A-21-year-old man exhibited lack of efficacy during treatment with heparin for pulmonary embolism [dosage not stated]. The man presented to the emergency department with 3 days of fever, shortness of breath, cough, pleuritic chest pain, and 1 day of lightheadedness with near syncope and worsened dyspnoea. At the time of the presentation, he was febrile, normotensive, tachycardic and hypoxic. The severe acute respiratory syndrome coronavirus 2 nasopharyngeal qualitative polymerase chain reaction (PCR) was positive. Electrocardiogram (ECG) demonstrated evidence of right heart strain, a pattern associated with acute pulmonary embolism. His ultrasound scan showed right ventricular free wall hypokinesis with preserved apical contractility consistent with pulmonary embolism. Initial D-Dimer, troponin-T and pro-BNP-NT levels were elevated. A computed tomography pulmonary angiogram (CTPA) demonstrated pulmonary emboli. He was admitted to the intensive care unit (ICU) on high flow nasal cannula and started receiving treatment with IV heparin [unfractionated heparin] infusion. He was persistently hypotensive and hypoxic. He was presumptively diagnosed with massive pulmonary embolism. He was urgently underwent catheter-directed thrombolysis (CDT). The treatment with IV heparin infusion was continued. Following CDT, he improved clinically and was transferred to the general medicine floor on hospital day 3. On hospital day 4, he was initially normotensive, and developed hypotension and acute respiratory failure, which required a non-re-breather mask. Subsequently, he was intubated and developed cardiac arrest regaining spontaneous circulation after one electrical shock and two rounds of cardiopulmonary resuscitation. Vasopressor support was initiated with phenylephrine, epinephrine, norepinephrine and vasopressin. Recurrence of pulmonary embolism was suspected, and his pulmonary angiogram demonstrated bilateral pulmonary emboli with interval worsening of clot burden. Repeat CDT of the bilateral pulmonary arteries was performed with continuous systemic IV heparin infusion. Over the next 48h, ventilation parameters improved, and vasopressors were discontinued. Repeat pulmonary angiography demonstrated significant improvement in the right central pulmonary embolism. Venous duplex ultrasound was then performed demonstrating non-occlusive thrombus of the right femoral vein and popliteal vein. An inferior vena cava filter was placed.The man's anticoagulation regimen was transitioned from heparin to enoxaparin sodium [enoxaparin] due to concern for failure of anticoagulation. He was successfully weaned from extracorporeal membrane oxygenation (ECMO) on hospital day 10 and extubated on hospital day 31. He received treatment for septic shock treated with broad spectrum antibiotics and developed a right thigh haematoma and compartment syndrome required surgical debridement on hospital day 40. Postoperatively, he was transitioned to rivaroxaban. On hospital day 52, he was discharged to an acute rehabilitation facility. He returned home ...