TNO Finger Pressure Reference Guide - Integral Process

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TNO Finger Pressure Reference Guide 14A servo-controller system usually defines a target value or setpoint and a measuredvalue that is compared with this setpoint. In the servo-controller the setpoint is thesignal of the plethysmogram (unloaded diameter of the artery) that has to beclamped. The measured value comes from the light detector. The amplifieddifference between setpoint and measured value, ‘the error signal’, is used tocontrol a fast pneumatic proportional valve in the frontend unit. This proportionalvalve, modulates the air pressure generated by the air compressor, and thus causeschanges in the finger cuff pressure.FLSV (PG)C 2SWPCDA PID PAC 1M (CP)EPTCompr.Figure 2.5 Block diagram of the volume clamp method of Peñáz.The original block diagram of the method as developed by Peñáz is visualized inFigure 2.5. The arrows indicate a closed servo control loop. The signal V (PG) ofthe photo-plethysmograph, consisting of a light source (L), and photocell (PC)built in a cuff (S) around the finger (F), is compared to a fixed setpoint value (C1).A difference between signal and setpoint, is amplified by DA, PID and PA, anddrives the proportional valve (EPT). Cuff pressure is monitored by the manometer(M). If the switch SW is closed, the servo feedback loop is opened, and a steadycuff pressure (C2) can reach the finger.
TNO Finger Pressure Reference Guide 152.6 Physiocal algorithmUnfortunately, defining the correct unloaded diameter of a finger artery is notstraightforward. An additional problem is that changes in hematocrit, and changesin stress and tone of smooth muscle in the arterial wall affect the unloadeddiameter. Therefore, the unloaded diameter is usually not constant during ameasurement and has to be verified at intervals. Periods of constant cuff pressureare used to adjust the correct unloaded diameter of the finger artery based on thesignal from the plethysmograph in the finger cuff. A disadvantage of this method isthat the measurement of blood pressure is temporarily interrupted during such aperiod. Therefore, the frequency and duration of the periods of interruption arekept to a minimum.It is obvious from the previous sections that the unloaded diameter is close to theaverage diameter at a pressure level where the amplitude of the pulsations in theplethysmogram is largest. However, using only the amplitude of theplethysmogram is relatively inaccurate. The Physiocal algorithm [WESSELING1995] in Portapres and Finometer, developed by the Dutch physicist K.H.Wesseling, not only uses the amplitude, but also interprets the shape of theplethysmogram during periods of constant cuff pressure. By analyzing theplethysmogram at two or more pressure levels, the Physiocal algorithm, explorespart of the pressure-diameter relation and is able to track the unloaded diameter ofa finger artery even if smooth muscle tone changes. The periodic interruption of afinger blood pressure measurement, with constant cuff pressure levels, is furtherreferred to as ‘Physiocal’.Figure 2.6 shows a typical finger plethysmogram at five cuff pressure levels. Themean blood pressure was about 100 mmHg.• When cuff pressure is below mean blood pressure the arterial diameter isrelatively large and therefore the plethysmographic signal is low. Sincecompliance is low in this state the amplitude of the pulsation is small and theplethysmogram only varies significantly during diastole.• When cuff pressure is above mean blood pressure the artery is collapsedduring a substantial part of every heart beat. The plethysmographic signal ishigh (no absorption of arterial blood), and only varies significantly duringsystole.• When the cuff pressure level equals the average intra-arterial pressure theplethysmogram resembles an inverted pressure waveform. The end-diastolicperiod of the plethysmographic waveform is analyzed during a Physiocal.When the top of the plethysmographic signal at end-diastole is too sharp, meancuff pressure is below mean arterial pressure, when it is too flat, cuff pressure
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