A 60-year-old woman presents with sudden onset pleuritic chest pain and shortness of breath. She is pale and sweaty. Observations: BP 80/50 mmHg, pulse 130/min, respiratory rate 32/min, SpO2 88% on 15 L oxygen via non-rebreathe mask. ECG shows sinus tachycardia. Massive pulmonary embolism is suspected. What is the most appropriate immediate management?