Indirect measurement (NIBP) - using Rheographic technique G R O U P 2 B : R A T N A A / P C H O W 1 0 1 1 5 1 8 6 5 N U R U L E L I N A B T R A M L I 1 0 1 1 5 1 0 3 3 W A N A Z A N I B . W A N M U S T A F A 1 0 1 1 5 1 9 3 8 N A R M T H I P A / P S A I M 1 0 1 1 5 1 7 4 5 M U H A M A D R I F A ’ E S H U I B 1 0 1 1 5 1 7 2 4
The Rheographic Method What is Rheography? Recording changes in electrical impedance between electrodes placed on opposite site of a part of the body.
The Rheographic Method A fully automatic apparatus for measuring systolic and diastolic blood pressures has been developed using the ordinary Riva-Rocci cuff and the principle of rheographic detection of an arterial pulse. The change in impedance at two points under the occluding cuff forms the basis of detection of the diastolic pressure. In this method, a set of 3 electrodes, which are attached to the cuff are placed in contact with the skin.
A good contact is essential to reduce the skin electrode contact impedance. Electrode B which acts as a common electrode is positioned slightly distal from the mid-line of the cuff. Electrode A and C are placed at a certain distance from the electrode B, one distally and the other proximally. A high frequency current source operating at 100kHz is connected to the electrodes A and C.
When we measure the impedance between any 2 electrodes before pressurizing the cuffs, it shows modulation in accordance with the blood flow pulsations in the artery. Thus, arterial pulses can be detected by the demodulation and amplification of this modulation. When the cuff is inflated above the systolic value, no pulse is detected by the electrode A. The pulse appears when the cuff pressure is just below the systolic level.
The appearance of the first distal arterial pulse results in an electrical signal, which operates a valve to fix a manometer pointer on the systolic value. As long as the pressure in the cuff is between the systolic and diastolic values, differential signal exists between the electrodes A and C. This is because of blood flow is impeded underneath the occluding cuff and the pulse appearing at the electrode A is time delayed from the pulse appearing at C.
When the cuff pressure reaches diastolic pressure, the arterial blood flow is no longer impeded and the differential signal disappears. A command signal is then initiated and the diastolic pressure is indicated on the manometer.
Advantages In the rheographic method of measuring blood pressure, the cuff need not be precisely positioned as in the case with the Korotkoff microphone, which is to be fixed exactly above an artery. Also the readings are not affected by ambient sounds.
Other technique (photo-electric plethysmograph) This techniques is similar to the rheographic method of measuring arterial blood pressure non-invasively. It can be use to detects cardiac volume pulse and transduces it to an electrical signal. The rate of pulsatile blood-volume change in peripheral site, such as the earlobe or finger tip is used for indicating the magnitude of the arterial systolic blood pressure after calibration against a standard blood pressure measuring means.
The correlation between the rate of peripheral pulsatile blood-volume change and the arterial systolic blood pressure is more close if these quantities are measured during a portion of each cardiac cysle i.e. period of diastole or systole and for just one instant during this period.
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