12 lead help?

[mediKate]

Hey, does anybody know of any good websites/textbooks that really explain 12 leads well? I've tried searching online and come up with nothing all that helpful. They were covered in our EMT training (how to put them on, basic patterns, red flags, etc.) but interpretation was not covered very in depth, and I am having a lot of trouble with them. For instance, bundle branch blocks and "positively" identifying the location of an AMI.
Any ideas would be appreciated.

 

[JPINFV]

Dubin's Rapid Interp is generally a good go-to book for the first time through. Very repetitive and hand holding, which can be a good thing.

http://www.amazon.com/Rapid-Interpretation-EKGs-Sixth-Dubin/dp/0912912065

 

[usafmedic45]

Pretty much in the field, Dubin's will teach everything one could conceivably need to know to be well ahead of the curve.

 

[systemet]

Tim Phalen's 12-lead in ACS is also really good for a basic introduction. You could probably pick up either the 2006 or 1996 editions, and they'd still be useful, and a little cheaper.

http://www.amazon.com/12-Lead-ECG-A...7854/ref=sr_1_1?ie=UTF8&qid=1315720112&sr=8-1

 

[ArcticKat]

http://www.physio-control.com/learning/clinical-topics/stemi.aspx

 

[mediKate]

Thanks a lot, guys! I will check those out.
Truth to tell, a book on 12 leads has got to be one of the few things that I honestly don't care if it is "hand-holding" or repetitive. The more dumbed down the better, as far as I am concerned. 12 leads are just one of those things that you need to make sure you have a REALLY good grasp of the basics before moving on. Or else you end up like me...

 

[medic417]

http://www.multileadmedics.com/

Catch a Bob Page seminar to go along with his book.

 

[systemet]

Just to attempt to answer some of your questions quickly:

One of the keys to understanding 12-lead is understanding why we see different ECG recordings when we view the heart from different angles. This is also related to the concept of a mean QRS vector, and something called "axis deviation". We can discuss these, but for the moment, we'll try and keep it relatively simple.

Any ECG lead defines a plane (or perhaps more accurately a "line segment") through the heart. We typically define a horizontal plane as 0 degrees -- this corresponds to the position of lead I. We see any depolarisation that occurs in the direction of the positive electrode (in this case the left arm), as a positive deflection on the recording. We see anything that moves towards the negative electrode as a negative deflection. You're probably already aware of this.

What usually isn't covered in EMT class, is that any signal that occurs at an angle of 90 degrees, is lost completely. This represents a depolarisation in the vertical plane. But we don't see a signal corresponding to this activity in lead I, because there is no horizontal component. The closer the angle of a given depolarisation to 0 degrees, the more of a positive signal we observe. The closer the angle to 90 degrees, the less we see. And then as we move from 91 degrees to 180 degrees, we see a progressively more and more negative component. In Lead I.

While this might seem overly complicated, the idea is that any given lead picks up signals from one region of the heart better than other regions. If we want to know what's happening in the lateral wall of the left ventricle, lead I is a great place to look. If we want to know what's happening in the inferior wall / apex of the heart, then lead I is not the best place to look. So any single lead only gives us a limited amount of information.

Our limb (I O, II, +60, III, + 120) and augmented leads (aVR - 150, AVL - 30, AVF +90) give us a view of the heart in the frontal plane. Think of a man standing in a guillotine. The guillotine falls, this is the plane we're looking in. We see the lateral wall (I, aVL), and the inferior wall (II, III, aVF) [simplified].

However, this gives us very little information about the anterior wall, for example. So to do that, we use the chest or precordial leads (V1-V6). We could set these (or close approximations) up like the limb leads, by placing negative electrodes on the back, and positive electrodes on the front of the chest. Instead we do a little bit of electrical trickery and we join the signals from the left foot, right arm and right shoulder together to form a "virtual electrode". This can be thought of as lying in the center of the heart, and forms the negative electrode for the chest leads. This is also known as "Wilson's central terminal".

The precordial leads then give us a view from the center of the heart through the front of the chest. These allow us to see the septum (V1, V2), the anterior wall (V3,V4), and the lateral wall (V5, V6) [If you doubt this, consider the direction of a line drawn from the center of the heart towards V6, and compare how that would lie relative to lead I].

So all in all, these give us a view of the following regions of the heart:

inferior: II, III, aVF
anterior: V1-V4 (septal V1-V2)
lateral: I, aVL, V5, V6.

We primarily obtain information from the left ventricle, as it's mass is much larger, and dominates the signal recorded. The advantage to having multiple leads, is we can now localise a change to a given region of the heart.

For example, if we know that ST elevation is an indicative of myocardial infarction, and we see elevation in V1 through V4, we can say that we have evidence for anteroseptal ischemia. We can identify the area where the myocardium is dysfunctional. This may allow us to guess which coronary artery, or which branch of a given coronary artery is occluded, and may cause us to opt for different treatment pathways, e.g. RVI versus left ventricular MI.

---------------------

Regarding bundle branch block, our primary indication that a BBB is present is that the QRS is >0.12 seconds. We then look at lead V1 to determine whether we are dealing with a left or a right bundle branch block.

The simplest method is to identify the end of the QRS complex, where it meets the ST segment. This is called the J point. We then move backwards from the J point, into the QRS, and determine whether the last part of the QRS slops downwards from the J point (left bundle branch block), or upwards (right bundle branch block).

The clinical relevance of this is that LBBB can indicate a new MI, or (if pre-existing), can obscure evidence of a new MI. It makes identification of criteria for thrombolytics / angiography more challenging (but not impossible). The presence of a LBBB almost always suggests some sort of underlying ischemic heart disease. [Although this may not be the patient's current problem.]

In contrast, in the setting of RBBB, we can still rely on interpretation of the ST segment to determine whether an MI is occuring, without having to do any correction. Unlike LBBB, RBBB is quite common in patients with pulmonary disease.


Hope this helps. Feel free to ask further questions here or by pm.

 

[systemet]

http://lanoswww.epfl.ch/personal/schimmin/uni/ecglex/ekg.htm

This is an ok website.

 

[mediKate]

Aahhh, systemet, why could you not have been one of my instructors???? Thanks for the great reply! This makes so much more sense when it is explained like that.

(P.S. loved the guillotine...)

 

[dixie_flatline]

My pop's a cardiologist. I asked him about EKG books not too long ago in preparation for medic classes and he recommended Marriott's Practical Electrocardiography first, and Dubin's as a close second.

 

[systemet]

Aahhh, systemet, why could you not have been one of my instructors???? Thanks for the great reply! This makes so much more sense when it is explained like that.

(P.S. loved the guillotine...)

Glad to be of help. Like I said, feel free to pm me or post here if you have any other questions, or you want something clarified.

Good luck.