2.2. Principle of TPTD Measurements with the PiCCO Device

To perform TPTD measurements, a bolus of 20 mL of cold, isotonic saline is injected through a CVC or CDC. A sensor at the injection site registers the drop in temperature and starts the measurement process. The cold bolus moves through the right atrium, right ventricle, the lungs, the left atrium, and left ventricle; enters the systemic circulation; and is detected by an arterially inserted catheter. The consecutive change in blood temperature is detected by a thermistor-tipped catheter, which is usually placed through the femoral artery in the descending aorta. In this way, a thermodilution curve is recorded, and with its help, the cardiac output can be determined. Using the thermodilution curve, the patient’s volume parameters can be calculated semi-automatically with the help of various algorithms [11]. Cardiac parameters (cardiac output (CO), cardiac index (CI), cardiac functional index (CFI)), as well as parameters of blood volume and edema like GEDV, ITBV, and EVLW, are calculated using three main values derived from a contour analysis of the thermodilution curve: mean transit time (MTt), downslope time (DSt), and area under the curve (AUC). MTt describes the time until half of the injected saline bolus has passed. The DSt describes the duration of the exponential decrease in the thermodilution curve. The AUC reflects the estimated CO. The difference between MTt and DSt multiplied by CO results in the GEDV. The EVLW can be calculated from the difference of the ITTV (MTt × CO) and ITBV (1.25 × GEDV). In the PiCCO^® algorithm, real bodyweight is used to calculate the cardiac parameters. For volume parameters, on the other hand, the predicted bodyweight (PBW) and the predicted body surface area (BSA) are used.