Viva F6(ii)

What are Wests' zones?

• On the basis of its perfusion the lung can be divided into four discrete areas:
  • Zone 1, where alveolar pressure is higher than arterial or venous pressure;
  • Zone 2, where the arterial pressure is higher than alveolar and venous, a relationship which changes during the respiratory cycle
  • Zone 3, where both arterial and venous pressure is higher than alveolar
  • Zone 4, where the interstitial pressure is higher than alveolar or pulmonary venous pressure.

  • In fewer words,

    • Zone 1: P_A > P_a > P_v
    • Zone 2: P_a > P_A > P_v
    • Zone 3: P_a > P_v > P_A
    • Zone 4: P_a > P_i > P_v > P_A

  • Where:

    • P_A = alveolar  pressure
    • P_a = arterial pressure
    • P_v  = venous pressure
    • P_i = interstitial pressure

What is West's Zone 1?

• "...that part of the lung above the level at which arterial and alveolar pressure are equal".
• Alveolar pressure exceeds pulmonary arterial and venous capillary pressure
• Little gas exchange takes place
• Blood flow is limited and probably cyclical (i.e. only systolic, and dependent on the phase of the respiratory cycle)
• some of Zone 1 is probably going to be dead space, but really, all the definition asks for is for the alveolar (atmospheric) pressure to be higher than the alveolar capillary pressure. 
• Zone 1 conditions are generally said to be absent from the normal lung under physiological conditions.
  • Even in the upright person, with the hilum 15cm below the apex of lung, a low-ish pulmonary arterial systolic pressure of 20mmHg (27 cm H₂O) will still be high enough at the apex (13 cm H₂O) to overcome atmospheric pressure.

What can increase the size of Zone 1?

• Haemorrhagic shock or hypovolemia generally
• Positive pressure ventilation: increased positive alveolar pressure can push blood out of the lung, creating a Zone 1. 

What would this look like clinically?

• Worsening hypoxia with increased PEEP
• Improving hypoxia with supine positioning
• Improved hypoxia with fluid resuscitation

What is West's Zone 2?

• "Zone 2 is that part of the lung between the levels at which arterial and alveolar pressure are equal, and venous and alveolar pressure are equal".
• Pulmonary arterial pressure exceeds alveolar pressure
• Alveolar pressure exceeds pulmonary venous pressure
• Blood flow is therefore dependent on the gradient between alveolar and pulmonary arterial pressure
• In diastole, particularly in conditions of hypovolemia, pulmonary arterial pressure may also be lower than the alveolar pressure, which means flow would only occur during systole

What is West's Zone 3?

• "Zone 3 is that part of the lung below the level at which venous and alveolar pressures are equal."
• Both pulmonary arterial and pulmonary venous pressure exceeds alveolar pressure
• Flow is proportional to the gradient between pulmonary arterial and pulmonary venous pressure (alveolar pressure does not play much of a role unless it exceeds pulmonary venous pressure)
• Blood flow to this zone exceeds the blood flow to all the other zones

What is West's Zone 4?

• Atelectatic or oedematous lung at the very base of the chest cavity, where interstitial fluid pressure exceeds pulmonary venous pressure.
• "...flow is reduced ... because the calibre of the extra-alveolar vessels is narrowed by the increased interstitial pressure, as a consequence of the reduced expansion of the dependent zones."