This is a pilot study of the Flow Medical Catheter, a catheter-based device used to deliver clot-dissolving medication directly to a blood clot in the lungs (pulmonary embolism). The study evaluates whether the device can be used safely and as intended in adults with intermediate-risk acute pulmonary embolism. All participants receive treatment with the study device; there is no comparison group.

This single-arm, pilot study evaluates the safety and feasibility of the Flow Medical Catheter for catheter-directed thrombolysis in adults with acute intermediate-risk pulmonary embolism. Participants undergo the catheter-directed thrombolysis procedure with the study device and are followed for procedure-related safety outcomes during the early post-procedure period. Eligibility criteria, study intervention, and outcome measures are described in the corresponding structured sections of this record.

Inclusion Criteria:

Subjects must meet ALL of the following criteria to be eligible:

1. Age 18 to 75 years, inclusive.
2. Clinical signs and/or symptoms consistent with acute PE with symptom onset ≤14 days prior to baseline CTPA.
3. CTPA demonstrating acute PE with a filling defect in at least one main or lobar pulmonary artery.
4. RV/LV diameter ratio ≥0.9 on baseline CTPA (site read acceptable for eligibility determination).
5. Medically eligible for CDT and standard anticoagulation per institutional guidelines and the treating investigator's judgment.
6. Willing and able to provide written informed consent and comply with all study procedures and follow-up requirements.

Exclusion Criteria:

Subjects meeting ANY of the following criteria are not eligible:

1. High-risk (massive) PE: Sustained SBP <90 mmHg for ≥15 consecutive minutes, >40 mmHg drop from baseline, vasopressor requirement, or cardiac arrest requiring cardiopulmonary resuscitation during the immediate hospitalization.

2. Respiratory insufficiency: FiO₂ requirement >40% or >6 LPM supplemental oxygen to maintain SpO₂ >90%.

3. Prior PE: Prior PE ≤180 days from index procedure.

4. Absolute contraindications to thrombolytic therapy:

   1. Any prior intracranial hemorrhage (no time limit)
   2. Known structural cerebrovascular lesion or arteriovenous malformation (no time limit)
   3. Known malignant intracranial neoplasm (no time limit)
   4. Ischemic stroke or TIA within 3 months of baseline CTPA
   5. Intracranial or intraspinal surgery within 2 months of baseline CTPA
   6. Significant closed-head or facial trauma within 3 months of baseline CTPA
   7. Suspected aortic dissection
   8. Active bleeding or bleeding diathesis (excluding menses)
   9. Active intracranial or intraspinal disease
   10. Irreversible neurological compromise

5. Relative contraindications to thrombolytic therapy:

   1. Severe uncontrolled hypertension at presentation (SBP >180 mmHg or DBP >110 mmHg)
   2. Major surgery within 3 weeks of baseline CTPA
   3. Major trauma (non-intracranial) within 3 months of baseline CTPA
   4. Traumatic or prolonged CPR (>10 minutes) within 3 weeks of baseline CTPA
   5. Recent internal bleeding within 4 weeks of baseline CTPA
   6. Noncompressible vascular puncture site(s)
   7. Active peptic ulcer
   8. Ischemic stroke more than 3 months prior to baseline CTPA

6. Laboratory exclusions (within 6 hours of index procedure, not correctable prior to procedure):

   1. Platelet count <100,000/µL
   2. Hemoglobin <8.0 g/dL
   3. INR >2.0; or INR >1.5 if currently receiving warfarin
   4. aPTT >50 seconds in the absence of anticoagulant therapy
   5. eGFR <30 mL/min/1.73 m²

7. Cardiac and pulmonary exclusions:

   1. Known LVEF ≤30% by any imaging modality
   2. Decompensated heart failure at presentation requiring inotropic or diuretic escalation unrelated to the index PE procedure
   3. History of chronic lung disease requiring supplemental oxygen therapy at baseline
   4. Known chronic severe pulmonary hypertension not attributable to the index PE episode (documented PASP >70 mmHg by right heart catheterization or echocardiography, or documented CTEPH)
   5. Profound bradycardia requiring temporary pacing or inotropic support

8. Procedural and anatomical exclusions:

   1. Known or suspected clot in transit (right atrium or right ventricle), IVC clot, or intracardiac thrombus
   2. Intracardiac lead in the right ventricle or right atrium placed ≤180 days prior to the index procedure
   3. Actively progressing cancer actively treated with chemotherapy
   4. Anatomic or clinical conditions precluding safe venous access or catheter delivery to the pulmonary arteries
   5. BMI >45 kg/m² if, in the judgment of the investigator, body habitus poses unacceptable risk for vascular access, catheter delivery, or procedural bleeding management

9. Device and drug hypersensitivity:

   1. Allergy or hypersensitivity to rt-PA (alteplase)
   2. Heparin allergy or history of heparin-induced thrombocytopenia (HIT)
   3. Known hypersensitivity to nitinol (nickel-titanium alloy)
   4. Known anaphylactic reaction to radiographic contrast agents not amenable to premedication

10. Prior thrombolytic use or glycoprotein IIb/IIIa inhibitor use within 3 days of baseline CTPA.

11. Use of a direct oral anticoagulant (DOAC) within 48 hours of the index procedure, unless the investigator judges procedural bleeding risk to be acceptable and documents this determination, with rationale, in the subject's medical record.

12. Pregnancy. A pregnancy test is required for all women of childbearing potential prior to enrollment; point-of-care testing is acceptable. Patients with a positive pregnancy test are excluded. Postpartum patients are eligible if all other criteria are met.

13. Life expectancy <90 days in the investigator's judgment.

14. Concurrent enrollment in another interventional clinical investigation.

15. Previous enrollment in this study.

16. Absolute contraindication to anticoagulation, or inability to receive post-procedure anticoagulation per institutional standard of care.

17. Unable or unwilling to provide written informed consent.

18. In the opinion of the investigator, the subject is at unacceptably high risk for bleeding complications or is otherwise not a suitable candidate for the study