A 4-month-old infant with Tetralogy of Fallot presents with sudden-onset severe cyanosis, rapid breathing, and loss of consciousness during feeding. On examination, the child is in acute distress. The structure marked **A** (pulmonary stenosis/RVOT obstruction) is the primary determinant of cyanosis severity in this condition. Which of the following is the MOST appropriate immediate management to reverse this acute hypercyanotic episode?