26 August ~ 1 September 2012 (Week 4)
(Research)

Tittle of activity

- Design procedure of ppg .

Objectives 

-To know what is the design procedure ?

Content / Procedure

Search from internet


1.infrared emitter

               Infrared was chosen as the light source because infrared is well absorbed by blood and very weakly absorbed by other tissues and fluids in the body.  This means that the blood volume changes can be very easily observed.  The iron content in the blood reflects the infrared waves.  During systole, the heart pumps blood through the body.  This causes the volume of blood in the tissue to increase [by sara sieg and bill ziebold project 38. ppg).  This results in more absorption of the infrared light.  During diastole, the heart muscle relaxes and fills with blood.  This results in a lower volume of blood in the tissues.  When there is less blood in the tissues, there is less absorption of the transmitted signal.  Therefore, the output of the photoplethysmograph should emulate the waveform shown in Figure 1 :

The frequency of the pulse rate is dependent on other physiological factors.  When the body is at a higher temperature, or there are increased amounts of physical activity or stress, the pulse rate increases.  Also, the amount of blood pumped through the body per minute increases.  This will result in a stronger output signal on the photoplethysmograph.  The contrary is also true.  If the tissue being measured is elevated above the heart or is very cold, the blood flow will be significantly decreased.  This will result in a marked decrease in the output waveform of the photoplethysmograph.

2.Infrared Receiver



An infrared receiver was chosen to be matched to the infrared emitter.  This was done to ensure that the receiver would be able to detect the signal from the emitter.  The matched infrared receiver was
also chosen to minimize the amount of ambient light that would be detected.  Any detected ambient light would interfere with the output of the photoplethysmograph and appear as noise.  This would make it hard to differentiate the output signal from the changes in blood volume.  It is also beneficial to use an infrared emitter because changes in blood oxygen content are very prominent in the visible light region.  Restricting the light that is detected will minimize any measurement of oxygen changes in the blood.  The receiver was placed so that the light from the emitter was transmitted, as opposed to reflected, through the skin and tissues.  This was done to prevent any interference from the emitter.  When the receiver is placed next to the emitter on the same surface, the receiver may pick up some of the emitted signal before it travels through the body as well as the reflected signal.  This leads to a very confusing and inaccurate waveform.  Also, the signal to wave ratio of a reflection mode photoplethysmograph is much lower than the transmission mode photoplethysmograph [by sara sieg and bill ziebold project 38. ppg].  This occurs because more of  the signal is transmitted than reflected.


3.Device Placement
The placement of the emitter and receiver on the body is also an important aspect of the photoplethysmograph.  The areas chosen were the earlobe and the area between the thumb and the index finger.  They were chosen because of the consistency of the tissues and also because it would be relatively easy to hold the emitter and receiver in place.  These areas on the body can also be held relatively still to reduce motion artifact which would distort the output signal.  Another area of interest was the tip of the index finger.  This area was rejected because of the presence of bone that would reflect a higher proportion of the wave.  This would significantly reduce the strength of the signal that could be received.  To make measurements on this part of the body, it would be necessary to use a receiver in reflection mode.  Another part of the body considered was the nasal septum.  This area of the body is often used in the clinical setting.  The nasal septum is ideal because even if the body goes into shock, there will still be blood flow to the nasal septum.  When the body goes into shock, blood flow is significantly reduced to the extremities.  However, during shock, blood still flows to the brain and other major organs including the nasal septum [The placement of the emitter and receiver on the body is also an important aspect of the photoplethysmograph.  The areas chosen were the earlobe and the area between the thumb and the index finger.  They were chosen because of the consistency of the tissues and also because it would be relatively easy to hold the emitter and receiver in place.  These areas on the body can also be held relatively still to reduce motion artifact which would distort the output signal.  Another area of interest was the tip of the index finger.  This area was rejected because of the presence of bone that would reflect a higher proportion of the wave.  This would significantly reduce the strength of the signal that could be received.  To make measurements on this part of the body, it would be necessary to use a receiver in reflection mode.  Another part of the body considered was the nasal septum.

Result/Procedure 

got the knowledge of the design procedure .