7th February 2024

West’s zones

Dead space and shunt

When considering ventilation and perfusion of the lung, ventilation-perfusion ratio, dead space, and shunt come to everyone’s mind.

First, some definitions:

Dead space is the volume of inspired air that does not participate in gas exchange. In terms of the V/Q ratio, this translates to V of 1, Q of 0, and V/Q ratio of ∞.

A shunt is a volume of blood that enters systemic circulation without participating in gas exchange. In terms of the V/Q ratio, this translates to V of 0 and Q of 1. Hence V/Q ratio would be 0.

Dead space and shunt can be considered the opposite extremes of the V/Q ratio.

The normal V/Q ratio is around 0.8.

Dead space

Dead space is then classified further into anatomical and alveolar dead space.

Anatomical dead space = volume of air in the conducting airway.

Alveolar dead space = volume of air in the alveoli that is not perfused.

Physiological dead space = anatomical dead space + alveolar dead space.

Factors that will increase dead space include:

• Bronchodilation
• Pulmonary emboli (blood/air)
• Hypovolaemia/Hypotension

Shunt

Shunt can be classified as intrapulmonary shunt and anatomical shunt.

Intrapulmonary shunts include:

• Physiological hypoxic pulmonary vasoconstriction
• Pathological factors (e.g. consolidation in chest infection)

Anatomical shunts include:

• Physiological: Thebesian veins, bronchial vein
• Pathological: cardiac anomalies resulting in R-L shunt (e.g. ventricular septal defect)