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AVNRT

Paroxysmal Supraventricular Tachycardia

Sun, 04/12/2015 - 13:59 -- Dawn

This ECG is from a man in his 60's who is experiencing chest discomfort and palpitations.  The onset of the rapid heart rate and the symptoms was sudden, while he was at rest.  The rate did not slow when he was placed on oxygen, given IV fluids, and rested further. The rate is 177 / min.  

The rhythm is AV nodal reentry tachycardia (AVNRT), which is one of the rhythms that falls into the category of paroxysmal supraventricular tachycardia (PSVT).We can see signs of retrograde P waves in some leads (II, III, aVF, V1).  AVNRT is caused by a reentry circuit in the AV node.

 

Some instructors teach students that sinus tach is approximately 100-150 per minute, and atrial tach is usually 150-250 per minute.  If students only learn about differentiating these two rhythms by the rate difference, it will cause later problems.  Of course, there is actually an overlap in rates between the two rhythms.  For example, a febrile, dehydrated infant could easily reach this rate and be in sinus rhythm.  A young, healthy person on a treadmill could, too.  Clues to the ectopic origin of this rhythm are:  sudden onset (unfortunately, not witnessed here), regular rhythm with unwaivering rate, and the patient's situation (symptoms while at rest, no obvious reason for sinus tach).  Of course, we need to teach to the level of our students' abilities.  Consider whether they just memorizing rhythms criteria now, or are they learning about re-entry?

ECG Basics: Paroxysmal Supraventricular Tachycardia Treated With Adenosine

Sun, 12/28/2014 - 17:43 -- Dawn

This series of ECG rhythm strips shows a paroxysmal supraventricular tachycardia successfully treated with adenosine.  The patient was complaining of a rapid heart rate and palpitations, but was hemodynamically stable.  It is not known whether any parasympathetic stimulation, such as a Valsalva maneuver or carotid sinus massage, was used initially.   

The first rhythm strip shows a PSVT, presumably AV nodal reentrant tachycardia, at a rate of about 215 per minute.  (We originally indicated a rate of 240 per minute, but this was a typo). Using the simplist method of determining rate, the six-second method, we see 21, but almost 22, QRS complexes in six seconds. Differential diagnosis would include sinus tachycardia, but this rate is too fast for sinus tach, especially in a resting patient.  Also, sinus tach would slow down as the patient is rested or made more comfortable, and this rate did not vary.  Also, when confronted with a supraventricular tachycardia, one should also consider atrial flutter and atrial fibrillation.  This is somewhat slow for atrial flutter with 1:1 conduction, and that rhythm is much more rare than AVNRT. It is too regular for atrial fibrillation.  So, we are left with the probable diagnosis of paroxysmal supraventricular tachycardia.  The “paroxysmal” part is presumed since AVNRT has an abrupt onset, and the patient’s symptoms started suddenly. 

The second rhythm strip shows what happened after adenosine was administered.  The patient received first a 6 mg dose, rapid IV push.  When that was not effective, he received 12 mg rapid IV push.  The rhythm strip is typical of the first minute or so after adenosine administration.  Adenosine can cause transient AV blocks, escape rhythms, and ectopic irritability.  The half-life of adenosine is only 6 seconds, so the dysrhythmias and uncomfortable symptoms are short-lived.  In this strip, we see frequent PVCs and runs of V tach.  

Supraventricular Tachycardia

Thu, 12/04/2014 - 23:47 -- Dawn

This ECG shows AV nodal reentrant tachycardia in an elderly man.  Clinical information is not available.  AVNRT is the rhythm most often associated with the term, "supraventricular tachycardia".

Although we can't see the beginning of this rhythm, one of the identifying features of SVT is an abrupt (paroxysmal) onset.  In patients with AVNRT, there are two pathways in the AV node, a pathway with fast conduction and a long refractory period, and a pathway with slow conduction and a short refractory period.  Normal sinus impulses travel down the fast pathway and into the ventricles, but also start up the slow pathway in a retrograde direction. The retrograde impulse and the normal impulse traveling down the slow pathway collide, cancelling each other out.  If a PAC occurs, it will travel down the slow pathway while the fast pathway is still refractory.  By the time the impulse reaches the end of the slow pathway, it finds the fast pathway no longer refractory, and travels back up to the atria.  This forms a circular movement (circus movement) of the impulse, and it repeats itself rapidly until interrupted.  When each impulse reaches the ventricles, it travels into the interventricular conduction system and causes ventricular depolarization and contraction, usually at a rate of 140 - 250+.  Unlike sinus tachycardia, AVNRT does not adjust its rate according to the needs or activity of the patient.

There are many forms of supraventricular tachycardia, and they are not always easy to differentiate based on ECG criteria alone.  AVNRT of the type described above is the most common PSVT in structurally normal hearts.  For more information on supraventricular tachycardia, go to Life in the Fast Lane.  For a discussion on clinical management, we recommend Dr. Grauer's ECG Video 6 - Rhythm Mgmt-Part 3.

