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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