called “shunted blood ” The total quantitative amount of shunted blood per minute is called the physiologic shunt

11. Physiological Shunt Vs Physiological Dead Space Physiological shunt includes wasted blood . physiological dead space includes wasted air. Both wasted blood and wasted air exist on either side of alveolar membrane and both affect the ventilation-perfusion ratio.

12. Physiologic Shunt Whenever VA/Q is below normal, there is inadequate ventilation to provide the oxygen needed to fully oxygenate the blood flowing through the alveolar capillaries. The fraction of venous blood passing through pulmonary capillaries which doesn’t get oxygenated is called “shunted blood ” The total quantitative amount of shunted blood per minute is called the physiologic shunt

13. Calculation of Physiologic Shunt Where