Final answer:
For an unresponsive, apneic 3-year-old child with a shockable rhythm on the cardiac monitor, continue CPR and consider defibrillation with a biphasic manual defibrillator, dosed at 2-4 J/kg, in conjunction with chest compressions and ventilation.
Step-by-step explanation:
When a 3-year-old unresponsive, apneic child is brought to the emergency department (ED), and the cardiac monitor shows an arrhythmia, the most appropriate therapy is to continue cardiopulmonary resuscitation (CPR) with bag-mask ventilation as currently being administered. Since a biphasic manual defibrillator is available and the child's estimated weight is 15 kg a defibrillation shock may be indicated if the rhythm is deemed shockable, such as ventricular fibrillation (VF) or pulseless ventricular tachycardia (VT).
The exact dosage of the defibrillation shock must be carefully calculated based on ACLS pediatric guidelines typically starting at 2-4 J/kg for a biphasic defibrillator indicating an initial dose of 30-60 J for a 15kg child. Immediate defibrillation is critical to attempt to restore a regular rhythm and should be integrated with high-quality chest compressions and bag-mask ventilation as parts of the resuscitation efforts.