Inclusion Criteria:

1. Pediatric patients age from 1 month to less than 18 years who need sedation to tolerate mechanical ventilation and will remain ventilated for ≥ 24 hours
2. Receiving IV sedation by infusion or bolus for ≤72h to aid mechanical ventilation (but will be recruited as early as possible)

Exclusion Criteria:

1. Current use of inhaled prostacyclin (high viscosity of prostacyclin impairs vaporizer device) or continuous salbutamol nebulization (impairs gas analyzer function)
2. Family history or personal history of malignant hyperthermia (MH)
3. Allergy to isoflurane, sevoflurane or other volatile inhaled anesthetic gas
4. Metabolic, mitochondrial or myopathy disease e.g., Duchenne muscular dystrophy (volatiles may trigger malignant hyperthermia like reaction)
5. Moribund with expected survival < 24h
6. Known pregnancy or lactation
7. Suspected or evidence of high intracranial pressure (ICP)
8. Not a 'closed ventilator circuit' with potential for gas leak (e.g. uncuffed ETT, bronchopleural fistula, tracheal/trachea-esophageal fistulas, or other sources of "gas leak" from patient circuit that cannot be removed)
9. Prior enrollment in the ABOVE trial