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 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:
- 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:
- 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
Opening of a non-dependent hemithorax causes:
- Increased compliance and FRC of the non-dependent lung
- Reduced compliance and FRC of the dependent lung
- Dunn, PF. Physiology of the Lateral Decubitus Position and One-Lung Ventilation. Thoracic Anaesthesia. Volume 38(1), Winter 2000, pp 25-53.
- Graph from Benumof JL, ed. Anesthesia for thoracic surgery. 2nd ed. Philadelphia: WB Saunders Company, 1995.
- ANZCA August/September 2015