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.
This series of three ECGs is from a 60-year-old man who was brought to the Emergency Department after being involved in a motor vehicle accident. No injuries were found, but the patient was severely intoxicated by alcohol consumption. He was conscious but agitated.
ECG NO. 1 15:07:23
The first ECG was taken by fire-rescue personnel at the scene of the accident. His hemodynamic status was stable, and the rate was not addressed in the field. ECG No. 1 shows a supraventricular rhythm at 161 bpm, with a narrow QRS and P waves visible before each QRS.
A notable feature of this ECG are the left axis deviation, by default diagnosed at left anterior hemiblock (left anterior fascicular block). The .10 second QRS width is typical of LAHB, as is the rS pattern in Lead III.
Also noted is the unusual R wave progression in the precordial leads. The R waves are prominent in V2, and then fail to progress across the precordium, and the S waves persist. This is probably due to the hemiblock. We do not know this patient’s medical history, except that he self-described as an “alcoholic”. LAFB can be associated with coronary artery disease.
ECG NO. 2 15:20:38
Now being evaluated in the Emergency Dept., we see the patient's heart rate is 163 bpm. Some variability in the rate was noted with patient agitation and activity, so it was determined that the rhythm was probably sinus tachycardia. There were no other significant changes in the ECG from the first one. Unfortunately, we no longer have access to lab results, so we do not know his electrolyte or hydration status. Labs confirmed ETOH intoxication.
ECG NO. 3 15:43:26
Narrow-complex tachycardias can be very confusing to students of basic-level ECG. There are very many rhythms that fall into the broad category of narrow-complex tachycardia. We usually further divide them into sinus tachycardia and other "supraventricular tachycardias". The basic student will want to make this distinction, as well as be able to differentiate atrial fib and atrial flutter from the other SVTs. The more advanced student will want to go into more detail about which mechanism for supraventricular tachycardia is present.
Just the basics, please. When the tachycardia is regular, it is most important to determine whether it is a SINUS TACHYCARDIA or a SUPRAVENTRICULAR TACHYCARDIA. (Yes, we are aware that sinus rhythms are supraventricular, but the term "supraventricular tachycardia" or "SVT" is usually reserved for the fast, regular rhythms that are not sinus.) So, what clues will be most helpful to our beginner students?
Rate SVTs tend to be faster than sinus tachycardia. More importantly, they are fast regardless of the patient's situation. Sinus tachycardia almost always is reacting to the patient's situation. For instance, a 22-year-old woman resting in a chair with a heart rate of 150 is likely to have an SVT. A 22-year-old woman who is running in a 10 k marathon race and has a heart rate of 160 is responding appropriately to an increased need for oxygen and nutrients to her cells. Sinus tachycardia will ususally be 160 or less, and have an obvious reason for being, such as fever, pain, anxiety, exercise, hypovolemia, hypoxia, or drugs. Unfortunately, many beginning students are told that any narrow-complex tachycardia with a rate of 150 or less is sinus, and over 150 is SVT. While they may be right most of the time, or on the written test they are about to take, this rule should not be applied in "real life". Sinus rhythms can go over 150, and SVTs can be slower than 150. So, what other clues should we be teaching beginners?
Consider the clinical situation Look for an obvious cause for sinus tachycardia. If none is found, strongly consider SVT. Remember that pediatric patients have faster heart rates, especially infants. If the strip is on a test, with no clinical information, consider these:
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.
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