Position and Ventilation

Explain the effect of changes in posture on ventilatory function

Altered patient position can cause significant changes to V/Q matching. In general:

  • The dependent lung has increased perfusion independent of anaesthesia, paralysis, or ventilation method
  • The independent lung has progressively more ventilation through each of:
    • Spontaneous ventilation
    • Controlled ventilation
    • Thoracotomy

Factors Affecting Distribution of Blood Flow

Key factors:

  • Gravity
  • HPV
  • Anatomy Blood flow is greater in central than peripheral portions.
  • Airway pressures
    Due to effects on:
    • Volume
      • Altering vascular distension and subsequently PVR
      • Alterations is extra-alveolar and intra-alveolar pressures at FRC may alter regional blood flow.
    • Right ventricular afterload
      • Lung volume
  • When both lungs are being ventilated, V/Q matching can be improved with selective application of PEEP to the dependent lung, which improves compliance

Factors Affecting Distribution of Ventilation

Key factors:

  • Gravity
    Intrapleural pressure is more negative in the elevated portions of the lung, due to the weight of the lung pulling down. This results in:
    • Larger intra-alveolar volume of the upper portion of the lung, and reduced compliance
    • Smaller intra-alveolar volume of the dependent portion of the lung, and improved compliance
    • Which results in the upper portion of the lung being higher on the compliance curve
    • Reducing the relative contribution of this portion of the lung to a tidal volume, and relatively reduced ventilation
      These changes may be exaggerated in disease, e.g. COPD.

Lateral Decubitus

Effects depends on the method of ventilation:

  • In the spontaneously ventilating patient:
    • Dependent lung ventilation and perfusion improves by ~10%
      Due to the effect of gravity.
    • Dependent lung corresponds more to West Zone 3
    • Non-dependent lung corresponds more to West Zone 2
  • In the anaesthetised patient:
    • FRC ↓ by ~20% in both lungs due to reduced muscle tone
      Each lung therefore changes in compliance:
      • Non-dependent lung falls to the steep portion of the curve, and compliance improves
      • The dependent lung falls from the steep position to the lower shallow segment, and compliance decreases
        Ventilation is now directed preferentially to the non-dependent lung.
      • Perfusion is unchanged
  • In the paralysed and positively-pressure ventilated patient:
    • FRC reduces further due to cephalad movement of:
      • Mediastinum
      • Abdominal organs
    • Ventilation is further directed to the non-dependent lung
    • Distribution of perfusion is unaffected, but the application of PEEP increases dead space and further worsens V/Q matching


Opening the non-dependent hemithorax causes:

  • No change in perfusion pressure
    Unless mediastinal structures move.
  • Variable effects on the lung:
    • If lung isolation:
      • Lung will collapse under its own elastic recoil
        Improved V/Q matching, as there is no ventilation, and perfusion will decrease as the lung collapses.
    • If no lung isolation:
      • Expansion of the lung, now unrestrained by the chest wall
        Worse V/Q matching.


  1. Dunn, PF. Physiology of the Lateral Decubitus Position and One-Lung Ventilation. Thoracic Anaesthesia. Volume 38(1), Winter 2000, pp 25-53.
  2. Graph from Benumof JL, ed. Anesthesia for thoracic surgery. 2nd ed. Philadelphia: WB Saunders Company, 1995.
  3. ANZCA August/September 2015
Last updated 2019-12-09

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