Chapter 40: Ventilation and Perfusion

INTRODUCTION

Ventilation is the movement of air between the atmosphere and alveoli, and the distribution of air within the lungs to maintain adequate levels of oxygen and carbon dioxide in the blood. Perfusion is delivering of blood to capillary beds in the lungs. The relationship between ventilation and perfusion is important in maintaining adequate levels of oxygen in the blood. The normal ratio of ventilation (V) and perfusion (Q) is 0.8.

VENTILATION

Ventilation involves two phases, inspiration and expiration. Spontaneous ventilation starts with the expansion of the chest wall, generating a negative pressure gradient for air to follow. Expiration is a passive process. As the diaphragm relaxes, chest volume decreases, increasing air pressure inside the lungs. Higher-pressure air inside the lungs rushes out towards lower pressure air outside. Once an air mixture reaches the alveoli, each molecule follows a gradient for passive diffusion. Following concentration gradients from high to low, oxygen diffuses from alveoli to lungs and carbon dioxide from blood to alveoli.

Minute Ventilation (V_E)

V_E is the total volume of air expired in 1 minute, and is the product of the respiratory rate (RR) and tidal volume (V_T). Normal tidal volume is roughly 7 mL/kg of body weight.

Alveolar Ventilation (V_A)

V_A is the volume of gas expired from the alveoli to the outside of the body per minute. It is equal to tidal volume (V_T) minus the anatomical dead space. Anatomical dead space is the portion of the airways that does not participate in gas exchange, including conducting airways such as the nose, pharynx, larynx, and trachea and conducting bronchi.

where the anatomical dead space is about 150 mL in an average adult, or 2.2 mL/kg.

Alveolar Gas Equation (AGE)

The AGE relates the alveolar partial pressure of oxygen (P_AO₂), the fraction of inspired gas that is oxygen (F_iO₂,), the arterial partial pressure of carbon dioxide (P_aCO₂), and the respiratory quotient (R). It allows the calculation of the alveolar partial pressure of oxygen from data that is practically measurable.

where P_aCO₂ approximates P_ACO₂ due to the rapid diffusion of CO₂, approximately 40 mmHg, F_iO₂ is the fraction of inspired oxygen, 0.21 if breathing room air, P_atm is the atmospheric pressure, 760 mmHg at sea level, P_H2O is the water vapor pressure, 47 mmHg, and R is the respiratory quotient, CO₂ eliminated/CO₂ consumed (V_CO2/V_O2) = 0.8.

In a normal individual breathing room air:

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