Question 10

Describe the ventilation / perfusion (V/Q) relationships in the upright lung according to West’s zones (40%). Explain the physiological mechanisms responsible for these relationships (60%).

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College Answer

This is a core aspect of respiratory physiology, and a detailed understanding of this topic is crucial to the daily practise of intensive care. As such the answers were expected to be detailed. Strong answers included precise descriptions of the zones of the lung as described by West and related these to the V/Q relationship in the upright lung. Generally, most candidates scored well in this section. Diagrams were of varying value. However, an impression from the examiners was that candidates spent too much time on this first section and ran out of time for a detailed answer in the second section. The answers to the second section seemed rushed and were often lacking in detail with many incorrect facts. This question highlights the importance of exam technique preparation in the lead up to the written paper.

Discussion

VQ relationships in West's zones:

  • Zone 1:alveolar pressure is higher than arterial or venous pressure;
  • Zone 2: alveolar pressure is lower than the arterial but higher than the venous pressure
  • Zone 3: both arterial and venous pressure is higher than alveolar
  • Zone 4: the interstitial pressure is higher than alveolar and pulmonary venous pressure (but not pulmonary arterial pressure)

Physiological mechanisms responsible for this:

  • Regional differences in perfusion 
    • Perfusion of the lungs occurs at a low pressure
    • Hydrostatic pressure of the column of blood therefore has a significant influence
    • This is affected by:
      • posture (upright vs supine)
      • gravity
      • Lung volume (atelectasis increases pulmonary vascular resistance)
      • Hypoxic pulmonary vasoconstriction
      • Gravity (affects the direction of the hydrostatic gradient)
      • Pulmonary vascular architecture (some lung units are structurally advantaged)
  • Regional differences in ventilation
    • Changes in the shape of the thoracic cavity occur unevenly (i.e. the base expands more than the apex)
    • Thus, regional ventilation differences develop (base usually better ventilated than apex)
    • This is affected by:
      • Gravity (the weight of the lung) which produces a vertical gradient in pleural pressure
      • Posture, which changes the direction of this vertical gradient
      • Anatomical expansion potential (i.e. bases have more room to expand than apices)
      • Lung compliance (more compliant lung regions, eg. lung bases, will be better ventilated at any given traspulmonary pressure
      • Pattern of breathing (voluntary deep vs. automatic diaphragmatic vs. mechanical ventilation)

References

West, John B. "Ventilation-perfusion relationships." American review of respiratory disease 116.5 (1977): 919-943.

West, J. B. "Regional differences in the lung." Postgraduate medical journal 44.507 (1968): 120.

Galvin, I., G. B. Drummond, and M. Nirmalan. "Distribution of blood flow and ventilation in the lung: gravity is not the only factor." British journal of anaesthesia 98.4 (2007): 420-428.

Shykoff, Barbara E., Albertus Van Grondelle, and H. K. Chang. "Effects of unequal pressure swings and different waveforms on distribution of ventilation: a non-linear model simulation." Respiration physiology 48.1 (1982): 157-168.

Glenny, Robb W., Steven McKinney, and H. Thomas Robertson. "Spatial pattern of pulmonary blood flow distribution is stable over days." Journal of Applied Physiology 82.3 (1997): 902-907.