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ECG Basics: Second-degree AV Block, Type I

This two-lead rhythm strip shows a normal sinus rhythm at about 63 bpm.  The P waves are regular. After the sixth P-QRS, there is a non-conducted P wave.  The normal rhythm then resumes.  The two most common reasons for a non-conducted P wave in the midst of a normal sinus rhythm are 1) non-conducted PAC, and 2) Wenckebach conduction. The first is easy to rule out.  The non-conducted P wave is not premature, so it is not a PAC.  The second one is a little harder when we only have a short strip to look at.  We are conditioned to look for progressively-prolonging PR intervals until a QRS is "dropped".  In this case, the progression is in very tiny increments that are hard to see unless you zoom in and measure.  But they ARE progressively prolonging.  An easy hack:  measure the last PRI before the dropped beat and the first one after the pause.  You will see that the cycle ends on a longer PRI (about .28 seconds) and the new cycle starts up with a PR interval of about .20 seconds.  Fortunately, this conduction ratio will have very little effect on the patient's heart rate.

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ECG Basics: Paroxysmal Supraventricular Tachycardia Converting to Sinus With PACs

This strip shows a supraventricular tachycardia, rate 196 bpm, after adenosine was administered to the patient.  The PSVT breaks, and an irregular rhythm composed of sinus beats and premature atrial contractions ensues.  This is common after medical cardioversion. The patient later settled into a normal sinus rhythm.  The abrupt change from a fast, regular rhythm to a slower, irregular rhythm is evidence that the tachycardia was due to a reentrant circuit, and not sinus tachycardia.

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ECG Basics: Sinus Rhythm With Non-Conducted PACs

This is a good strip to demonstrate the change in the appearance of a T wave when a premature P wave occurs on the preceding T wave.  The PACs found the atria ready to depolarize and produced a P wave that landed on top of the preceding T wave, making it appear taller than the others.  The PACs also reset the sinus node, causing a slight delay before the next sinus discharge.  The PACs occurred while the ventricles were still refractory, so no QRS complexes followed.

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ECG Basics: Second-degree AV Block, Type II

This rhythm strip was obtained from a man who was suffering an acute inferior wall M.I.  There are ST elevation and hyperacute T waves.  The rhythm is SINUS ARRHYTHMIA WITH SECOND-DEGREE AV BLOCK, TYPE II.    There is also first-degree AV block.

There are more P waves than QRS complexes, with a 3:2 ratio.  The atrial rate varies between 55 -68 beats per minute.  The sinus rate speeds slightly after the dropped QRS in each group. The ventricular rate is about 40 bpm, with grouped beating. (Regularly irregular.)

The PR intervals are steady at 226 ms (slightly prolonged).

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ECG Basics: Multifocal Atrial Tachycardia

Multifocal atrial tachycardia is diagosed when an irregular atrial rhythym is over 100 beats per minute.  It is caused by multiple competing atrial pacemaker sites.  There need to be at least three different P wave morphologies to diagnose MAT.  The PR intervals may vary also.   It is nearly always seen in very sick patients, often with chronic obstructive pulmonary disease and/or respiratory failure.

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ECG Basics: 2:1 AV Block

Second-degree AV block can either be Type I (Wenckebach) or Type II.  In either case, some P waves are conducted to the ventricles, and some are not. Type I blocks usually occur in the AV node, and are often benign. Type II blocks are more often "sub-Hisian", or fascicular blocks, and are more likely to progress to higher levels of AV block and bradycardia.  When a second-degree AVB is conducted in a 2:1 ratio, it is difficult to differentiate Type I from Type II.  Features that favor the diagnosis of Type I are narrow QRS complex and the non-conducted P waves land on the previous T waves - during the refractory period of the ventricles.

Type II blocks are more likely to have a wide QRS with a bundle branch block morphology.  That is because Type II blocks often reflect serious fascicular disease.  A typical Type II block is a persistent bifascicular block (ex: RBBB and left anterior hemiblock)) with an intermittent block in the third fascicle.  Another way to think of it is an intermittent tri-fascicular block. If that one remaining fascicle stops conducting, the patient will be in complete heart block.

Signs of Type II blocks include the wide QRS and also two or more non-conducted P waves in a row.  Also, P waves that are "out in the open", away from the refractory period, but fail to conduct are an ominous sign.

One strategy for reacting to a 2:1 block is to first assess the ventricular rate (54 bpm in this example).  Determine if it is adequate for the patient's hemodynamic stability.  If not, act to increase the rate.  Otherwise, it may be prudent in the stable patient to watch the rhythm strips for a while.  Sometimes, two p waves in a row will conduct - unmasking either progressive prolongation of the PR interval (Type I) or stable PR intervals (Type II). 

