The purpose of this study is to test a new way to measure the oxygen level in the mixed venous blood system (the system that carries blood collected from all over the body to the heart), which is a very important monitoring tool for the overall oxygen delivery to all our tissues. This new way measures the oxygen level in the mixed venous blood from outside the body, instead of measuring from the inside through a catheter (a thin, flexible plastic tube) that is placed by breaking the skin.