A Study to Evaluate the Safety of Withholding Anticoagulation in Patients With Subsegmental PE Who Have a Negative Serial Bilateral Lower Extremity Ultrasound (SSPE)

• Interobserver agreement for SSPE diagnosis on CTPA (local Vs. central interpretation) [ Time Frame: 90 day follow-up ] [ Designated as safety issue: Yes ]
• Death due to PE [ Time Frame: 90 day follow-up ] [ Designated as safety issue: Yes ]
  Definition of death due to PE Certain: hypotension, hypoxia, cardiac arrest with no other explanation other than PE and autopsy or radiographic confirmation Highly probable: criteria for certain but another disease could have caused the death Probable: other cause suspected based on clinical evidence but 100% certainty not available Unlikely: all other cases
• Death probably due to PE [ Time Frame: 90 day follow-up ] [ Designated as safety issue: Yes ]
  Certain: hypotension, hypoxia, cardiac arrest with no other explanation other than PE and autopsy or radiographic confirmation Highly probable: criteria for certain but another disease could have caused the death Probable: other cause suspected based on clinical evidence but 100% certainty not available Unlikely: all other cases
• Major bleeding [ Time Frame: 90 day follow-up ] [ Designated as safety issue: Yes ]

  Major bleeding will be defined according to previously published criteria (51):

  1. Fatal bleeding; OR
  2. Symptomatic bleeding in a critical area or organ, such as intracranial, intraspinal, intraocular, retroperitoneal, intraarticular or pericardial, or intramuscular with compartment syndrome; OR
  3. Bleeding causing a fall in hemoglobin level of 20 g/L (1.24 mmol/L) or more, or leading to transfusion of two or more units of whole blood or red cells
• Minor bleeding [ Time Frame: 90 day follow-up ] [ Designated as safety issue: Yes ]
  Minor bleeding will be defined as any bleeding not meeting the requirements of a major bleeding event.

Blood clots in lung arteries (pulmonary embolism) are usually detected using a radiological test called computed tomography (CT scan). As technology advances, the CT scans are able to detect smaller and smaller blood clots. Over time, the frequency of blood clots in the pulmonary arteries has increased significantly (CT scan are now detecting very small blood clots that the investigators could not see before). As a result, more and more people are on blood thinners to treat these small blood clots but their true clinical significance is unknown.

The management of blood thinners is costly and also utilizes scarce healthcare resources. These blood thinners need to be monitored with frequent blood work. Furthermore, every year, approximately 3 percent of patients on blood thinners will have a major bleeding event requiring medical attention.

The investigators don't think that treating these small blood clots in the pulmonary arteries detected on CT scan is worth the risk of bleeding from the blood thinners.

The main goal of this study is to find out if it is safe to not treat very small blood clots in the pulmonary arteries.

The investigators plan to follow 270 patients with small blood clots in their lungs for 90 days. These patients will not be treated with blood thinners but will be followed closely with other non-invasive tests to avoid progression or recurrence of blood clots.

Consecutive patients with symptomatic, isolated SSPE* (any number), that are newly diagnosed by computed tomographic pulmonary angiography will be eligible to participate in the study.

Inclusion Criteria:

Patients with newly diagnosed isolated SSPE* (any number) that are managed as outpatients will be eligible to participate in the study.

*Isolated SSPE is defined as CTPA demonstrating an intraluminal filling defect in a subsegmental artery with no filling defects visualized at more proximal pulmonary artery levels.

Exclusion Criteria:

1. Proximal lower extremity (popliteal vein or above) or upper extremity (subclavian vein or above) DVT.
2. Need for long term oral anticoagulant therapy for reasons other than VTE.
3. Hospitalized at the time of SSPE diagnosis or admitted to hospital with SSPE diagnosis.
4. Requiring oxygen therapy to maintain an O2 saturation over 92%
5. Previous history of DVT or PE
6. Geographically inaccessible for follow-up
7. Active Malignancy (defined as other than basal-cell or squamous cell carcinoma of the skin; cancer within the past 6 months; any treatment for cancer in the past 6 months; or recurrent or metastatic cancer)
8. Pregnancy (or within 6 weeks postpartum)
9. Have received more than 48 hours of anticoagulants
10. Unable/refuse to sign informed consent
11. Asymptomatic SSPE (e.g. SSPE is an incidental finding on a CT scan conducted for reasons other than suspected PE)