Describe West's zones of the lung and explain the mechanisms responsible for them
West's Zones take into account the effect of alveolar pressure on pulmonary blood flow. The lung is divided into four zones:
- West Zone 1: PA > Pa > Pv
Alveolar pressure exceeds arterial pressure.
- The alveolus compresses the capillary, and no blood flow occurs
- As there is ventilation but no perfusion, this can also be thought of as dead space
- This occurs when:
- Alveolar pressure is high
- Arterial pressure is low
- West Zone 2: Pa > PA > Pv
Arterial pressure exceeds alveolar pressure, which exceeds venous pressure.
- Blood flow occurs intermittently during the cardiac cycle
- Alveolar pressure acts as a Starling resistor
Flow is proportional to the Pa - PA gradient.
- When Pa falls below PA (e.g. in diastole), then no blood flow will occur
- West Zone 3: Pa > Pv > PA
Arterial pressure exceeds venous pressure which exceeds alveolar pressure.
- West Zone 4: Pa > Pi > Pv > PA
Interstitial pressure acts as a Starling resistor for pulmonary blood flow.
- It is seen when interstitial pressure is high (e.g due to pulmonary oedema).
- Brandis K. The Physiology Viva: Questions & Answers. 2003.
- Chambers D, Huang C, Matthews G. Basic Physiology for Anaesthetists. Cambridge University Press. 2015.