 Near infrared spectroscopy NIRS can be used to measure the absorption of electromagnetic energy in the near infrared range 650 to 1000 nm by in vivo chromophores. When assessing regional tissue oxygenation, NIRS assesses the amount of emitted energy absorbed in the range associated with hemoglobin 760 nm and with deoxyhemoglobin 800 nm. The ratio of these amounts of absorbed light can be used to generate an index of tissue oxygenationdeoxygenation. Indocyanine green dye is a synthetic chromophore that absorbs energy in the infrared spectrum 805 nm. Guenette et al. measured the accumulation of ICG in respiratory muscles using NIRS while the concentration of ICG in the arterial blood was measured using photodensitometry. This technique has the advantage of being able to quantify absolute muscle blood flow with simultaneous measures of cardiac output, thereby allowing the calculation of absolute regional blood flows and absolute rates of regional oxygen uptake. The practicality of this technique in healthy volunteers is complicated by with the need for blood withdrawals and re-infusions as well as arterial cannulation and the measurement of cardiac output. 