a method for determining the blood constituents of a patient comprising coupling an oximeter sensor arrangement to a tissue region of the patient ; passing first and second lights through the patient &# 39 ; s tissue region for a first period of time while the venous blood in the tissue region has a first volume and for a second period of time while the venous blood in the tissue region has a second volume , the first light being substantially in a red light range and the second light being substantially in an infrared light range ; detecting a red light signal and an infrared light signal , the red and infrared signals having at least first and second frequencies ; computing a first ratio of the red and infrared signals at the first frequency ; computing a second ratio of the red and infrared signals at the second frequency ; comparing the first and second ratios to determine a first blood constituent .