ECG Basics: Paroxysmal Supraventricular Tachycardia

Fri, 11/21/2014 - 21:07 -- Dawn

This two-lead rhythm strip clearly shows the transition from normal sinus rhythm to a paroxysmal supraventricular rhythm.  In this case, the arrhythmia is AV nodal reentrant tachycardia, AVNRT.  The rate of the first rhythm, NSR, is around 75 per minute.  The fourth beat on the strip is a PAC which initiates the paroxysm of tachycardia lasting 12 beats.  The arrhythmia terminates spontaneously at that point.  The tachycardia rate is about 150/min.

The topic of supraventricular tachycardias can be a very complex one to teach.  For an excellent example of a concise lesson geared toward Primary Practice physicians, go to Dr. Grauer's VIDEO - Part III of his Arrhythmia series.

To cover the important points for the beginner-level student:

  *  It can be difficult to determine a rhythm is SVT if the rhythm is near 150 bpm and you DON'T see the beginning or end of the arrhythmia.  If the onset (or offset) is sudden, then this is not a sinus rhythm.  The sinus node speeds and slows more gradually - it doesn't change rates in one heartbeat.  This strip has an excellent view of BOTH the onset and the offset.

  *  The faster the rate, the more likely we are looking at a PSVT rather than sinus rhythm.  If a sinus tachycardia exists, we can almost ALWAYS see the reason for it in the patient's clinical situation.  We may see fever, dehydration, bleeding, fear, pain, exercise.  Therefore, a patient at rest with a rate of 150 would be suspect for PSVT.  A patient on a treadmill for 5 minutes would be considered to have a sinus rhythm.

  *  Any patient with a rate around 150 per minute should be evaluated for ATRIAL FLUTTER with 2:1 conduction.  Atrial flutter often conducts at that ratio, because a rate of 150 is fairly easy for the AV node to conduct, whereas the instrinsic rate of atrial flutter (250-350) is not.  A 12-lead ECG makes it easier to search for tell-tale flutter waves.

Supraventricular Tachycardia

Thu, 01/02/2014 - 13:36 -- Dawn

This example of supraventricular tachycardia is from a 51-year-old woman who presented to the Emergency Dept. with a complaint of palpitations.  We do not have any more clinical information for her.  We know this tachycardia is supraventricular because the QRS complexes are narrow.  The term, "supraventricular tachycardia", or "SVT" actually describes many different rhythms with many different mechanisms and causes.  It would help a great deal to see the beginning (or end) of this rhythm to determine if the onset was sudden (paroxysmal) - or gradual.  Sinus rhythms tend to speed up gradually - picture a patient on a treadmill getting a faster and faster heartrate.  Take the patient off the treadmill, and the rate gradually decreases.  This reflects normal function of the sinus node.

SVT with paroxysmal onset indicates a re-entry mechanism, where the impulse travels from the atria to the ventricles by one pathway, but is able to return in a retrograde fashion, following a second pathway in the atria or AV node, and reenter the ventricles, depolarizing them antegradely, causing another QRS.  The atria are usually depolarized retrogradely as well.   Reentry can occur in the sinus node, in the AV node, or around the AV node via accessory pathways in the atria.  For a very clear and concise discussion of AV nodal reentry tachycardia (AVNRT), go to this Life in the Fast Lane link.  There is also a great discussion of AVNT and pre-excitation syndromes, including Wolff-Parkinson-White syndrome, at LITFL.

In this ECG, we do not know the patient's clinical situation,  and we haven't seen the onset of this rhythm.  The rate is 166 / min.  In order for her sinus node to naturally reach this rate, we can assume she would have some visible reason for tachycardia:  extreme anxiety, blood loss, hypoxia, exercise, etc.  An "inappropriate" sinus tachycardia is a possibility, but not the most common thing.  If she describes this rhythm as having a sudden onset, the most common and most likely diagnosis is AVNRT, also called SVT or PSVT.  

The presence of retrograde P waves can aid in the diagnosis of AVNRT.  Retrograde P waves are negative in Leads II, III, and aVF.  They can occur before, during, or after the QRS.   In this ECG, we cannot see P waves, either before or after the QRS.  Some of the QRS complexes (limb leads) have a small notch at the end, but it does not look like a typical retrograde P wave in II or aVF.

If sinus tachycardia can be ruled out, it is safe to treat this rhythm with Valsalva maneuvers and adenosine, and then investigate the cause.

ECG Basics: Supraventricular Tachycardia

Fri, 05/17/2013 - 22:20 -- Dawn

This strip is from a patient who experienced a sudden onset of palpitations and rapid pulse while at rest.  It shows a narrow-complex tachycardia, specifically a paroxysmal supraventricular tachycardia.  The subject of supraventricular tachycardias is a fascinating one, and is covered extensively throughout this website.  The mechanisms of SVT are many, and can be complex for the beginning student to understand.  Search the search terms on the left side of the page for entries from ASK the EXPERT and JASON's BLOG for more advanced information about SVT.

