What does a normal ventilation scan combined with an abnormal perfusion scan suggest?

A normal ventilation scan combined with an abnormal perfusion scan primarily indicates the presence of a pulmonary embolus. In this context, the ventilation scan assesses how well air is reaching the alveoli in the lungs, and when it shows normal results, it implies that the airways are clear and functioning appropriately. Conversely, an abnormal perfusion scan indicates issues with blood flow in the pulmonary circulation, specifically altered perfusion due to blockage.

When a pulmonary embolus occurs, it can block blood flow to specific areas of the lung, resulting in mismatched ventilation (which remains normal) and perfusion (which is decreased or abnormal). This is a classic presentation in diagnosing pulmonary embolism, as the ventilation scan reflects the ability to ventilate the alveoli, while the perfusion scan reveals that blood flow is compromised.

The other conditions do not typically present this specific pattern. For example, Chronic Obstructive Pulmonary Disease (COPD) generally leads to both ventilation and perfusion abnormalities. Asthma exacerbation usually involves problems with airflow but can also affect blood flow; however, it would not typically show a significant mismatch as seen with a pulmonary embolism. Interstitial lung disease tends to affect both ventilation and perfusion due to the diffusion limitations and structural changes