V/Q ratio Flashcards
How is the pulmonary ventilation/perfusion scan carried out?
A pulmonary ventilation/perfusion scan is actually 2 tests. They may be done separately or together.
During the perfusion scan, a technician injects radioactive albumin into your vein. You are placed on a movable table that is under the arm of a scanner. The machine scans your lungs as blood flows through them to find the location of the radioactive particles.
During the ventilation scan, you breathe in radioactive gas through a mask while you are sitting or lying on a table under the scanner arm.
What is ventilation-perfusion (V/Q) mismatch?
An imbalance between air reaching the alveoli (ventilation) and blood reaching the alveoli (perfusion), leading to impaired gas exchange
What is an A-a gradient?
A–a gradient tells you if oxygen is making it from the alveoli into the blood — useful in hypoxemia.
Difference between V/Q mismatch and FEV1/FVC ratio
V/Q mismatch describes the physical imbalance between air and blood flow in the lungs — often causes a raised A–a gradient.
FEV₁/FVC ratio is a lung function test metric, used to differentiate obstructive vs restrictive lung disease — not a direct measure of gas exchange.
How is V/Q mismatch detected clinically?
Using V/Q scanning or ABG showing hypoxemia with a raised A–a gradient.
N.b. A–a gradient tells you if oxygen is making it from the alveoli into the blood — useful in hypoxemia.
What is a common cause of V/Q mismatch?
Pulmonary embolism
How does V/Q mismatch cause hypoxemia?
Areas with low V/Q can’t properly oxygenate blood, and high V/Q areas can’t compensate due to the shape of the oxygen-hemoglobin dissociation curve
Which condition leads to dead space ventilation (V/Q = ∞)?
Pulmonary embolism, where alveoli are ventilated but not perfused
Which condition is associated with a true shunt (V/Q = 0)?
ARDS or pulmonary edema, where alveoli are perfused but not ventilated.
What causes a low V/Q ratio?
Perfusion without ventilation (shunt-like state), seen in COPD, asthma, or pneumonia
What causes a high V/Q ratio?
Ventilation without perfusion (dead space), seen in pulmonary embolism
What is the normal V/Q ratio?
Approximately 0.8, indicating slightly more perfusion than ventilation in healthy lungs
