Continuous Heart rate Monitoring – Optical Method

Heart rate reveals some important characteristics about the health and well being of human beings. Heart rate is number of heartbeats per minutes which is also the Pulse of human beings. Heart rate is measured in order to aid in diagnosing many heart diseases among other diseases. Other than due to diseases, variation in heart rate from normal rate could be an indicator of poor or unhealthy life style, high stress or anxiety levels etc.

It is useful rather important to continually measure the heart rate all times. Thanks to advents in the electronics it is today quite possible to develop a wearable electronic device that could measure heart rate and such device could also talk to smart phone through an App thus making it possible to analyse and transmit the heart rate parameters. Many portable electronic heart rate monitoring devices are available today some of them are wearable too. Making a wearable heart rate monitor though require precision analogue circuitry, filtering components and software

Measure heart rate with light

For continual measurement of heart rate ECG may not be a best solution because it is not comfortable to wear ECG device for extended hours. For continual measurement device should be portable, wearable like a wrist band etc. PPG (photoplesmythography) is an optical measurement method which is used for heart rate measurement and pulse oximetry by medical professionals. It measures the change in the volume of blood as it distends the arteries and arterioles in the subcutaneous tissue. It is implemented in a form of a finger clip. A beam of light is made to penetrate in to the skin from one side of the finger using an LED and with the help of a photodiode it measures the changes in the light transmission through the finger on the other side. At each pulse, there is a sharp fall in the light detected by the photosensor because increase in the volume of the blood will absorb more light. The output from the photodiode will be a saw-tooth curve as shown below with its base frequency corresponds to the heart rate.

PPG works in two modes, one is transmission mode and the other is reflection mode. The transmission method is explained above. As it can be seen that continual measurement using transmission method could be cumbersome as user will have to wear a clip like device on a finger or on earlobe. However in reflection method a wrist band like device could be worn that indicates the volume change of the radial artery. A block diagram of a reflection method is given below. The only disadvantage is that it is slightly less accurate.

There are several advantages of continual heart rate monitors. Athletes use them all the time to keep a check on their performance & health. Old people or ill people (having heart related illness) would be extremely benefitted by this device. And lastly in modern days people are becoming more & more health conscious. They may find this device useful in keeping check on their health and it could motivate them to become more active!

By - Minaxi Dassi – Asst. Prof. ECE, Chitkara University H.P.

References:

http://www.newelectronics.co.uk