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:

Onset and offset   Since we develop sinus tachycardia as a reaction to some other condition, the onset of the faster rate will be gradual.  That is, each beat will be closer to the last until maximum rate is reached.  This may take only a few beats, but there will be a gradual lengthening of the R-to-R intervals.  SVT, on the other hand, will usually begin very abruptly, with a premature atrial contraction (PAC).  From that beat forward, there is a fast, regular rhythm.  We call this paroxysmal supraventricular tachycardia, because it begins paroxysmally.  These rhythms usually END abruptly, as well.  If we are fortunate to see the onset or offset of the tachycardia, we will know whether it is sinus or ectopic in origin.

P waves     Sometimes, a tachycardia is so fast that P waves are buried in the preceding T waves and we can't evaluate them.  This can make it difficult to differentiate between sinus tachycardia and PSVT.  It helps to have multiple leads, especially a 12-lead ECG, because P waves show up better in some leads than in others.  Suggest to your students that they check Leads II and V1 if they have the option.  PSVT rhythms are often REENTRANT rhythms, caused by a circular conduction pathway that allows one impulse to circle around and reenter the ventricles.  These rhythms often have retrograde P waves, which will be negative in the inferior leads (II, III, aVF).  SVTs may also have P waves that are after the QRS.  Also, the P waves in an ectopic tachycardia will usually look different than the sinus P waves.  So, if we catch the onset of the tachycardia, and it is sudden, with a change in the appearance of the P waves, we are certain to have a PSVT.

Response to treatment.   Sinus tachycardia may respond temporarily to a Valsalva maneuver, or bearing down, but it will return because the cause of the sinus tachycardia is still present.  Supraventricular tachycardia often is stopped by a Valsalva maneuver or carotid sinus massage.  Sinus tachycardia usually responds promptly to addressing its cause - relieving pain, reducing fever, calming anxiety, etc.

It helps to give the students factual information, even when it is necessary to simplify.  That way, when they go on to more advanced training, they do not have to "unlearn" factoids they have memorized.  I have had to help students "unlearn" the 150 per minute "rule" hundreds of times.  And, thanks to the widespread use of rhythm generators for training, many people firmly believe that "sinus tach has a P wave and a T wave and SVT has only a T-P".