What To Learn From an Essentially "Normal" ECG
Jerry W. Jones, MD FACEP FAAEM 


ECG 1

First, I want to say that this is a pretty good ECG for an 88 y/o woman! However, there are a few things to discuss. This post is for the intermediate-level ECG "nerds" but you newbies follow along!

First, there is sinus rhythm. We know this because the P waves are upright in both Leads I and II. There is also a POS/NEG biphasic P wave in Lead V1 which is supportive of sinus rhythm. The rate is regular except for one beat near the beginning of the ECG.

PEARL: If you want to check the regularity of a rhythm - especially if you suspect that it is subtlety irregular - focus on the T-P intervals and not so much on the R-R intervals. It is often easier to detect rate changes by the T-P segments due to the bordering T and P waves which will make it easier to detect a rate variation.

The mean QRS axis (ÂQRS indicates "mean QRS axis") in the frontal plane is normal. We know this because the QRS complexes in Leads I and II are both positive (upright). To determine a normal QRS axis in the frontal plane, we use Leads I and II because we now consider a normal ÂQRS to be between +90° and -30°. Many, many years ago, a normal ÂQRS was from +90° and 0°, but we changed during the 1960's. So if you are trying to learn to read ECGs from a website or online course that teaches you to look at Leads I and aVF to determine if the ÂQRS is normal, be assured that they haven't updated their information in almost 60 years! You might also wonder what other misinformation they are disseminating.

The machine has calculated the ÂQRS to be +59°. While I NEVER recommend relying on the ECG machine's interpretations, I think we can usually accept the measurement data that it gives. Occasionally, you will find a ventricular rate that is off because the machine has counted very tall T waves as QRS complexes or that the rate is reported as half of what it actually is because of a QRS alternans, but generally I find the measurements reliable.

This ÂQRS results in Lead II having the depolarization (QRS) with the largest (tallest) amplitude. Note that I did NOT say "largest R wave." That's because sometimes the Lead with the largest QRS is negative and consists mostly of an S or QS wave. Since Lead II has the largest QRS, the Lead that is 90° from it (Lead aVL) consequently has the smallest QRS. It is for this reason that Lead aVL is a very dangerous lead. Most mean QRS axes cluster around +60° which happens to be the axis for Lead II. That means Lead aVL will often be very small and difficult to interpret - especially if it involves any deviation of the ST segment - either elevation or depression. Lead aVL can be very dangerous because an acute MI may be overlooked. An acute STEMI cannot always produce 1 mm of ST elevation in that lead. Always be careful with Lead aVL!

If you look in the Lead V5 rhythm strip at the bottom of the ECG, you will note that this lady has some ST flattening and many of the T waves have an acute, sharp origin of the T wave from the ST segment. That is NOT normal. There should be a gentle rise of the ST segment and the ST segment and T wave onset should blend together imperceptibly. In other words, you should NOT be able to se the precise origin of the T wave. This pattern in Lead V5 - and elsewhere on the ECG - is very suggestive of chronic ischemia.

Now let's address the premature ectopic beat. Is it a premature atrial complex (PAC), a premature junctional complex (PJC) or a premature ventricular complex (PVC)?

A premature atrial complex will always be preceded by a P' wave. A P' wave is any P wave that did not originate in the sinoatrial node. With a PAC, the P' wave is usually upright but when the ectopic P' originates in the lower right atrium, it will be recorded as a retrograde P' wave and it will be inverted (negative) in the inferior leads (Leads II, III and aVF). Regardless, it will always precede the QRS complex. Note that the P' wave IS the PAC - the QRS has nothing to do with it. Many PACs are not followed by a QRS.

A junctional premature complex will always try to produce a P' wave and a QRS... and it will succeed as long as there is no block between the junction and the atrium or the junction and the ventricle. Most of the time, the atrial impulse will arrive in the atria at the same time the ventricular impulse arrives in the ventricles causing the P' wave to be hidden within the QRS. The P' wave of a junctional beat may appear before the QRS, during the QRS (in which case you can't see it) or after the QRS. The P' wave, however, will always be recorded as retrograde, i.e., inverted in the inferior leads.

