After Swan developed the initial balloon tip, Ganz used Fronek's idea and added a small thermistor (temperature probe) about 3 cm behind the tip. Either cold 10 ml of saline (0.9% NaCl) under 10° Celsius or room temperature (not as accurate) is injected into an opening in the right atrium. As this cooler fluid passes the tip thermistor, a very brief drop in the blood temperature is recorded. A recent variation in design is the incorporation of a heating coil on the catheter (30 cm from the tip, residing in the atrium area) which eliminates the cold fluid bolus, a major factor in human technique variation.
By attaching both the injector site and the ventricular thermistor to a small computer, the thermodilution curve can be plotted. If details about the patient's body mass index (size); core temp, Systolic, diastolic, central venous pressure CVP (measured from the atrium by the third lumen simultaneously) and pulmonary artery pressure are input, a comprehensive flow vs pressure map can be calculated.
In crude terms, this measurement compares left and right cardiac activity and calculates preload and afterload flow and pressures which theoretically if stabilized or adjusted with drugs to either constrict or dilate the vessels to raise or lower the pressure of blood flow to the lungs, respectively, in order to maximize oxygen for delivery to the body tissues.
The ability to record results is not a guarantee of patient survivability. The true art remains with the consultant physician or intensivist in balancing fluid load, so much so that the introduction of a balloon catheter, which is usually yellow, has been nicknamed "The kiss of the yellow snake".