For the beginner, it is important to teach the difference between sinus tachycardia and "supraventricular tachycardia".  Of course, sinus tachycardia IS supraventricular - but current convention has us using the term "SVT" for atrial or junctional tachycardias, and especially for reentrant tachycardias.  Beginner students should understand the function of the sinus node, and it's ability to control the heart rate, based on direction given by the nervous system.  The sinus node increases and decreases the rate incrementally, or more gradually than the onset and offset of a reentrant tachycardia.  The appearance of a sudden onset of regular tachycardia following a PAC, producing a rhythm with a distinctly faster rate than the original sinus rhythm, is a sure sign of SVT.  When the onset or offset are caught on the rhythm strip, our job is SO much easier!

The heart rate helps with the diagnosis.  SVTs tend to be faster than sinus tachycardias.  SVTs tend to be faster than 150/min, while sinus rhythms TEND to be slower than 150.  And patients with sinus tachycardia usually have a readily determined reason for the tachycardia, such as fever, pain, fear, hypovolemia, hypoxia, or exertion.  So, a patient on a treadmill for an exercise stress test might very well have a heart rate over 150 / min.

When your students master the understanding of the different behaviors of the sinus node and the reentrant rhythms, and how important patient presentation is to the diagnosis, you will want to add atrial flutter and atrial fibrillation.  When these are mastered, be sure to remind your students that atrial flutter can conduct 2:1, and will mimic sinus tach.  A good rule of thumb is: Under 150/min.:  look for sinus tach first.  Around 150 / min.: look for atrial flutter with 2:1 conduction.  Over 150/min.: suspect PSVT.   

 

How do you teach the concept of reentry in PSVT to students with only a basic understanding of dysrhythmias?

Sun, 03/04/2012 - 02:13 -- Dawn

Our expert today is Dr. Ken Grauer. He is a frequent contributer to the ECG Guru.

 

KEN GRAUER, MD is Professor Emeritus (Dept. Community Health/Family Medicine, College of Medicine, University of Florida in Gainesville).
Dr. Grauer has been a leading family physician educator for over 30 years. During that time he has published (as principal author) more than 10 books and numerous study aids on the topics of ECG interpretation, cardiac arrhythmias, and ACLS (including an ongoing Educational ECG Blog.

 
Answer:
PSVT is a reentry tachycardia. This arrhythmia carries many names, one of which is AVNRT = AV Nodal Reentry Tachycardia - recognizing that in the vast majority of cases, there is reentry occurring in or around the AV node. The AV node is not a homogeneous structure - instead functionally (and anatomically on a microscopic level) - there are 2 basic pathways. One of these conducts "fast" - and the other "slower". Conduction preferentially goes down the "fast" pathway (thereby 'blocking' and preventing conduction down the slow pathway). But if for any reason (like a PAC) the fast pathway is "blocked" - then conduction of the impulse will have to go down the "slow" pathway. If the timing is just right - conditions may be set up that allow "reentry" within the AV node - with the impulse going down the slow pathway and up the fast pathway. Less commonly, reentry within the AV node may be set up in which the impulse goes down the fast pathway and back up the slow pathway.

 

Think of the phenomenon of reentry as comparable to the situation when 50 young children are all holding hands and running around in a circle while holding hands. All it takes is for one disgruntled person to stick out their leg - and ALL 50 of the children who are holding hands will fall down. So it is with reentry - it is a circuit that is set up by fortuitous circumstances of conduction speed, refractory period duration, and usually a precipitating premature impulse. How do we treat AV Nodal Reentry Tachycardia (PSVT)? Either by vagal maneuvers or medication such as adenosine, diltiazem/verapamil, or beta-blockers - ALL of which at least transiently alter conduction properties within a portion of the AV node. Just like the disgruntled citizen who stuck out their leg and tripped up all 50 children - all it takes is brief alteration/interruption of the conduction circuit (by meds or vagal maneuver) to terminate an AV Nodal Reentry Tachycardia.

 

Simplistic illustration of the concept of reentry appears in the Figure below (excerpted from pp 567-574 of Grauer K, Cavallaro D: ACLS: Comprehensive Review [Vol 2] - 3rd Edition, Mosby Lifeline, St. Louis - 1993). For a more detailed look at reentry - visit: https://www.kg-ekgpress.com/reentry-svt/ - where you can download a pdf of the above 7 pages.

 

Ken Grauer, MD ([email protected])

 

 

Exerpted from pp 567-574 of Grauer  K, Cavallaro D: ACLS: Comprehensive Review (Vol. 2) - 3rd Edition, Mosby Lifeline, St. Louis - 1993.

 

 

 

 

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