This study uses a non-invasive (external) ultrasound test called Transcranial Doppler (TCD) to measure how blood flows through the major arteries of the brain of children receiving care in the Pediatric Intensive Care Unit (PICU). TCD is already used for clinical reasons in many children, but it is not part of routine monitoring for every critically ill child. By using TCD, the investigators hope to better understand how brain blood flow changes during illness. They will also track children's functional status from before their illness to the time of PICU discharge to explore how brain blood flow patterns relate to neurological outcomes.125 participants will be enrolled and will be on study while in the PICU, estimated to be 3-14 days.

Current clinical management of hemodynamics in critically ill children relies primarily on systemic parameters (e.g., blood pressure, heart rate) rather than direct assessment of cerebrovascular regulation. This approach may overlook individual variations in cerebral autoregulation (CA) capacity, potentially leading to suboptimal cerebral perfusion.

By prospectively evaluating TCD-derived CA indices in intubated pediatric patients with arterial blood pressure monitoring, this study aims to:

1. Establish normative Cerebral blood flow velocity (CBFV) ranges for critically ill children
2. Describe CBFV patterns across critically ill pediatric populations with varying neurologic risk profiles
3. Characterize the prevalence of CA dysfunction in a heterogeneous PICU population
4. Explore associations between CA/CBFV and functional outcomes

These data could:

1. Provide age-stratified CBFV reference values for the PICU setting
2. Identify patients at highest risk of neurological morbidity using population-specific thresholds
3. Inform future trials of autoregulation-guided hemodynamic management

Inclusion Criteria:

• Has at least one Parent or Legal Guardian who is willing and able to provide informed consent in English
• Mechanically ventilated (Endotracheal tube)
• Indwelling arterial catheter
• Current patient in the American Family Children's Hospital PICU.
• 1 day to 17.5 years of age

Exclusion Criteria:

• Subjects with a known or obvious anatomic barrier preventing safe application of the transcranial Doppler probe (e.g., cranial dressings, cranial fixation hardware, ventricular access devices positioned over insonation sites, or bilateral craniectomies)
• Patient with known chronic hypertension
• Admission FSS score > 16
• Tracheostomized patient
• The designated sub-investigator (Dr. Talal Al Hendawi) and PI (Dr. Neil Munjal) are unavailable to perform the Transcranial Doppler scan at the time of eligibility. All study scans are performed by either Dr. Al Hendawi or Dr.Munjal to ensure standardization and data integrity. Therefore, if they are unavailable (e.g., due to clinical service, leave, or other scheduling constraints), the patient will not be enrolled in the study.
• Not suitable for study participation due to other reasons at the discretion of the investigators

Mechanically ventilated pediatric patients (1 day-17.5 years) admitted to the PICU who meet eligibility criteria