Position and ventilation

Explain the effect of changes in posture on ventilatory function

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

Lateral Decubitus

In the lateral position in a spontaneously ventilating patient:

  • Dependent lung ventilation improves by ~10%
    Due to impaired compliance of the non-dependent lung (it hyperinflates) and improved compliance of the dependent lung (it is less expanded).
    • Dependent lung corresponds more to West Zone 3
    • Non-dependent lung corresponds more to West Zone 2
  • Dependent lung perfusion improves by ~10%
    Due to the effect of gravity.

In the lateral position in a positive-pressure ventilated patient:

  • The majority (~55%) of the tidal volume is delivered to the non-dependent lung
  • The majority of pulmonary blood flow is delivered to the dependent lung
  • The compliance of the dependent lung falls due to compression from the:
    • Mediastinum
    • Abdominal organs
      These move cephalad in a paralysed patient.
  • The dependent lung typically receives greater blood flow due to the effect of gravity
    • This may worsen V/Q matching
    • Blood flow is also affected by:
      • HPV
      • Anatomical factors
        Blood flow is greater in central than peripheral portions.
      • Lung volume
        Alterations is extra-alveolar and intra-alveolar pressures at FRC may alter regional blood flow.
  • When both lungs are being ventilated, V/Q matching can be improved with selective application of PEEP to the dependent lung, which improves compliance

Thoracotomy

Opening of a non-dependent hemithorax causes:

  • Increased compliance and FRC of the non-dependent lung
  • Reduced compliance and FRC of the dependent lung

References

  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 2017-09-20

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