Zone 1 2 3 4 of west
zones of West: zone I-PA>Pa>Pv zone II-Pa>PA>Pv zone III-Pa>Pv>PA zone IV- Pa>Pi>Pv>PA where: pa is alveolar pressure, pa is arterial pressure, Pv is venous pressure, and pi is interstitual pressure. Normal physiologic optimalzone is 3. In zone 3 arterial pressure exceeds venous pressure maintaing forward flow of blood and both arterial and venous pressures exceed the alveoli pressure. Blood flow is contunual throughout the alveolar ventilation. Disease states such as ards and cHF may increase the interstitual pressures impeeding flow flow leading to VQ mismatch. Over distension of alveoli also may increase alveolar pressures impeeding blood flow leading to VQ mismatch.