The Finapres (Finger Arterial Pressure) device is based on the finger clamp method introduced by Penaz in 1973. The volume of blood in the finger varies with the cardiac cycle. A small cuff placed around the finger is used to keep the blood volume of the finger constant. An infrared photo-plethysmograph detects changes in the volume of blood within the finger with each cardiac cycle. A controller system alters the pressure in the cuff accordingly, to keep the volume of blood in the finger constant. The applied pressure waveform correlates with the arterial blood volume and, therefore, with the arterial blood pressure. This applied pressure is then displayed continuously, in real time, as the arterial blood pressure waveform.