Chapter 137

### 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:

MV = VT × RR

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:

VA = (VT − VD) × RR where VD is dead space ventilation

or

VA = Vco2/PAco2

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/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).

###### FIGURE 137-1

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. 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 ...

Sign in to your MyAccess profile while you are actively authenticated on this site via your institution (you will be able to verify this by looking at the top right corner of the screen - if you see your institution's name, you are authenticated). Once logged in to your MyAccess profile, you will be able to access your institution's subscription for 90 days from any location. You must be logged in while authenticated at least once every 90 days to maintain this remote access.

Ok

## Subscription Options

### AccessAnesthesiology Full Site: One-Year Subscription

Connect to the full suite of AccessAnesthesiology content and resources including procedural videos, interactive self-assessment, real-life cases, 20+ textbooks, and more