West's Zones

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: P_A > P_a > P_v
  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
      • PEEP
    • Arterial pressure is low
      • Shock
        • Hypovolaemia

• West Zone 2: P_a > P_A > P_v
  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 P_a - P_A gradient.
    • When P_a falls below P_A (e.g. in diastole), then no blood flow will occur

• West Zone 3: P_a > P_v > P_A
  Arterial pressure exceeds venous pressure which exceeds alveolar pressure.
  • Blood flow occurs throughout the cardiac cycle
    Flow is proportional to the P_a - P_v gradient.
  • For an accurate measure of PCWP, a PAC must be placed in West Zone 3 (so there is a continual column of blood)
  • This tends to happen naturally as the majority of pulmonary flow is to this region

• West Zone 4: P_a > P_i > P_v > P_A
  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).

References

1. Brandis K. The Physiology Viva: Questions & Answers. 2003.
2. Chambers D, Huang C, Matthews G. Basic Physiology for Anaesthetists. Cambridge University Press. 2015.