A 6-month-old girl from Delhi with known tetralogy of Fallot (diagnosed on antenatal ultrasound) presents to the emergency department with acute onset of severe cyanosis, rapid and labored breathing, and altered sensorium. The mother reports the baby was playing normally 2 hours ago. On examination, the infant is deeply cyanosed (SpO2 65%), tachypneic (RR 60/min), and unresponsive to commands. The heart rate is 180/min. Chest X-ray shows a 'boot-shaped' heart. There is no evidence of infection or metabolic derangement. What is the most appropriate immediate management?