Editing Cardiac output (section)

==== Finapres methodology ====

In 1967, the Czech physiologist Jan Peňáz invented and patented the [[Continuous noninvasive arterial pressure|volume clamp method]] of measuring continuous blood pressure. The principle of the volume clamp method is to dynamically provide equal pressures, on either side of an artery wall. By clamping the artery to a certain volume, inside pressure—intra-arterial pressure—balances outside pressure—finger cuff pressure. Peñáz decided the finger was the optimal site to apply this volume clamp method. The use of finger cuffs excludes the device from application in patients without vasoconstriction, such as in sepsis or in patients on vasopressors.{{citation needed|date=June 2015}}

In 1978, scientists at BMI-TNO, the research unit of [[Netherlands Organisation for Applied Scientific Research]] at the [[University of Amsterdam]], invented and patented a series of additional key elements that make the volume clamp work in clinical practice. These methods include the use of modulated infrared light in the optical system inside the sensor, the lightweight, easy-to-wrap finger cuff with [[velcro]] fixation, a new pneumatic proportional control valve principle, and a set point strategy for the determining and tracking the correct volume at which to clamp the finger arteries—the Physiocal system. An acronym for physiological calibration of the finger arteries, this Physiocal tracker was found to be accurate, robust and reliable.{{citation needed|date=June 2015}}

The Finapres methodology was developed to use this information to calculate arterial pressure from finger cuff pressure data. A generalised algorithm to correct for the pressure level difference between the finger and brachial sites in patients was developed. This correction worked under all of the circumstances it was tested in—even when it was not designed for it—because it applied general physiological principles. This innovative brachial pressure waveform reconstruction method was first implemented in the Finometer, the successor of Finapres that BMI-TNO introduced to the market in 2000.{{Citation needed|date = June 2015}}

The availability of a continuous, high-fidelity, calibrated blood pressure waveform opened up the perspective of beat-to-beat computation of integrated haemodynamics, based on two notions: pressure and flow are inter-related at each site in the arterial system by their so-called characteristic impedance. At the proximal aortic site, the 3-element [[Windkessel effect|Windkessel]] model of this impedance can be modelled with sufficient accuracy in an individual patient with known age, gender, height and weight. According to comparisons of non-invasive peripheral vascular monitors, modest clinical utility is restricted to patients with normal and invariant circulation.<ref name="de Wilde">{{cite journal | vauthors = de Wilde RB, Schreuder JJ, van den Berg PC, Jansen JR | title = An evaluation of cardiac output by five arterial pulse contour techniques during cardiac surgery | journal = Anaesthesia | volume = 62 | issue = 8 | pages = 760–68 | date = August 2007 | pmid = 17635422 | doi = 10.1111/j.1365-2044.2007.05135.x | doi-access = free }}</ref>