A 28-year-old woman presents to the emergency department with sudden onset pleuritic chest pain and breathlessness. She returned from a trip to Thailand 3 days ago. She takes the combined oral contraceptive pill. Observations: HR 115 bpm, BP 128/76 mmHg, RR 24/min, SpO2 94% on air. Her Wells score for pulmonary embolism is calculated as 6.5. What is the most appropriate immediate management step?