A 38-year-old female with no past medical history presents with sudden-onset chest pain, tachycardia, and breathlessness. She has recently returned from a long-haul flight. Her ECG shows sinus tachycardia with S1Q3T3 pattern. Her systolic blood pressure is 82 mmHg. What does the haemodynamic instability indicate about the severity of pulmonary embolism, and what treatment should be considered?