V/Q ratio Flashcards
(12 cards)
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 (radioactive xenon or technetium) 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
