Each year in the United States more than 30,000 children are admitted to intensive care units. The majority of these children have some degree of heart instability during their stay, yet there is currently no routine way to measure the actual amount of blood that the heart pumps. The ability to measure the amount of blood that the heart pumps accurately and easily at frequent intervals would be very helpful to the doctor caring for these children because many of them have poor heart function as a result of their illnesses. Current techniques used in adults to measure output of the heart are either not readily transferred to children or demand difficult invasive procedures. Because of this, the amount of blood that the heart pumps cannot be measured with enough frequency to help guide care. Despite this reality, accurate measurements of the amount of blood that the heart pumps in these patients at crucial points in their illnesses would allow for more accurate use of potentially harmful procedures and could possibly improve the outlook for these children. Likewise, being able to correctly measure blood volume could provide a better way to estimate the pressure on the heart and improve treatment.