The World Health Organization (WHO) declared the 2019 novel coronavirus (COVID-19) a pandemic on March 11, 2020. As of 19 July 2020, there have been 14.3 million confirmed cases and over 600,000 confirmed deaths. Up to 14% of infected patients develop interstitial pneumonia, which may evolve to acute respiratory distress syndrome.

COVID-19 associated pulmonary arterial microthrombosis and coagulopathy has prompted physicians to implicate pulmonary embolism (PE) as a potential cause for acute respiratory deterioration.

Literature review reveals few studies of varying size, quality and design. Recent meta-analysis reports venous thromboembolism in approximately 20% of COVID-19 patients. There has yet to be a case-controlled study which proves and quantifies the associated between COVID-19 and PE.Confirming and quantifying this association has numerous clinical implications for the treatment of critically unwell patients with COVID-19 infection. For example, clinicians will be more inclined to investigate and treat sudden deteriorations with the knowledge that pulmonary embolism is the commonest cause for said deteriorations.

A retrospective case-control study of all patients who had a CT pulmonary angiogram (CTPA) at NHS Grampian between 3rd March and 20th June 2020 will be conducted.

CTPA reports and test results of Reverse Transcriptase-Polymerase Chain Reaction (RT-PCR) for COVID-19 will be reviewed via electronic medical records.

Patients with no COVID-19 RT-PCR test done within 7 days of the CTPA or the same clinical episode will be excluded. Patients who had repeat CTPAs, who are already known to have a PE, and who had CTPAs which are considered to be radiologically inadequate will be excluded.

Patients will be categorised into cases (positive PE) and controls (negative PE).

CTPA images of cases will be reviewed to collect further data about severity, distribution and right heart strain.

Patients' exposure status will also be determined (positive or negative for COVID- 19 infection).

The primary outcome is the number of pulmonary embolism cases related to COVID19.

The secondary outcomes are the severity, distribution of pulmonary embolism as determined by calculated Qanadli score. Presence of right heart strain.

Further data for the multivariate analysis will be obtained from the patient's electronic medical record. Specifically, we will collect data on risk factors for COVID-19 infection and PE to correct for confounding.

Statistical data analysis by various hypothesis tests and multivariate logistic regression will be conducted to determine the odds ratio of developing a PE, given positive COVID-19 infection.

Patients with a positive PE will have their scans reviewed to quantitatively assess their clot burden and degree of right heart strain. The former will be done by calculating a Qanadil score. The latter will be done my measuring the right ventricle- left ventricle ratio. This will allow for a comparison between COVID-19 infected and non-infected patients with PE.

Inclusion Criteria:

-Patients who had a CT pulmonary angiogram at NHS Grampian between 3rd March and 20th June 2020.

Exclusion Criteria:

• Patients with no COVID-19 RT-PCR test done within 7 days of the CT pulmonary angiogram or the same clinical episode.
• Duplicate CT pulmonary angiograms.
• Patients who are already known to have a pulmonary embolism.
• Patients who had radiologically inadequate CT pulmonary angiograms.
• Patients younger than 16 years old.
• Patients who have incomplete or unavailable notes.

Acute hospital inpatients