A premature ventricular beat is easily recognized because it is wide and bizarre appearing. If it manages to cross through the AV node into the atria it will produce a retrograde P' wave. The P' wave will always follow the PVC.

Here's one last PEARL: look closely at the T wave that precedes the premature QRS complex in Lead V5. Compare it to the T wave of the preceding beat. The T wave that precedes the premature QRS is larger. That may appear very subtle to you - but not to me! (OK... it really IS subtle.) My point is that you need to train your eyes so that they can detect these subtleties more easily. It takes a little practice, but it isn't difficult.

Why is the T wave that is hiding the P' wave larger? An upright P' wave (or a true P wave, for that matter) consists of positive voltage. An upright T wave also consists of positive voltage. Any deflection that is above the baseline has positive voltage by definition. We add the voltages and we get a larger positive quantity, so the T wave appears larger. But what if the P' were inverted, meaning it had negative voltage? We would have to subtract that negative voltage from the positive voltage of an upright T wave which would result in a smaller T wave manifesting less positive voltage.

I hope you learned some basic electrocardiography from this post. If you would like to learn more, visit my website at: https://medicusofhouston.com. If you hover the cursor over "HOME" in the menu, you will see a submenu with "Dr. Jones's ECG Blog" containing lots more articles on ECG interpretation.

I teach advanced electrocardiography because when someone's LIFE is in YOUR hands - introductory knowledge is simply not enough!




Comments

Post a Comment

Popular posts from this blog

Image
  Analyzing an Acute MI “Jones’s Rule” and Primary Repolarization Changes Jerry W. Jones, MD FACEP FAAEM Figure 1 This is another 12-lead ECG randomly selected from my collection. There is a bit of baseline wander, but that’s life! It was recorded on a 68 y/o Hispanic female in the emergency department who was complaining of chest pain. There is no other information about her. First, the absolute basics. We can see by the upright P waves in leads I and II that sinus rhythm is present. The pos/neg biphasic P wave in Lead V1 is also supportive of sinus rhythm. The P wave in Lead V2 is basically isoelectric – but that’s OK, as long as it isn’t inverted or biphasic. There is 1:1 sinoatrial conduction and the P-R intervals are normal and remain the same. The rate is quite variable and may represent a sinus arrhythmia. Also, as we will see in a moment, there is also a good reason for increased parasympathetic influence on the rate. No apparent lethal dysrhythmias and no blocks are not...
Read more
Image
  Notes on a Randomly-Selected ECG Jerry W. Jones, MD FACEP FAAEM Figure 1 What is the rhythm on this ECG? Don’t be too quick to respond because I just asked you a trick question! Which rhythm are we discussing – the atrial rhythm or the ventricular rhythm ? Although this looks like a “single” ECG, it is really two ECGs – an atrial ECG and a ventricular ECG – one superimposed upon the other. Let’s begin with the atrial rhythm. Do you see any P waves? I don’t. That – by definition – is either atrial fibrillation , sinus arrest or third degree SA block . Now let’s consider the ventricular rhythm. It appears irregular . It is always best to assume that a ventricular rhythm that is based on an ectopic junctional or ventricular escape pacemaker will be very regular – even though it may be very slow. That isn’t always the case, but it will be about 99% of the time in my experience. OK… let’s put these two observations together: if the atrial rhythm (or lack of an atrial ...
Read more
Image
  Are you treating myocardial infarctions? Why? That's DEAD tissue! You should be treating myocardial ischemia! What's the difference between ST elevation and ST depression? Does ST depression ALWAYS represent a subendocardial ischemia? Does  ST elevation ALWAYS  represent an acute transmural ischemia? Which blood test do you use to confirm an acute transmural ischemia...         ...troponins, or a CK-MB? If you base you diagnosis of an acute transmural ischemia on the presence of ST elevation according to the STEMI protocol, then you are basing your diagnosis on a CK-MB, because that's the test that was used to "prove" that the ST deviation was or was not a STEMI! The STEMI protocol was based on just ONE article that used CK-MB assays as proof of acute transmural ischemia. Is there a problem with that? Well, no... if you don't mind missing about 25% of acute transmural ischemias that should have gone directly to the cath lab but instead were...
Read more