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: