Wests Lung Zones

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  • Perfusion in normal upright lung varies from apex to base by about 3x due to hydrostatic pressure differences
  • During exercise these regional differences become less due to flow being redirected towards mid & upper zones
  • Lung volume alters vessel resistance:
  • Alveolar vessels are compressed by increasing volumes, increasing vascular resistance
  • Extra-alveolar vessels are tractioned by increased volumes, decreasing vascular resistance
  • For flow to occur, Pa must be at least several cmH2O > Pv
  • Regional perfusion variation with hydrostatic pressure gives the 3 west zones:
  • Zone 1: PA > Pa > Pv - No flow possible! Does not occur under normal conditions, but may occur if arterial pressure is reduced (eg. haemorrhage) or if alveolar pressure is raised (IPPV). Zone 1 lung is alveolar dead space.
  • Zone 2: Pa > PA > Pv - Flow proportional to Pa - PA gradient
  • Zone 3: Pa > Pv > PA - Flow proportional to Pa - Pv gradient
  • Zone 2 acts as a starling resistor (waterfall effect), as flow is determined by a-A pressure gradient - not the normal a-v gradient
  • Zone 2 - Pa increases down the zone causing recruitment of additional vessels
  • Zone 3 - normal a-v gradient determines flow. Distension of already opened capillaries is the main way of increasing flow, although there is some new recruitment
  • Zone 4 - a region of reduced blood flow due to narrowing of extra-alveolar vessels at low inflation states due to a lack of distension by surrounding lung tissue
  • There are also random anatomical variations in lung vessel/capillary arrangements, and some decrease in perfusion towards the lung peripheries