VENTILATION-PERFUSION RATIO (V/Q RATIO)

Ventilation (V) – 

• Amount of air reaching alveoli. 

Perfusion (Q) –

• Amount of blood reaching alveoli via capillaries.

Two variables (V & Q)

• Main determinants of concentration of blood oxygen (O₂) & carbon dioxide (CO₂).

V/Q ratio:

• V/Q ratio = Amount of air reaching alveoli per minute / Amount of blood reaching alveoli per minute.

Calculation method:

• Cardiac output is approximately about 5.0 L/min

• Usually, pulmonary perfusion/ blood flow equals cardiac output level. 

Hence,

• Pulmonary perfusion (Q) – 

– 5.0 L/min

• Alveolar ventilation (V) – 

– 4.2 L/min

• Ratio (Normal) = 

– Ventilation / Perfusion (V/Q) ratio = 

– 4.2/ 5 = 0.8.

• Overall Ventilation-Perfusion (V/Q) ratio value – 0.8

• V/Q ratio can be measured with V/Q scan

Interpretations of V/Q ratio values:

• Value is 0.8 

– Means Normal V/Q ratio

– Ventilation-Perfusion matching

If V/Q is > 0.8 – 

• Means more ventilation than perfusion.

If V/Q is < 0.8 – 

• Means poor ventilation 

• Ventilation & perfusion values does not match.

• Referred as V/Q mismatch

Variations in V/Q ratio:

• Typically, V/Q ratio of each alveolus must be 0.8

• Yet, ratio is uneven in normal lungs.

Factors influencing:

1. Position of lung:

• Ratio varies depending on position.

• Lung is centered & vertically around heart.

• A part of lung is superior to heart & another part is inferior. 

• This majorily impacts on V/Q ratio

2. Gravity:

• Main reason for V/Q ratio differences between base & apex of lung.

• Affects perfusion more than ventilation.

Effect of gravity on variations bt. basal & apical V/Q ratio:

• Both ventilation & perfusion are decreased on moving up from base to apex.

• More pronounced decrease in perfusion observed.

– i.e., Base is more perfused than apex.

Effects on apex:

• Due to gravity, apical alveoli are both underventilated & under-perfused

• To be precise, it is more under-perfused than underventilated

• Hence, V/Q is maximum at apex

• Implying maximum O₂ tension at apex

Effects on base of lung:

• Conversely, basal alveoli are both overventilated & over-perfused.

• Precisely, more over-perfused than overventilated

• Hence, V/Q is minimum at base of lung.

• Implying maximum CO₂ tension at base

Values:

• V/Q at apex 

– About 3.0 (maximum)

• V/Q at base of lung 

– About 0.6 (minimum)

3. Postural variations:

In erect position:

• Lung apex – Higher V/Q ratio

• Base of lung – Lower V/Q ratio

• Due to gravitational force.

In supine position:

• When supine, blood flow is uniform throughout lungs.

• Thus, perfusion = ventilation (approximately)

• Hence, V/Q ratio approximately equals 0.8, all over lungs.

COMPARISON Bt. PO₂ & PCO₂ conc. IN BASE & APEX:

• PO₂ & PCO₂ concentrations in well-perfused basal alveoli – 

Low PO₂ & high PCO₂ 

• Approximately same as PO₂ & PCO₂ levels in pulmonary artery 

• PO₂ & PCO₂ concentrations in poorly perfused apical alveoli – 

High PO₂ & low PCO₂

• Approximatelysame as PO₂ & PCO₂ levels in inspired air.

Reason:

• This variation in PO₂ & PCO₂ levels is mainly due to, 

– Ventilation-perfusion mismatch

In apex: 

• Generally, apex is ventilated more than perfused.

• Hence, less O₂ transferred from alveoli to blood.

• So, High PO₂ & low PCO₂ (levels close as in inspired air)

In base:

• Generally, lung base is perfused more than being ventilated.

• Hence, more O₂ transferred from alveoli to blood.

• So, Low PO₂ & high PCO₂ (levels close as in pulmonary artery)

Conditions associated:

High V/Q ratio –

• Pulmonary embolism & COPD

Low  V/Q ratio – 

• Asthma & Pulmonary hemorrhage

Applied physiology:

1. Shunt

• An area with perfusion but no ventilation.

• V/Q = zero.

2. Dead space

• An area with ventilation but no perfusion.

• V/Q undefined, though approaching infinity.