Lung zone and pneumonic
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The lung has distinct zones based on the relationship between arterial pressure (Pa), alveolar pressure (PA), and venous pressure (Pv). The classic mnemonic is the West Zones (1-4), describing blood flow distribution. Zone 1: PA > Pa > Pv (no flow), Zone 2: Pa > PA > Pv (intermittent flow), Zone 3: Pa > Pv > PA (continuous flow), Zone 4: Pa > Pv > PA but with extra-alveolar vessel compression (reduced flow).
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West zones of lung blood flow distribution diagram
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West Zones of the Lung
The Classic Diagram
From Morgan & Mikhail's Clinical Anesthesiology, 7e - Figure 23-15. Left (A): Classic 4-zone model in upright lung. Right (B): "Onion-like" in vivo perfusion scanning showing central-to-peripheral distribution in addition to the gravitational gradient.
The Core Concept
Pulmonary blood flow is non-uniform. Dependent (lower) areas of the lung receive more blood flow than non-dependent (upper) areas, due to a gravitational gradient of ~1 cm H₂O per cm lung height. The low-pressure pulmonary circulation is very susceptible to this gravity effect. The interplay between three pressures determines flow in each region:
| Symbol | Meaning |
|---|---|
| Pa | Pulmonary arterial pressure |
| PA | Alveolar pressure |
| Pv | Pulmonary venous pressure |
| PIS | Pulmonary interstitial pressure (Zone 4) |
The 4 West Zones
Zone 1 - Apex (Top) - "Collapse" / Dead Space
Pressure relationship: PA > Pa > Pv
- Alveolar pressure exceeds arterial pressure - capillaries are compressed and collapsed
- No blood flow - creates alveolar dead space (ventilated but not perfused)
- In healthy people, Zone 1 is very small (Pa is just barely able to reach the apex)
- Enlarges with: positive pressure ventilation (raises PA), hypovolemia/hemorrhage (reduces Pa)
Mnemonic for Zone 1: "No flow at the TOP" - Dead zone, Dead space
Zone 2 - Middle - "Waterfall" / "Sluice"
Pressure relationship: Pa > PA > Pv
- Arterial pressure now exceeds alveolar, so capillaries open
- But PA > Pv, so the vessel behaves like a Starling resistor (collapsible tube in a pressure chamber)
- Flow is determined by Pa - PA (NOT Pa - Pv)
- Also called the "waterfall effect" - flow is independent of downstream (venous) pressure
- Blood flow increases going downward because Pa increases with gravity while PA stays constant
- Comprises most of the lung in upright position
Mnemonic for Zone 2: "Waterfall" - flow depends on Artery minus Alveolus
Zone 3 - Base - "Distention" / Continuous Flow
Pressure relationship: Pa > Pv > PA
- Both Pa and Pv exceed alveolar pressure - continuous blood flow
- Flow is now determined by Pa - Pv (the normal driving pressure)
- Blood flow still increases going downward, but now because of progressive capillary distension (and recruitment) as transmural pressure rises - not because of increased driving pressure
- Largest zone; most perfused; most efficient gas exchange
Mnemonic for Zone 3: "Distention" - flow depends on Artery minus Vein (normal)
Zone 4 - Most Dependent Base - "Interstitial pressure"
Pressure relationship: Pa > PIS > Pv > PA
- The extra-alveolar vessels (not alveolar capillaries) become narrow because alveoli at the very base are less expanded (due to the weight of the lung above)
- Increased contribution of extra-alveolar vascular resistance = reduced blood flow despite high Pa
- Flow depends on Pa - PIS (interstitial pressure, not alveolar)
- Worsened by: low lung volume, interstitial edema (cuffing of vessels), vasoconstrictors (serotonin)
- Improved by: vasodilators (isoproterenol), increased lung volume
Mnemonic for Zone 4: "Interstitial squeeze" - flow drops at the very Bottom
Master Mnemonic Table
| Zone | Location | Relationship | Flow | Key Feature |
|---|---|---|---|---|
| 1 | Apex | PA > Pa > Pv | None | Dead space |
| 2 | Mid | Pa > PA > Pv | Intermittent / "Waterfall" | Pa - PA drives flow |
| 3 | Base | Pa > Pv > PA | Continuous | Pa - Pv drives flow; distension |
| 4 | Most dependent | Pa > PIS > Pv > PA | Reduced | Extra-alveolar compression |
Useful Mnemonics
"1 has Nothing, 2 is Waterfall, 3 is Distended, 4 is Interstitial"
For pressure order memory (Zones 1 → 3):
- Zone 1: Alveolar biggest → "1 is the lone alveolus crushing everything"
- Zone 2: Artery > Alveolus > Vein → "AaV = Artery Alone Victorious (over alveolus and vein)"
- Zone 3: Artery > Vein > Alveolus → "AVA = Artery Vein Alveolus (both vascular > alveolar)"
"No Water Distends Interstitium" → Zone 1 (No flow), 2 (Waterfall), 3 (Distention), 4 (Interstitial pressure)
Clinical Relevance
| Clinical Scenario | West Zone Effect |
|---|---|
| Positive pressure ventilation (PPV) | Expands Zone 1 (raises PA) → more dead space |
| Hypovolemia / shock | Expands Zone 1 (reduces Pa) → dead space ↑ |
| PA catheter tip placement | Ideal position = Zone 3 (wedge pressure accurately reflects LAP only in Zone 3) |
| Interstitial pulmonary edema | Expands Zone 4 upward; may invert flow (apex > base) |
| Lateral decubitus position | Dependent lung = Zone 3-dominant (better perfused); useful for unilateral lung disease |
| Mitral stenosis (chronic) | Zone 4 expansion → blood flow inverted (apex > base on scan) |
Sources:
- Morgan & Mikhail's Clinical Anesthesiology, 7e, pp. 943-944
- Murray & Nadel's Textbook of Respiratory Medicine, pp. 226-228 (West JB, Dollery CT, Naimark A, J Appl Physiol 1964)
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