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.
- Electrical Impedance Changes
- Strain Gauge or microphone (mechanical)
- Optical change (change in density)
- 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.
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.
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.
In transmittance method, LED and photoresistor are attached in an enclosure. Then it is fitted to patient’s finger. Light is transmitted through patient’s fingertip. The resistance of photoresistor change. The amount of light reacting with determines the resistance of photoresistor. When heart contracts, blood flow to the extremities and increases the blood flow in fingers. Due to this optical density is altered. Light transmission through finger reduces but photoresistor resistance increases. In this setup, photoresistor is connected as voltage divider circuit. Therefore, it produces a voltage that varies with blood in the finger. The voltage follows the pressure pulse. The waveform is displayed on an oscilloscope.
In this case, photoresistor is placed adjacent to excitation lamp. When light emits from the LED part of the light is reflected and the remaining is scattered from the skin. The scattered light falls on the photoresistor. The blood saturation of capillaries determines the amount of light reflected. Voltage drop is connected across the photoresistor and it acts as voltage divider. It varies in proportion to blood volume changes in blood vessels.
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.