F3v: Explain the significance of the vertical gradient of pleural pressure and the effect of positioning

Intrapleural pressure

Definition: pressure in space between visceral and parietal pleura (between lungs and chest wall)
Normal value: -5cm H2O average
- Value is negative due to: recoil of lungs, recoil of chest wall, pressure exerted by lymphatic system
- Note there is a difference between intrapleural pressure at apex compared to base – this is known as vertical gradient

Normal values:
- Apex: -10cm H2O
- Midzone: -5cm H2O
- Base: -3cm H2O
Differences in pleural pressure important due to intrapleural pressure relationship with transpulmonary pressure and compliance of alveoli
Example of upright patient
- Intrapleural pressure more negative in non-dependent lung at FRC
  - Apical alveoli = non-dependent areas = large intra alveolar volume (maximally inflated)
  - Basal alveoli = dependent areas = small intra alveolar volume (relatively deflated)
  - Consequence: basal alveoli more compliant and therefore have improved ventilation compared to apical alveoli

Positioning affects gravitational pull, mediastinal content and abdominal content and therefore it influences the vertical gradient
Overall it impacts alveoli compliance

Vertical gradient is greatest
As described above:
- Apex: maximally inflated. Less compliant. Will have minimal change in ventilation with increased pleural pressure
- Base: relatively deflated. Most compliant. Will have improved ventilation with increased pleural pressure

Vertical gradient reduces by 30%
- Due to gravitational force maldistribution being much less
- Pleural pressure more evenly distributed
- Decreased FRC and improved compliance of alveoli throughout

Vertical gradient decreases and is the smallest compared to any other position (2cm H2O)
Compliance improved, lung resistance decreases
Overall pleural pressure in lung unit more homogenous and therefore less likely to develop ventilator associated lung injury from overdistension
This position is typically used as a rescue measure for patients with issues refractory hypoxaemia

Different between lung units due to mediastinal content
In dependent lung – less negative
- Improved compliance in these lung areas and therefore improved ventilation
- Perfusion also affected and improves VQ mismatch lung unit
In non-dependent lung – more negative
- Decreased compliance, decreased ventilation, decreased perfusion

Author: Suzanne Luong