is most commonly a result of atelectasis, partial or complete airway obstruction.
The other extreme, a V/Q ratio of infinity, is dead space ventilation.
V Alveolar gas exchange
A Composition of gases
The composition of gases in a mixture can be described by their fractional composition or their partial pressures.
The composition of gas within the alveoli is determined by the movement of gas into and out of the alveoli via the airways or across the alveolar capillary membrane and into or out of the pulmonary capillaries.
B Movement of gases
Bulk transfer describes the movement of gases during inspiration and expiration within the proximal large airways.
Diffusion is the passive movement of gases down the concentration gradient in the distal small airways to the alveolus. It is the process by which gases move (i) in and out of the alveoli into the terminal airways, (ii) across the alveolar capillary membrane, and (iii) between the blood and tissues.
C Factors influencing diffusion
The surface area available for diffusion.
The physical properties of the gas.
The thickness of the air‐blood barrier.
The driving pressure of the gas between the alveolus and capillary blood as described by Fick's law of diffusion.Fick's Law Vgas = the volume of gas transferred across a membrane or barrier.A = the area available for diffusion.T = the membrane thickness.D = a diffusion constant that is dependent on the physical properties of the gas.P1‐P2 = the partial pressure difference of the gas across the membrane.Note: CO2 is approximately 20 times more soluble than O2, and therefore its diffusion across a membrane is less likely to be impaired, relative to O2, by a change in membrane thickness.
In the normal lung, equilibration of O2 and CO2 across the alveolar capillary membrane occurs within 0.25 seconds; approximately one third of the time the blood is in the capillary.
D Carbon dioxide
Carbon dioxide is the end product of aerobic metabolism. There is a continuous gradient of CO2 from the mitochondria in peripheral cells to venous blood and then to the alveolar gas.
Carbon dioxide is transported in the blood in several forms including:Dissolved in physical solution (~5%).As carbonic acid (~90%).Combined with proteins (~5%) such as carbaminohemoglobin.
Carbon dioxide moves from the blood to the alveoli in its dissolved form only.
Alveolar CO2 partial pressures are directly proportional to CO2 production and indirectly proportional to alveolar ventilation. VaCO2 = Rate of CO2 production.VA = Alveolar ventilation.
Clinically, the adequacy of alveolar ventilation is evaluated by measuring arterial CO2 partial pressures (PaCO2).
The normal values for PaCO2 and PACO2 are between 35 and 45 mmHg.
E Oxygen
The partial pressure of oxygen in the alveoli can be determined using a simplified version of the alveolar gas equation:FiO2 = Fraction of inspired oxygen ≈ 0.21.PB = Atmospheric pressure (760 mmHg at sea level).PH2O = Water vapor pressure (mmHg) in airway (~50 mmHg at body temp of horse).R = Respiratory gas exchange ratio (~0.8).
This calculation emphasizes the significance of FiO2 and PaCO2 on the alveolar gas partial pressure.
Clinically, this equation highlights the significance of O2 supplementation for patients with impaired ventilation.
Example 1. Horse breathing room air (21% O2) with a PaCO2 = 35 mmHg.
Example 2. Horse breathing 100% O2 with a PaCO2 = 35 mmHg.
Example 3. Anesthetized horse hypoventilating on room air (PaCO2 = 70 mmHg)
Although this horse is hypoxemic, a PaO2 of <60 mmHg is fairly typical for anesthetized horses breathing room air.
This example emphasizes how an increase in PaCO2 affects PAO2 and hence PaO2.
Example 4. Anesthetized horse hypoventilating on 100% oxygen (PaCO2 = 70 mmHg)
This example emphasizes the importance of providing O2 during anesthesia to prevent hypoxemia.
F Alveolar‐arterial oxygen gradient [P(A-a)O2]
A small gradient in oxygen partial pressure normally exists between the alveoli (A) and the arterial blood (a).
This gradient is due to:Normal physiologic shunting of blood through bronchial and coronary veins that drain deoxygenated blood directly into the left side of the heart.Normal ventilation‐perfusion gradients within the lung.
The magnitude of the gradient (A‐a) can be calculated using the alveolar gas equation and by measuring the PaO2.
Knowledge of the magnitude of the difference in the P(A‐a)O2 can indicate whether a functional deficit in O2 exchange exists.
The significance of a calculated gradient is, however, dependent on the FiO2.
Examples:A normal horse breathing room air, the P(A‐a)O2