A 60-year-old man presents with sudden onset pleuritic chest pain and shortness of breath. He is tachycardic at 130 beats/min, hypotensive at 80/50 mmHg and hypoxic despite high-flow oxygen. ECG shows sinus tachycardia and right heart strain; bedside echocardiography suggests a large pulmonary embolism with right ventricular dilatation. What is the most appropriate immediate management?