Abstract
In the upright position, ventilation and perfusion both increase from the top to the bottom of the lung. This was previously attributed to the effect of gravity (the so-called gravitational model), but it is now thought that structural similarities between the pulmonary arteries and bronchioles contribute (see Chapter 15).
What are the West zones of the lung?
In the upright position, ventilation and perfusion both increase from the top to the bottom of the lung. This was previously attributed to the effect of gravity (the so-called gravitational model), but it is now thought that structural similarities between the pulmonary arteries and bronchioles contribute (see Chapter 15).
J. B. West built on a gravitational model of ventilation and perfusion. This assumes that capillary blood flow to the alveolus is dependent on the pressure of the gas within the alveolus. This is particularly important in anaesthesia, as positive-pressure ventilation significantly alters alveolar pressure. West divided the upright lung into three vertical zones, numbered 1 (at the apex) to 3 (at the base). The arterial, venous and alveolar pressures differ in each zone, which has implications for the V̇/Q̇ ratio.
How do the changes in arterial, venous and alveolar pressures affect alveolar perfusion?
The variation in alveolar perfusion in the three West zones (Figure 16.1) is most easily explained by starting from West zone 3, at the base of the lung:
In West zone 3, Pa > Pv > PA (a: arterial; v: venous; A: alveolar). Both arterial and venous pressures are greater than alveolar pressure. This is because of the effects of gravity on alveolar volume. The lungs are suspended superiorly in the chest from the large airways and therefore there is little weight acting upon the base of the lung. For this reason, the basal alveoli are not particularly distended and thus sit upon a more compliant part of the pressure–volume loop. As the alveoli occupy a small volume, they exert minimal extramural pressure on the pulmonary vasculature. Capillary blood flows continuously throughout the cardiac cycle – flow is dependent on the arterial–venous pressure difference, which is generated by the right ventricle. West zone 3 is how normal, healthy lungs behave below the level of the hilum.
In West zone 2, Pa > PA > Pv. As the lung is ascended, there is an increasing effect of the weight of the lung. Alveoli are pulled open and become less compliant. PA therefore increases and the lung exerts increased extramural pressure on the pulmonary vasculature: alveolar pressure thus exceeds venous pressure, causing compression of the venous end of the pulmonary capillary. Capillary blood flow is therefore dependent on the arterial–alveolar pressure difference. Systolic pulmonary arterial pressure is greater than alveolar pressure, but diastolic pulmonary arterial pressure is not – blood therefore only flows through the pulmonary capillary during systole. The intermittent nature of blood flow causes a mismatch between alveolar ventilation and perfusion. Thus, the V̇/Q̇ ratio is higher in West zone 3, with an increased alveolar dead space and, consequently, wasted ventilation. West zone 2 is how normal lungs behave between the apex and the hilum.
In West zone 1, PA > Pa > Pv. Alveolar pressure exceeds systolic pulmonary arterial pressure as the alveoli are maximally distended by the weight of the whole lung. The pulmonary capillary is completely compressed by the alveolus, with alveolar perfusion ceasing. The apical alveoli are still ventilated – V̇/Q̇ ratio is therefore high, with a high alveolar dead space. West zone 1 does not exist in normal lungs.
