Alveolar pressure dominates at the apex, arterial pressure at the base
Mnemonic
Three zones, defined by the relationship between alveolar (A), arterial (a) and venous (v) pressures:
- Zone 1: PA > Pa > Pv. No flow, so alveolar dead space. Does not exist in health, but appears with hypotension or positive pressure ventilation
- Zone 2: Pa > PA > Pv. Flow depends on the arterial to alveolar difference, the waterfall effect
- Zone 3: Pa > Pv > PA. Continuous flow, the normal state at the base
“Zone 1 is dead space, zone 3 is the base”, and the whole lung shifts towards zone 3 on lying down and towards zone 1 on standing or with raised airway pressure.
A fourth zone at the extreme base has increased interstitial pressure and therefore reduced flow.
The practical consequences: a pulmonary artery catheter must sit in zone 3 for the wedge pressure to reflect left atrial pressure, and prone positioning in ARDS works partly by making ventilation and perfusion more uniform.
Expansion
The zones are defined by the relationship between alveolar (A), arterial (a) and venous (v) pressures:
- Zone 1 (apex): PA > Pa > Pv. Capillaries are compressed and there is no flow; this is alveolar dead space
- Zone 2 (mid): Pa > PA > Pv. Flow is intermittent, determined by the arterial to alveolar difference, described as a waterfall effect
- Zone 3 (base): Pa > Pv > PA. Continuous flow determined by the arteriovenous difference, as in a normal circulation
Zone 1 does not exist in the healthy upright lung, because pulmonary artery pressure just exceeds alveolar pressure at the apex. It appears when arterial pressure falls (haemorrhage, hypotension) or alveolar pressure rises (positive pressure ventilation, high PEEP), converting apical lung into dead space.
A practical corollary: a pulmonary artery catheter must be sited in zone 3 for the wedge pressure to reflect left atrial pressure, since only there is the vascular column continuous.