Image challenge: acute shortness of breath with widespread T wave inversion

A 72-year-old woman presented with sudden-onset shortness of breath. There was no associated chest pain, nausea or sweating. She had had breast cancer 14 years previously, treated by wide local excision, radiotherapy and tamoxifen. She gave no history of cardiac or pulmonary disease and had no current risk factors for venous thromboembolism. On examination she had good air entry to both lungs and normal breath sounds. Heart rate was 88/min with blood pressure 133/73 mm Hg. Her SpO2 on air was 92%–95% at rest. She had no lower limb swelling or tenderness. Revised Geneva score was 4 suggesting moderate clinical probability of pulmonary embolism. There was no suggestion of recurrent breast cancer clinically. Haemoglobin was 14.5 g/dL, renal function was normal and C-reactive protein was 9 mg/L (0–10). N-terminal pro-B-type natriuretic peptide was raised at 1158 pg/mL (0–150). Her chest X-ray (CXR) was normal and her ECG is shown below (figure 1).