Finapres systems are built on combinations of four technologies.
Volume clamp with physiocal technology The volume-clamp method was first introduced by Czech physiologist Prof. J Peñáz in 1967. With this method, finger arterial pressure is measured using a finger cuff and an inflatable bladder in combination with an infrared plethysmograph, which consists of an infrared light source and detector. The infrared light is absorbed by the blood, and the pulsation of arterial diameter during a heart beat causes a pulsation in the light detector signal.
The first step in this method is determining the proper unloaded diameter of the finger arteries, the point at which finger cuff pressure and intra-arterial pressure are equal and at which the transmural pressure across the finger arterial walls is zero. Then the arteries are clamped (kept at this unloaded diameter) by varying the pressure of the finger cuff inflatable bladder using the fast cuff pressure control system.
A servo-controller system usually defines a target value or set point and a measured value that is compared with this set point. In the servo-controller the set point is the signal of the plethysmograph (unloaded diameter of the arteries) that must be clamped. The measured value comes from the light detector. The amplified difference between the set point and measured value, "the error signal", is used to control a fast pneumatic proportional valve in the Frontend Unit. This proportional valve modulates the air pressure generated by the air compressor, thus causing changes in the finger cuff pressure in parallel with intra-arterial pressure in the finger so as to dynamically unload the arterial walls in the finger. The cuff pressure thus provides an indirect measure of intra-arterial pressure.
Defining the correct unloaded diameter of a finger artery is crucial for the accuracy of the measurement. Changes in hematocrit, stress and the tone of smooth muscle in the arterial wall will affect the unloaded diameter. Therefore, the unloaded diameter is usually not constant during a measurement and must be verified at intervals. Periodically, constant cuff pressure are used to adjust the correct unloaded diameter of the finger artery based on the signal from the plethysmograph in the finger cuff.
The Physiocal (abbreviation for Physiological Calibration) algorithm in Finapres devices, not only uses the amplitude, but also interprets the shape of the plethysmograph signal during periods of constant cuff pressure. By analyzing the plethysmograph signal at two or more pressure levels, the Physiocal algorithm, explores part of the pressure-diameter relation and is able to track the unloaded diameter of a finger artery, even if smooth muscle tone changes. The periodic interruption of a finger blood pressure measurement, with constant cuff pressure levels, is further referred to as "Physiocal".
Physiocal is the automatic algorithm that calibrates the finger arterial size: now the finger cuff air pressure equals the finger arterial blood pressure. Physiocal is built into the Finapres devices and users can determine whether they wish to use the Physiocal algorithm or switch it off during tests in which uninterrupted data-collection is required.
Brachial arterial reconstruction technology There is a delay of several dozen milliseconds between finger blood pressure pulsations and intra-brachial pulsations since the former travel further. In addition, their levels are generally lower and the waveforms appear more distorted, mainly due to reflections and pressure gradients.
To correct the distortion in finger pressure relative to brachial artery pressure, a frequency dependent filter can be used to restore the waveform at the brachial level. This brachial artery pressure reconstruction technique allows clinicians and researchers to obtain the brachial arterial pressure if they wish to perform a more precise measurement at the heart level. Waveform filtering is done in real-time.
The transfer function from brachial to finger resonates at about 8Hz . This causes oscillatory distortions of the finger wave. Distortions can be removed by a digital filter that has an anti-resonance at 8Hz . The two transfer functions compensate each other almost perfectly to produce a desirably flat overall transfer function.
Upper arm calibration technology The return to flow calibration is an individual upper arm calibration that ensures the accuracy required by the common blood pressure measurement standards like AAMI SP10 and BHS.
For this purpose, an arm cuff is wrapped around the same arm as the finger cuff and the operator can set the computer system to automatically inflate and deflate the arm cuff. When arm cuff pressure is supra-systolic, no pulsations can be sensed in the finger.
The first slight pulsation that passes under the arm cuff signals return to flow. It is sensed in the finger and detected by the software. The arm cuff pressure is read at that instant and the reconstructed brachial pressure is defined by this individual amount, thus improving bias and precision substantially.
The absolute accuracy with respect to blood pressure measurements has been found in comparative studies with intra-arterial blood pressure as the reference. The Finometer® has passed the AAMI/SP10 and BHS standards. The AAMI/SP10, for example, demands that findings between stethoscope measurements and the automated device are less than 5± 8 mmHg (average ± standard deviation). The BHS demands, for example, that at least 50% of the measurements between stethoscope and device are less than 5 mmHg and that the difference is less than 15 mmHg in 90% of the measurements.
Modelflo technology Modelflo is a model-based method and algorithm used to compute the aortic flow waveform from an arterial blood pressure pulsation by simulating a nonlinear, self-adaptive model of the aortic input impedance.