AVNRT

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.  

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.

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.