Non-invasive Central Venous Pressure Estimation in Pediatric Patients

This study is testing a non-invasive ultrasound technique to estimate central venous pressure (a measure of how much fluid is in the body's circulation) in children and young people, comparing it against the readings from a standard catheter already placed for medical care. **You may be eligible if...** - You are between birth and 21 years old - You already have a central venous catheter (a special IV line near the heart) placed as part of your standard medical care, OR you are undergoing a cardiac catheterization procedure **You may NOT be eligible if...** - You weigh less than 2.5 kg at enrollment - Both sides of your neck are inaccessible for ultrasound due to dressings or IV lines - You have skin conditions on the neck that prevent ultrasound - You are actively bleeding - You are clinically unstable, per your doctor's assessment Talk to your doctor to see if this trial is right for you.