In the respiratory system, ventilation/perfusion (V/Q) mismatch refers to the pathological discrepancy between ventilation (V) and perfusion (Q) resulting in an abnormal ventilation/perfusion (V/Q) ratio. Ventilation is a measure of the amount of inhaled air that reaches the alveoli, while perfusion is a measure of the amount of deoxygenated blood that reaches the alveoli through the capillary beds.[1] Under normal conditions, ventilation-perfusion coupling keeps ventilation (V) at approximately 4 L/min and normal perfusion (Q) at approximately 5 L/min. Thus, at rest, a normal V/Q ratio is 0.8.[2] Any deviation from this value is considered a V/Q mismatch. Maintenance of the V/Q ratio is crucial for preservation of effective pulmonary gas exchange and maintenance of oxygenation levels. A mismatch can contribute to hypoxemia and often signifies the presence or worsening of an underlying pulmonary condition.
In a condition such as pulmonary embolism, the pulmonary blood flow is affected, thus the ventilation of the lung is adequate, however there is a perfusion defect. Gas exchange thus becomes highly inefficient leading to hypoxemia, as measured by arterial oxygenation. A ventilation perfusion scan or lung scintigraphy can be used to diagnose areas of lungs being ventilated but not adequately perfused. This results in a raised Alveolar-arterial (A-a) gradient which is responsive to supplemental oxygen.
In conditions with right to left shunts, there are also ventilation perfusion defects with high A-a gradients. Hypoxemia is difficult to correct with supplemental oxygen and is associated with a widened A-a gradient. In cases of right to left shunts more of deoxygenated blood mixes with oxygenated blood from the lungs and thus to a small extent the condition might neutralize the high A-a gradient with pure oxygen therapy.Patient with parenchymal lung diseases will have an increased A-a gradient with moderate response to oxygen therapy.
A patient with hypoventilation will have complete response to 100% oxygen therapy.
To differentiate between a ventilation perfusion mismatch or not, a lung scan is performed. The ventilation and perfusion are measured separately. If both scans are done simultaneously then it is called as V/Q scan. Ventilation Scan is done first as it is easy to wash out tracer gas from lungs with the help of hyperventilation than clearing the tracer radioactive material from blood.
Indication: It is most commonly done in suspected case of pulmonary embolism.
Results:
Results are reported in following way:
Through ventilation and perfusion scans, the abnormal area of lung may be localized. A provisional diagnosis of COPD, asthma or pulmonary embolisms may be made. Treatment of these underlying conditions may address ventilation perfusion mismatch.
Management of the condition may vary. If ventilation is abnormal or low, increasing the tidal volume or the rate may result in the poorly ventilated area receiving an adequate amount of air, which ultimately leads to an improved V/Q ratio. Conversely, if perfusion scan is of low quality showing low perfusion to lung as in case of hypovolemia, treatment of the conditions is by giving it fluid and using inotropes in case of shock.