The Nexfin monitoring device relies on noninvasive pressure measurement using a finger-cuff manometer, combined with NIRS technology. The accuracy of the signal is dependent on the degree of vascular unloading, which takes an period before optimal unloading is reached. The evolution of the degree of inaccuracy caused by inclomplete unloading is quantified, and the absolute and relative inaccuracy of the nexfin as a function of the time is determined compared to the invasive blood pressure (IBP).