Ventilation to Perfusion Ratios & Defects
The ventilation/perfusion (V/Q) ratio is an indication of how well alveolar ventilation matches pulmonary capillary perfusion.
Due to gravitational forces, the V/Q ratio ranges regionally in the lung, from 3.0 at the apex to 0.6 at the base.
Clinicians and physiologists typically use the average value for the entire lung as a reference point.
Healthy average alveolar ventilation rate is approximately 4 liters of air/minute.
Blood flow rate is approximately 5 liters of blood/minute.
A healthy V/Q for the entire lung is 0.8.
Inspired air flows through the
tracheobronchial tree and to the thin-walled, sac-like alveoli. Pulmonary capillaries are in close physical proximity.
When the V/Q is 0.8, ventilation and perfusion are well matched and optimal gas exchange occurs (not perfect, which would be V/Q = 1; we'll use the average for the entire lung).
The partial pressures of oxygen and carbon dioxide in the alveoli and pulmonary blood flow equilibrate (the specifics of gas exchange are discussed elsewhere).
V/Q mismatches, aka, defects, occur along a spectrum.
Two extremes of mismatches are shunts and dead space:
- Shunts occur when the rate of alveolar ventilation is zero; V/Q = 0
- Dead spaces occur where the rate of blood flow is zero; V/Q = infinity
Notice that the normal V/Q of 0.8 is between the two extremes (a V/Q of 1 would be perfect, but we are using the average value)
Mismatches between 0 and 0.8 reflect defects in
alveolar airflow.
Mismatches greater than 0.8 reflect defects in
pulmonary capillary blood flow.
A shunt is the most extreme alveolar ventilation defect; it occurs when air flow is blocked and the alveoli remain unventilated.
Unventilated alveoli cannot participate in gas exchange with the pulmonary capillaries, which remain de-oxygenated.
As a result, the partial pressures of oxygen and carbon dioxide of the pulmonary blood remain equal to that of mixed venous blood.
Airway obstruction is a common cause of shunts
Because airflow ceases, alveolar collapse, aka, atelectasis, can occur in all or a portion of the lung.
Without proper ventilation, the partial pressure of arterial oxygen is reduced (aka, hypoxemia).
In the case of shunts, hypoxemia cannot be reversed by administration of concentrated oxygen because oxygen does not reach the pulmonary blood flow.
In this case, the alveoli are partially ventilated, so some gas exchange occurs with the nearby pulmonary capillary.
When ventilation is less than perfusion, the partial pressure of pulmonary blood carbon dioxide increases because it is held within the body, and the partial pressure of oxygen decreases because the normal amount of fresh oxygen is not available.
Reduced perfusion causes an increased ventilation-to-perfusion ratio.
The partial pressure of pulmonary blood carbon dioxide decreases, while the partial pressure of oxygen increases.
Absence of blood perfusion produces a ventilation-to-perfusion ratio equal to infinity.
No gas exchange occurs.
As a result, the partial pressures of oxygen and carbon dioxide of alveolar gas remain equal to that of inspired air.
Pulmonary embolism is a common cause of alveolar dead space:
To minimize "wasted" ventilation, bronchiolar constriction diverts air from non-perfused alveoli.
