The ability to measure cardiac output (CO) accurately and reproducibly at frequent intervals remains elusive to the clinician caring for critically ill pediatric patients even though a large proportion of these children are known to have hemodynamic compromise as a result of their illness. Current techniques used in adults to measure CO are not suitable for routine use with pediatric patients. A new ultrasound dilution approach provides an opportunity to measure cardiac output and blood volumes in pediatric patients. The main aim of this study is to compare CO measured by the new method with the clinician's estimate and implied CO from the measurement of the arteriovenous oxygen content difference.

Inclusion Criteria:

• Patients with in situ central venous and arterial catheters
• Ability to draw blood from arterial and central venous catheters
• Presence of parent or guardian to provide consent

Exclusion Criteria:

• Patients over 16 years of age.

Pediatric patients (up to 16 years of age ) in the ICU with in situ central venous and arterial catheters.