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.
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.
