 Participation in this study will last up to 4 weeks. At an initial baseline visit, participants will undergo water emersion measurements of their arm and leg, nerve testing, and muscle strength testing. About 1 to 2 weeks later, participants will enter the pediatric intensive care unit for up to 36 hours. During the inpatient stay, participants will undergo the high-pressure, high-volume transvenous limb perfusion procedure with normal saline in one of their arms or legs. Throughout the hospital stay, breathing, heart rate, and blood pressure will be monitored. Medication will be available to control any discomfort or pain experienced by participants. Each subsequent participant will receive more fluid pumped at a higher pressure and with a tighter tourniquet than the previous participant, as long as no problems or adverse effects are detected. Some of the participants will undergo an MRI immediately after the perfusion procedure. About 1 to 2 weeks after the inpatient stay, participants will attend a follow-up visit that will include repeat nerve and muscle strength testing, a blood draw, photos of limbs, and an ultrasound of the leg blood vessels if the participants leg was used during the perfusion procedure. 