MINUTE AND ALVEOLAR VENTILATION
Minute ventilation (MV) is the amount of air inspired in one breath (tidal volume = VT) 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:
where VD is dead space ventilation
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 (VA).
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.
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 143-1).
West zones in the upright patient. (Reproduced with permission from Levitsky MG. Pulmonary Physiology, 8th ed. New York: McGraw-Hill; 2013.)
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. Extra-alveolar 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.