Augmented limb lead question

pumas

Hello,

To measure the augmented limb lead voltages aVr, aVl and aVf two resistors of value R are used on the (-) side and a resistance R/2 on the (+) side. Could anyone please explain me what is the purpose of the R/2 resistor. I attached a picture.


Thank you

Attached Files

Augmented limb lead voltages.doc (31.0 KB, 24 views)

marcusl

The directional sensitivity of the standard lead system to cardiac activation currents lies in the anterior or coronal plane, but is rather skewed: I is horizontal, II is approximately 60 deg (down and to left), III is 120 deg (down and to right). The augmented limb leads use the same electrodes (LA, RA and LL) but weight and combine them with resistor networks to produce sensitivity vectors that are equally spaced in angle. Diseases and abnormalities will often show up best in one lead or another, so the combination of all six is useful. See the lead sensitivities towards the beginning, and diagnostic examples towards the end, of this:

medresidents.stanford.edu/TeachingMaterials/EKG%20Basics/EKG%20Basics%20-%20Long.ppt

There are other lead systems, too. The Frank VECG (vector ecg) is notable because it includes a lead with sensitivity in the sagittal plane (front to back).

berkeman

Awesome post, marcusl. And thanks for the link to the PPT file -- very helpful.

marcusl

Glad to be of help :o)

berkeman

You were more than basic help, marcusl. I followed some of the tree links from your post and found great study materials. I'm an EMT outside of my regular EE job. Thank you very much.

Proton Soup

it's been a really long since i've studied this, and looking through an old book wasn't a complete help so take it for what it's worth. but you've essentially got a two-lead signal feeding into an instrumentation op-amp. and even though the input impedance to the amp is high, the signals are small and the isolation resistors are high also. the two R leads balance out the input bias current so that your equivalent input resistance there is R/2. and so the single resistor lead also becomes R/2.

anyhoo, it's the sort of thing that only biomedical instrumentation engineers worry about. i may have the course notes on it somewhere...

marcusl

This description does not get to the purpose of the resistor network, and in any case the reality is more complicated. First, the input impedance of the instrumentation amps used in this application are incredibly high and bias currents are incredibly low--10 Gohm and 500 pA are typical even for cheap plastic-packaged IC's--so resistors in the leads are not there for balance. In fact the I, II and III leads are usually direct-coupled. (Of course there are three amplifiers, one for each lead pair).

Since patient safety is critical, the amps are commonly followed by optically-coupled isolation amps eliminating any possible connection to earth ground or power even if the ECG machine is AC powered.

Finally, the input amps are ground-referenced to yet another electrode which is standardized to be the right leg. It is common to actually drive the right leg to minimize common-mode potentials, since the patient is floating with respect to earth ground.

Which brings us back to the resistors; they form a signal-weighting network that transforms the RA-LA-LL "coordinate system" to another coordinate system with increased sensitivity to certain abnormalities and disease states, as I stated earlier.

Proton Soup

if there's no current flow going on here, then how do you suppose the weighting of the resistors factors in? should be simple to show how changing R/2 to R will change it, no? i didn't see where your ppt presentation addresses this.

rather, i think it's about CMRR.

marcusl

These resistors aren't attached to the patient, they are located after the amplifiers in the ECG instrument. Their relative values are chosen to produce the desired output weighting.

Proton Soup

link me to a circuit that shows this.

marcusl

I suspect it's in Geddes and Baker, Applied Biomed Instrumentation, but I don't have my copy handy. You can also look at Fig. 2, p. 23 of
http://www.hpl.hp.com/hpjournal/pdfs...Fs/1991-10.pdf
showing one of the 3 channels in a clinical product.

Proton Soup

thanks. the book i'm looking at is Medical Instrumentation Application and Design, Webster, editor, 1992, so it's from about the same era as that HP equipment.

the first stage of that circuit appears to be an instrumentation amplifier, which is used primarily because it has very high Common Mode Rejection Ratio (CMRR) and impedance. the precision resistors probably correspond to the resistors in the instrumentation amp. they have to be laser trimmed to maximize CMRR.

http://en.wikipedia.org/wiki/Instrumentation_amplifier

the resistors that have to be trimmed in this stage are the R1 resistors in the above link. and when i say "instrumenation amp", i don't mean the whole network in the link above, just the two input amps, the R1's, and Rgain. the exact value of Rgain is not critical.

at this point they break the circuit up with muxes so that they can switch between other pairs of instrumentation amps and add the driven right leg circuit feedback (which also improves CMRR, fwiw). the part they have labeled as a differential amp corresponds to the amp with resistors R2 and R3 in the wiki link. resistor values aren't as critical now. and then they do some post processing on the signal.

if lead resistances are automatically chosen in a lead selector circuit, they are not shown in that HP literature figure. that figure appears to address only the ASIC they designed for the amplifier.

i'm sure the input(lead) resistors are still there no matter what. they are a low-tech failsafe to protect the patient in case you were to fry your amp and accidentally provide a low-impedance path to ground. making the aVr input R/2 simply balances the input impedance because the aVl and aVf combined in parallel work out to R/2 back to your signal reference.

marcusl

Thanks, I am familiar with instrumentation amps and CMRR. The authors clearly indicate in the article's text that lead weighting is done after the input amps in the "precision resistor network" in the figure. Having said that, however, I agree with you that the lead weighting resistors can be put before the amplifiers, and maybe they was a time (before my time) when they always were. Current limiting resistors are actually a must to prevent instrument damage should a patient be defibrillated, and I always included them. And if resistors are used at the patient ports for either of these reasons, then I agree that the impedances need to be balanced. I mistakenly thought you were saying that they had to be there to balance the input bias currents.

As a historical note, Burr-Brown's 3656 was the first isolation amp IC available, and it made ECG design easy when it came out. The isolation was good up to 8kV and its isolated power supply easily powered three instrumentation input amps. An instrument designed around this unit, with proper layout and a plastic case, offered excellent patient safety. I found that the data sheets are still available online:
http://www.datasheetcatalog.com/data...6/3656BG.shtml

Proton Soup

no, i just think it's the value it is to maximize CMRR.

and yeah, i remember Burr-Brown being very popular back then.

marcusl

Fair enough!
So you're an old-timer too!