Part Number: ADS1298
I am a student trying to understand the acquisition of ECG signals.
I have a basic query regarding the placement of the electrodes on human body for ECG measurement using ADS1298.
It is a single lead system. The two differential inputs are Right Arm and Left Arm.
But as per my limited understanding, the ECG potential should be measured between Right Arm and Left Leg (Lead II) or Left Arm and Right Leg (Lead III).
Can you please explain the justification for taking inputs from Right Arm and Left Arm with “right leg drive” to help reduce the CMRR requirements of the INA ??
does this imply....
Lead I = lead II - lead III
I mean, if lead II contains info for some set of abnormalities and lead III contains info for some set of abnormalities, the Lead I (= lead II - lead III) will contain info on both sets is the above argument correct ???
What are the benefits for taking Lead I measurement over taking a Lead II measurement ??
The three electrodes that are used for measurement are RA, LA, and LL. The RL electrode is only to provide bias to the patient and to cancel common-mode interference which is ubiquitous in ECG applications. There is a helpful application note that discusses the use of common-mode feedback in ECG systems here.
ECG leads are defined as the voltage between two electrodes. The reason doctors look at multiple leads to to see the electrical activity in the heart at different "angles" which can show different behavior in the heart. Not only do doctors look at the three limb leads, but in some cases they may look at three more "augmented" leads, which show the angles "in between" the limb leads and they may look at "chest leads" which show more angles. Academic cardiology may even study dozens of ECG leads that each show some different type of behavior. However, the knowledge of what each unique lead shows is the expertise of doctors and biology PhDs, not electrical engineers at Texas Instruments :-).
You are correct that Lead I = Lead II - Lead III. The concept of being able to calculate a third ECG lead from the first two is known as "Einthoven's triangle" which you can research online. It comes from the simple fact that Lead I = LA - RA, Lead II = LL - RA, and Lead III = LL - LA. By simple arithmetic we can see that Lead I = Lead II - Lead III, Lead II = Lead III - Lead I, and Lead III = Lead II + Lead I. This means that it is possible to measure two leads directly and calculate the third. Simply calculating the third lead is not "cheating". It is widely understood and accepted by doctors and medical device manufacturers.
In reply to Brian Pisani:
In reply to Pankaj Jha:
Thanks for the reply. As a student, it helps me a lot.
A follow-up question:
I have done some literature review and found that most of the capacitively -coupled amplifiers for ECG monitoring implement the same placement of electrodes, i.e. Lead I, as mentioned in the application note I had referred.
So my question is -- what is the justification of extracting only Lead I , if we need at least 2 lead info to get full ECG info ???
Is it that Lead I info is enough for some cardiac abnormalities ??? if yes, what is the domain of abnormalities that Lead I only info can cater ??
What are the tradeoffs of extracting only Lead I and extracting all the 3 leads ???
I do not believe I can answer those questions. I am an electrical engineer and an expert on the ADS1298 and how that device fits in ECG measurement applications, but I am not a cardiology expert. Your question can probably be answered in medical journals or textbooks, or by a physician.
From the electronics design perspective, I know there are some uses for only a single lead ECG as there are products that only support one lead. However, I do not know the clinical details of why they only use 1 lead versus more.
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