TNO Finger Pressure Reference Guide - Integral Process

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TNO Finger Pressure Reference Guide 102 Measurement principle2.1 IntroductionPortapres and Finometer use the volume-clamp method, originally described by theCzech physiologist Jan Peñáz [PEÑÁZ 1969, PEÑÁZ 1973], to measure bloodpressure in the finger in a manner similar to Finapres. In this method the diameterof an artery under a cuff wrapped around the finger is kept constant (clamped), inspite of the changes in arterial pressure during each heart beat. Changes in arterialdiameter, detected by means of an infrared photo-plethysmograph built in thefinger cuff, are opposed by a fast pressure servo controller that changes pressure inan inflatable air bladder, which is also mounted in the finger cuff. An expert systemis used, Physiocal [WESSELING 1995], to define and maintain the correct diameterat which the finger artery is clamped. The volume-clamp method, and thePhysiocal algorithm are addressed in this chapter.2.2 Finger cuffPortapres and Finometer use wraparound finger cuffs.adebcabcdeAir hose connectorCuff cable connectorCuff air bladderLight detector (Photo Diode)Light source (Light Emitting Diode,LED)Figure 2.1 Finger cuffThe main components of the finger cuff are an inflatable air bladder, and aplethysmograph consisting of a light source and a light detector. The light source is
TNO Finger Pressure Reference Guide 11a LED (Light Emitting Diode) emitting infrared light. The light detector in thefinger cuff is an infrared photodiode.The air bladder is connected to the Portapres/Finometer frontend unit via an airhose and both components of the infrared plethysmograph via a cuff cable. Thefrontend unit is connected via the frontend cable and air hose to the main unit andpump unit in the Portapres waist belt. Finometer uses a combined air hose andelectrical cable to connect the frontend.Figure 2.2 Frontend unit, finger cuffs and height correction unit.2.3 PlethysmographThis section shows an example of the signal coming from a finger infraredplethysmograph when the pressure in the finger cuff is kept constant. One of theheart beats in the figure is marked by two vertical lines, representing the beginsystole and end diastole of the beat. A sudden rise in finger intra-arterial pressureduring systole causes an increase in arterial diameter, which is detected as anincrease in light absorption and thus as a decrease in the signal detected by theplethysmograph. During the diastolic phase of a beat, when blood pressure declinesgradually, the blood is expelled from the artery and consequently the amount oflight detected by the photodiode will increase again.The mean plethysmogram level is further referred to as DC-plethysmogram, thepulsation of the plethysmogram as AC-plethysmogram. Portapres displays the DCand AC component of the plethysmogram during the start-up phase of ameasurement, and on request when the FINGER key is pressed (refer to thePortapres Model-2 User’s Guide). On the display of the Portapres control unit theDC-plethysmogram is abbreviated as V=, the amplitude of the plethysmogram asV:, where the ‘V’ is short for volume signal. Finometer users findplethysmographic information in the ‘Physiocal’ tab if Finometer is in ‘ResearchMode’.
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TNO Finger Pressure Reference Guide - Integral Process

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