Final answer:
For a patient aged 2 to under 12 years, the ROSC directive for a 0.9% NaCl fluid bolus involves a mixture of isotonic saline and Na lactate for fluid and electrolyte losses, with readiness for potential significant blood loss.
Step-by-step explanation:
The treatment plan for administering a 0.9% NaCl fluid bolus under the Return of Spontaneous Circulation (ROSC) medical directive for a patient aged between 2 years to under 12 years involves correction of dehydration through intravenous fluids. If there's a high excretion of fluid containing Na and HCO₃ leading to a significant fluid and electrolyte loss, the preferred treatment would be an intravenous administration of a mixture consisting of two-thirds isotonic saline and one-third Na lactate. Furthermore, for cases involving a risk of blood loss of at least 500 ml or 7 ml/kg of body weight in children, it is crucial to ensure appropriate access and the availability of fluids for administration.