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
    Via:
    • 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

Thoracotomy

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.

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