Subjects will breathe normally through a mask while we record how fast and how much air they are breathing. We will have them "re-breathe" some of the air they breathed out by adjusting the ventilator. During this time, we will use the air breathed out to calculate how much blood per minute the subject's heart is pumping. We will also measure how much blood the heart is pumping at this time by injecting fluid into the catheter in the neck and then drawing about 1 teaspoon of blood from the catheters in the neck and arm. We will compare the calculated and measured values of the amount of blood pumped out of the heart for accuracy.