Ventilation systems are difficult to completely disinfect. It is essential to rule out acute pulmonary embolism.
In many centres if a patient who is suspected to have a PE has a normal chest x-ray a ventilation scan is not done. In that situation a perfusion scan alone has good positive and negative predictive values. If the chest x-ray is abnormal either a CT pulmonary angiogram or V/Q scan is required.
Unfortunately in the setting of COVID-19 this is more complex because lung ultrasound and CT scans have demonstrated abnormalities in affected patients with normal chest x-rays.
why would you need to rule out PE? it's probably safer just to give full dose anticoagulation if you notice VTE signs. POCUS for DVT or right heart strain would make much more sense.
Patients with severe and critical COVID-19, the ones in whom you are likely to suspect PE, may become coagulopathic and develop DIC. I certainly would not recommend full dose anticoagulation without definitive diagnosis of VTE. POCUS for VTE is fine but right heart strain is non-specific and may be due to pulmonary hypertension, pulmonary embolism, RV infarction, chronic lung disease, pulmonic stenosis, bronchospasm and pneumothorax.
In this current pandemic setting, the threshold for CTPA has become a lot lower. Traditionally I have been performing VQ scanning for females < 55 years old with clear CXRs who cannot be cleared clinically (by PERC criteria) nor biochemically (by d-dimer). I think that the risk of droplet spread during ventilation scanning now outweighs the radiation exposure of CT.
Many centres have completely stopped doing V/Q scanning during the COVID-19 pandemic because of the risk of transmission to staff performing the tests.