West Zones

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2021B Q10

Exam question

Describe the ventilation / perfusion (V/Q) relationships in the upright lung according to West’s zones (40%). Explain the physiological mechanisms responsible for these relationships (60%).

CICMWrecks answer

Master answer

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

Diagram used in Pulmonary Circulation — Master Answer — Legacy CICMWrecks figure · Source verification pending.

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

Regional lung compliance — CICMWrecks original diagram

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

Vertical distribution of ventilation and perfusion

Diagram used in VQ – West’s — Master Answer — Legacy CICMWrecks figure · Source verification pending.

JC 2019

Answer by JC

Past papers

Exam appearances

1 appearance

Exam	Exact exam wording	Candidate success
2021B Q10	Describe the ventilation / perfusion (V/Q) relationships in the upright lung according to West’s zones (40%). Explain the physiological mechanisms responsible for these relationships (60%).	47%