Week 7 - Research on the Plethysmograph

As salam..

As the previous post, I'm still on the research the
circuit. For this week, I'm proceed to do a research
for the next circuit that might be suitable for the
measurement of heart rate and pulse oximeter.

I'm also asked the senior that had an experienced

in a project of pulse oximeter and heart rate. Mostly 

few senior that I met, told me that they are used 

plethysmograph. That was my first time heard that kind

of name. ISo that, I'm doing my research on that 

plethymograph circuit. What I found is :

Plethysmograph is an instrument for measuring changes in

volume within an organ or whole body (usually resulting 

in the amount of blood or air it contains). 

A photoplethysmograph (PPG) is a plethysmograph that 

uses optical techniques. A pulse oximeter measures oxygen 

saturation level (SpO2) and is also a PPG.

It can measure the change in the volume of arterial blood

with each pulse beat. This change in blood volume can be 

detected in peripheral part of the body such as the fingertip

or ear lobe using a technique called photoplethysmography.

The pulse oximeter that detects the signal is called a 

plethysmograph (or 'Pleth' for short).

The blood flow is normally shown as a waveform using a 

graph. It can provide useful information regarding the heart 

condition. Below is the different waveform with the status 

of heart beat.

Heart Rhythm

Pleth is useful to detect irregular heart rhythm. The regular

beating or contraction of the heart moves the blood throughout

the body. The heart beating pattern is clearly displayed by 

watching the pleth waveform. This is important to observe the 

pleth waveform because from that the doctor can know 

the condition of the patient's heart.

Signal Strength

Weak signal is indicated by the amplitude of the waveform. 

If the signal is too low, it would affect the accuracy and 

functioning of the pulse oximeter. If your oximeter is not 

giving the correct result, check if the signal strength is too 

low.

There are several causes for the weak signal:

• Low blood perfusion
• Dirty sensor or LED lights
• Improper positioning of the oximeter

All the info I found is help to understand what is actually 

Plethysmograph.. Next post will be about the circuit of

plethysmograph.