4- low VQ =shunt

KYJ-4 Respiratory Shunt

What is a Shunt?
Often nurses hear that the patient has a VQ mismatch or Shunt, with respect to respiratory nursing.

"V" – ventilation – the air that reaches the alveoli
"Q" – perfusion – the blood that reaches the alveoli
These two variables constitute the main determinants of the blood oxygen concentration.


Given that our ventilation to perfusion ratio is approximately 4 litres of air, coming into contact with about 5 litres of blood, our VQ (ventilation/perfusion) is mathematically calculated as 4/5 or 0.8.

In a situation that our VQ is very low, where more blood goes through the lungs than usually contacts air , it is referred to as a Shunt.   Think blood shunting through lungs but not being able to gas exchange.

Classic examples are foreign body airway obstructions, pulmonary oedema, pneumonia and restrictive airways diseases like asthma.

Blood continues to circulate (shunt) through the pulmonary vasculature but with limited air, gas exchange is adversely affected. The patient is classically hypoxaemic (low Spo2) and potentially acid optic due to inability to blow off building CO2.

Shunt = low VQ ratio.
More blood, less air.
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