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.

7.  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

8.  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.

10.  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.

12. 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

13. 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

14. 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.

15. 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

16. Calculation of Physiologic Shunt
Where