Most commonly, PVR is measured using the Fick method based on the principle that a known amount of a specific indicator is added to a volume of fluid. If the concentration before and after this addition is known, the volume of fluid can be calculated. The Fick method uses the physiological uptake of oxygen as an indicator. However, this method may be inaccurate in this patient population. Other techniques, such as the dye dilution technique or thermodilution technique suffer from their own limitations, such as reduced reliability in the presence of shunts or valvular regurgitation.