A 5-year-old girl with known Tetralogy of Fallot is brought to the emergency department with acute onset of severe dyspnea, increased cyanosis (SpO₂ 65%), and loss of consciousness. Her mother reports she was playing when she suddenly became very blue and unresponsive. On examination, the child is unconscious, with severe cyanosis and a weak pulse. The cardiac murmur is no longer audible. What is the most likely acute complication, and what is the immediate first-line management?