West Zones of the Lung

West zones of the lung

Blood flow throughout the lung is dictated by gravity, with a pressure gradient that is higher at the base than at the apex when in an upright position. The pressure relationships between the alveoli and the arteriolar and venous systems describe the West zones of the lung. In the upright patient, these three vertically divided zones explain how pressure relationship affect perfusion and ventilation.



Zone 1: PA > Pa > Pv. The alveolar pressure exceeds that of the arterial and venous pressure. Poor perfusion occurs as vessels collapse, leading to dead space. This zone doesn’t normally exist, as Pa should be > PA in normal, healthy lungs. It develops most commonly in patients on positive pressure ventilation.
Zone 2: Pa > PA > Pv. The arterial pressure will exceed that of the alveolar pressure but not the venous pressure. In this region, blood flows in pulses.
Zone 3: Pa > Pv > PA. Both the arterial and venous pressures exceed the alveolar pressure



Ventilation/perfusion ratio: 1 > 2 > 3. The vertical change in V/Q ratios in the lung is because although both ventilation and perfusion increase from top to bottom of the lung, perfusion increases much quicker than ventilation. Thus the V/Q ratio at the top of the lung is 3.3 whereas at the bases it is around 0.6

So, why is this important? Positioning can greatly affect ventilation and perfusion of the ill patient.

The lean-forward position is a preferred position for patients who have chronic obstructive pulmonary disease (COPD) because of the marked reduction in inspiratory effort. The lean forward position allows for maximal excursion of the diaphragm and less dependence on scalene and sternocleidomastoid muscles. Alternatively, patients with COPD may benefit from trendelenburg positioning as the diaphragm is allowed to elevate more adequately during expiration, than in patients in the supine position.

The prone position has also been found to increase ventilation and perfusion, for the functional reasons mentioned above. Careful monitoring is required in such instances because in the event of an emergency, the patient must be readily repositioned into supine for access and intubation.

Although it is not the preferred position for patients, ventilation, perfusion and gas exchange can be theoretically improved in the supine position by increasing cardiac output. However, due to a decreased functional residual capacity (FRC), patent airways and gas exchange are compromised. One needs to consider the age, weight, and smoking habits of the patient as these may also contribute to decreased FRC and airway closure.

Lateral positioning may also be used to affect gas exchange, particularly in those patients with unilateral lung disease. When laterally positioning the patient with unequally diseased lungs, care must be taken to keep the healthy lung in the inferior position. This will maximize perfusion to dependant areas and ventilation in non-dependant areas. Even in patients with healthy lungs or equally distributed lung disease, PaO2 can be increased in the right lung down, as compared to patients positioned with the left lung down. This is thought to be a result of the smaller left lung volume and cardiac compression.

Moral of the story is, think about the West zones, especially for ventilated patients, and maximize patient positioning prior to altering ventilator settings to improve ventilation and oxygenation.

Mina Saeed and Kathy Wollner, MS4