Canonical Question
Resp Mechanics – Positioning
Master answer
Vertical gradient of pleural pressure and alveoli
Inspiration
- Inspiratory muscle contraction → Overcomes resistance to insp flow → negative alveolae pressure -1cm H2O → VT 500ml
- Weight of lung on alveoli and pleura above:
- causes gradient of pleural pressure: Apex (-10cmH2O) Base (-2.5cm H2O)
- Apical alveoli larger
- But compliance lesser (due to baseline distension)
- Lesser ventilation for same pressure
- Apex and base on different part of compliance curve
Expiration:
- Gradient of pleural pressure smaller
- Apex: -4cm H2O → on steeper part of compliance curve → ventilation better
- Base +3.5cm H2O →airway closure → gas trapping & shunt
Ventilation, perfusion, V/Q matching
Vertical segments based on V and Q
- Both V and Q ↓ as you move from base to apex.
- ↓ total number of alveoli → ↓ diffusive area → ↓ V
- ↑ compression of intra-alveolar vessels → ↑ west zone 1 → ↓ Q
- V ↓’s slower than Q
| Level | V/Q | Ventilation | Perfusion | PO2 | PCO2 |
|---|---|---|---|---|---|
| Apex | ~3 High | Lower Alveoli largest (no wt of lung) | Very Low Blood hydrostatic pressure low (high gravity) | High 132 | Low 28 |
| At level of heart | ~1 | Slightly more ventilated (Optimized) | Optimized (less effect of gravity) | 108 | 39 |
| Base | ~0.67 towards mixed venous point | Well ventilated (in normal lung): small alveoli, more compliance (effect of wt of lung) | Highly perfused Maximal Hydrostatic pressure | relativly hypoxic 89 | relatively hypercapnoeic 42 |
West Zones:
- Divided into zones based on relationship between Alveolar pressure (PA) Arterial pressure (Pa) Venous pressure (Pv) Interstitial pressure (Pi)
- West Zones 1,2,3: Due to changes in hydrostatic pressure when pumping to top of lung vs. bottom of lung
| West Zone | Pressure Relationships | Physiology | Location |
|---|---|---|---|
| Zone 1 | PA > Pa > Pv | No flow of blood, as arterial pressure completely opposed by alveolar pressure | not seen in normal lung |
| Zone 2 | Pa > PA > Pv | Resistance to flow is determined by alveolar pressure (Starling resistor effect) | about 3cm above the heart |
| Zone 3 | Pa > Pv > PA | Resistance to flow is determined by venous pressure Venous pooling causes increased distension of pulmonary capillaries | majority of healthy lung |
| Zone 4 | Pa > Pi > Pv > PA | Low lung volume causes narrowing of extra-alveolar vessels | Lung bases at low lung volume or in pulmonary edema |
JC 2019
Exam appearances
| Exam | Exact wording | Relationship | Success |
|---|---|---|---|
| 2016A Q03 | Compare the physiology of the apex of the lung with the base of the lung in the upright position. | historical_member | — |