I will study 100 patients in two groups. The first group will consist of critically ill pediatric patients at the time of admission to the PICU. These 50 patients will have cardiac output estimated by three methods: Estimation by a "panel of experienced clinicians"; Calculation of the arteriovenous content difference; and Direct measurement using a new minimally invasive system based on assessing changes in ultrasound transmission in blood after intravenous administration of a small (< 5 ml) bolus of normal saline. Group two will consist of 50 PICU patients who therapeutically require either a fluid bolus (25 patients) or a single intravenous dose of diuretic (25 patients). These patients will be assessed in the same fashion at the outset, but will have ongoing measurements of cardiac output and central blood volume using the minimally invasive system and central venous pressure using conventional techniques at 3 intervals within the first hour.