Chapter 137: Ventilation and Perfusion

MINUTE AND ALVEOLAR VENTILATION

Minute ventilation (MV) is the amount of air inspired in one breath (tidal volume = V_T) multiplied by respiratory rate (RR), where:

Ventilation can also be expressed as alveolar ventilation, or the amount of air that enters the alveoli and is thus available for gas exchange. Alveolar ventilation can be expressed as:

or

This equation states that alveolar Pco₂ (PAco₂) is directly proportional to the amount of CO₂ produced by metabolism and delivered to the lungs (Vco₂) and inversely proportional to the alveolar ventilation (V_A).

VENTILATION/PERFUSION

Ventilation can be described as the amount of air that reaches the alveoli. Perfusion is the amount of blood that reaches the alveoli. Ideally, ventilation matches perfusion, which allows equal exchange of O₂ and CO₂. In reality, different anatomic regions of the lung receive unbalanced perfusion and ventilation due to gravitational and nongravitational forces.

Zones of Lung

An understanding of the west zones of the lung is essential to comprehend both lung perfusion and ventilation. West zones describe areas of the lung based upon variations in pulmonary arterial pressure (PAP), pulmonary venous pressure (PVP), and alveolar pressure (AP). These differences result from a 20 mm Hg increase in blood flow found in the base of the lung relative to the apex as a result of gravity in an upright patient. While this pressure gradient is less apparent in the supine position, gravitational forces still lead to a greater degree of perfusion in the posterior lung than the anterior aspect (Figure 137-1).

Although gravity has a major impact on regional lung perfusion differences, recent research has highlighted the influence of nongravitational forces. Specifically, intrinsic features of the lung during inspiration also play a role in altering lung perfusion. Extraalveolar vasculature expands with inspiration due to radial traction, which may lead to increased blood flow even as alveolar pressure increases.

The perfusion dynamics of each zone are as follows:

• Zone 1: AP > PAP > PVP

• Zone 2: PAP > AP > PVP

• Zone 3: PAP > PVP > AP

where AP = arterial pressure; PAP = pulmonary artery pressure; PVP = pulmonary vein pressure.

Zone 1 is defined by high AP, which may ...