A 38-year-old woman with asthma is admitted to ICU following a near-fatal asthma attack. She is intubated and mechanically ventilated. Ventilator settings: VCV, TV 400 mL, RR 12/min, PEEP 0 cmH2O, FiO2 0.4, I:E ratio 1:4. ABG: pH 7.25, PaO2 14.5 kPa, PaCO2 8.8 kPa, HCO3 24 mmol/L, lactate 1.8 mmol/L. Peak pressure is 48 cmH2O, plateau pressure is 22 cmH2O. She develops hypotension with systolic BP 70 mmHg. What is the most likely cause of her haemodynamic instability?