The patient in this example was having an inferior wall M.I.  The ST elevation will not always show up on a monitor strip, as it does here.  A 12-lead ECG is the minimum standard for evaluating for coronary artery disease and acute M.I.  It is possible that the 2:1 block will disappear when the atrial rate (about 108 here) is slowed.

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Normal ECG In A Young Adult

This ECG was obtained from a 24-year-old man who was seen in the Emergency Department for chest pain that was determined to be non-cardiac in origin.  He had a fever and cough, with pain on inspiration. His vital signs were within normal range, and he appeared well-perfused. There was no complaint of dizziness or syncope.

So, what does his ECG show?  The ECG should be interpreted in the context of the age and presentation of the patient.  He is young, and has been healthy all his life.  He is lean and reasonably fit.

The rhythm:  the rate is 81 bpm, and the rhythm is regular.  His P waves are upright in Leads I and II, and they are followed by QRS complexes.  The rhythm is NORMAL SINUS RHYTHM.

Intervals:  The PR interval is 137 ms (.137 seconds), and his QRS duration is 91 ms (0.9 seconds).  His QTc is 404 ms.  All are within normal range.

QRS frontal plane axis:  Normal axis, at around 30 degrees.  Lead II has the tallest QRS of the limb leads, which is an indication of axis in the normal range.  When the electrical axis travels towards Lead II, we can expect Lead aVL to be small, or even biphasic.

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ECG Basics: Ventricular Tachycardia

V tach is identified by:  wide QRS complexes (>.12 seconds), rate faster than 100 bpm.  In MONOMORPHIC V tach, all QRS complexes look alike.  There are other mechanisms of wide-complex tachycardia, but they can be difficult to differentiate from a single rhythm strip.  All WCT should be treated as V tach until proven otherwise.

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ECG Basics: Accelerated Junctional Rhythm Overriding Normal Sinus Rhythm

This strip shows a junctional rhythm at a rate of 110 beats per minute. The QRS complexes are slightly wide at .10 seconds (100 ms), and they are within the parameters for supraventricular rhythm. The term, "junctional tachycardia" could be used, also, but this is not likely a "reentrant" junctional tachycardia, which would be fast, regular, and have a sudden onset. That type of junctional tachycardia is a PSVT.   In this strip, we can see the underlying sinus rhythm in P waves that appear to pop up randomly.  However, if you march out the P waves, you will find that they are regular, at a rate of about 90 per minute.  The junctional rhythm has overrun the sinus rhythm.  Most of the P waves cannot conduct due to where they have landed - in the refractory period of the QRS.  The exception might be the P wave after the fifth QRS.  The sixth QRS might be conducted from that P wave.

When accelerated junctional rhythm is encountered, you should suspect DIGITALIS TOXICITY - the classic dysrhythmia associated with digitalis toxicity is accelerated junctional rhythm. Other causes in adults could be beta-agonist drugs such as adrenalin, cardiac infection, ischemia, or surgery.

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ECG Basics: Onset of Atrial Fibrillation

This strip shows the onset of atrial fibrillation.  A fib can be "paroxysmal," meaning that it has a sudden onset, but then stops spontaneously, usually within 24 hours to a week.  A fib can also be classified as "persistent", meaninging that the a fib lasts more than a week.  It can stop spontaneously, or be halted with medical treatment.  "Permanent" a fib is a fib that is resistent to treatment.  

The first three beats in this strip represent sinus rhythm at 75 beats per minute.  At the onset of atrial fibrillation with beat number four, the rhythm becomes irregularly irregular, and the rate is around 140-150 bpm. We can expect new-onset a fib to have a fast ventricular rate, as the atria are sending hundreds of impulses to the AV node every minute. The AV node will conduct as many of those impulses as it can to the ventricles.  Most AV nodes can easily transmit 130-160 bpm.  In a fib, the atria are quivering, not contracting. Because of this fibrillation of the atrial muscle, a fib has no P waves, and therefore, no "atrial kick".  The contribution of the atria to cardiac output (25-30%) is lost. An extremely fast rate can also lower output and overwork the heart, so one treatment goal for a fib is to lower the rate.  This can be done independently of attempts to convert the rhythm.

During a fib, blood clots can form in parts of the atria, especially the left atrial appendage.  If sinus rhythm is restored after these thrombi form, they can embolize and travel to the brain, causing stroke.  Before electively converting atrial fib to a sinus rhythm, the patient may need to be anticoagulated.



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