ADS1298: Confusion on the placement of the electrodes for ECG measurement

Part Number: ADS1298

Hi,

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 ??

Also since

RA-LL=lead II

LA-LL=lead III

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 ??

  • Hello Pankaj,

    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.

    Regards,

    Brian Pisani

  • In reply to Brian Pisani:

    Hi Brian
    Thanks a ton for your support and information.
    To clarify my doubt I would like to refer to the document slyt226.pdf itled " Getting the most out of your instrumentation amplifier design". See figure 5.
    It is a single lead system. The two differential inputs are Right Arm and Left Arm.

    This corresponds to which lead out of Lead I, Lead II and Lead III.???
    How is it related to Lead I, Lead II and Lead III ??
    How to derive the information of either/all of Lead I, Lead II and Lead III ??
  • In reply to Pankaj Jha:

    Hello Pankaj,

    Measuring the voltage between LA and RA, as in Figure 5 of the application note, only provides Lead I. With only 1 ECG lead, you cannot derive any others. You need at least 2 ECG leads to derive any others.

    Brian
  • In reply to Brian Pisani:

    Dear Brian,

    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 ???

     

  • In reply to Pankaj Jha:

    Hello Pankaj,

    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.

    Brian

  • In reply to Brian Pisani:

    Dear Brian
    Thanks for your support.
    Regards
    Pankaj
  • In reply to Brian Pisani:

    Dear Brian,

    Hello !!!

    I have another question on the same topic.

    Is the front end (like  ADS1298 ) which is meant to  measure Lead I, also suitable for measuring Lead II and Lead III ?? Or, do we have different types of front ends  for measuring each Lead???

  • In reply to Pankaj Jha:

    Hello Pankaj,

    The ADS1298 can be used to measure 12 leads of ECG using only one device. This is possible through the use of the on-chip WCT amplifier which generates the reference voltage for the chest leads and also by doing calculations to find the other leads.

    Lead I: IN1P = LA, ININ = RA
    Lead II: IN2P = LL, IN2N = RA
    Lead III: Calculate from Lead I and Lead II in MCU
    aVL: Calculate from limb leads
    aVR: Calculate from limb leads
    aVF: Calculate from limb leads
    V1: IN3P = C1, IN3N = WCT
    V2: IN4P = C2, IN4N = WCT
    V3: IN5P = C3, IN5N = WCT
    V4: IN6P = C4, IN6N = WCT
    V5: IN7P = C5, IN7N = WCT
    V6: IN8P = C6, IN8N = WCT

    You can look at the ADS1298ECGFE-PDK schematic to see a common way of connecting the device to the electrodes. The formulas for the various calculated leads can be found on the web.

    Regards,
    Brian Pisani