West zones

**Definition:** John West's model divides the upright lung into three (sometimes four) zones. Perfusion depends on the relative magnitudes of alveolar pressure (PA), pulmonary arterial pressure (Pa), and pulmonary venous pressure (Pv).

**Zone 1 (apex): PA > Pa > Pv** - Alveolar pressure compresses the capillaries, so there is no flow (alveolar dead space). - Does not exist in normal lungs, because Pa is enough to perfuse the apex. - Occurs when Pa falls (hemorrhage, hypovolemic shock) or PA rises (positive-pressure ventilation, high PEEP).

**Zone 2 (middle): Pa > PA > Pv** - Flow is determined by Pa − PA, not by Pa − Pv (the "waterfall" or "Starling resistor" effect). - Venous pressure does not affect flow here.

**Zone 3 (base): Pa > Pv > PA** - Flow is determined by Pa − Pv (the usual arteriovenous gradient). - Capillaries are distended and perfusion is continuous. - Zone 3 is the largest zone at rest. With exercise, recruitment and distension make more of the lung behave as Zone 3, so apical perfusion improves.

**Zone 4 (base, sometimes described):** Interstitial pressure rises, so resistance rises and flow falls. - Seen in pulmonary edema, low lung volumes (near residual volume), and hypoxic vasoconstriction. - Extra-alveolar vessels are compressed.

**Regional V/Q (upright):** - Both ventilation and perfusion are greater at the base, but perfusion increases more steeply than ventilation. - Apex: V/Q ≈ 3 (wasted ventilation, higher PaO₂, lower PaCO₂). This favors reactivation of *M. tuberculosis* (aerobic organism) at the apex. - Base: V/Q ≈ 0.6 (shunt-like, lower PaO₂, higher PaCO₂). - Overall V/Q ≈ 0.8.

**Clinical pearls:** - Mechanical ventilation with high PEEP increases Zone 1 and dead space, and can reduce cardiac output. - Hemorrhagic shock increases Zone 1. - In a supine patient, zones are distributed from anterior (Zone 1/2) to posterior (Zone 3). - Exercise lowers V/Q heterogeneity because of recruitment of apical capillaries. - Pulmonary vascular resistance (PVR) is lowest at functional residual capacity (FRC). - Pulmonary edema is more likely to appear at the bases, where hydrostatic pressure is highest.