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Pulse Rate Measurement

Published on 21/8/2018 & updated on 29/8/2018
When heart muscles contract, ventricles eject blood and a pressure is transmitted along the circulatory system. When the pressure travels through vessels, it causes vessel displacement. The displacement is measurable at various points in circulatory system. This pulse is felt when fingertip is placed over the radial artery on the wrist or other location where an artery passes through below the skin. Pulse pressure and waveform indicates blood pressure and flow. Plethysmograph is the instruments to measure arterial pulse. All Plethysmograph measurement technique responds to change in blood volume as a measure of blood pressure. Velocity of pulse is 10 -15 times faster than blood flow and they do not depend on each other. Various methods to measure pulse (volume) changes due to blood flow are as below.

  1. Electrical Impedance Changes
  2. Strain Gauge or microphone (mechanical)
  3. Optical change (change in density)
  4. Piezo Electric crystal

Electrical Impedance Method

It measures the impedance change between two electrodes. Impedance change occurs due to change in blood volume between them. Impedance change will be very small when compared to total impedance. AC current is applied between electrodes attached to the body. Alternating signal of frequency 10 – 100 KHz is applied to prevent electrode polarization.

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Pulse Rate Measurement

Mechanical Method

In this method, strain gauge is attached to rubber band wound around a limb or finger. When blood volume changes, band expands that results in resistance change of strain gauge. Crystal microphone is used instead of strain gauge for sensitive technique.

Photoelectric Method

Another commonly used technique is photoelectric method. Reflectance and Transmittance methods are used in general. A photoelectric transducer should be suitable for wearing finger or ear lobe. Signal from the photocell is amplified and filtered. The time between two pulses is measured.

Piezo Electric Method

It detects pulse at certain places where displacement of tissue above artery is involved. A sealed capsule consists of piezo electric crystal that undergoes displacement stress. A diaphragm is attached through which displacement is sent to tissue. The crystal is connected to ECG recorder to measure the pulse rate.

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