What heart rhythm is identified by a "narrow QRS complex"?

A narrow QRS complex indicates that the electrical activity of the heart is being conducted through the normal conduction pathway, primarily involving the atria and the bundle of His, rather than through the ventricles alone. In the context of tachycardias, a narrow QRS complex is typically seen in conditions where the QRS duration is less than 120 milliseconds, indicating a rhythm originating above the level of the ventricles.

Supraventricular tachycardia (SVT) specifically refers to a rapid heart rate (usually greater than 100 beats per minute) that arises from abnormal electrical activity in the atria or in the conduit pathways above the ventricles. Since this rhythm is a result of impulses conducted through the normal pathways without any delay, a narrow QRS complex is characteristic of SVT.

Atrial fibrillation may also have a narrow QRS, but it is defined by its chaotic atrial activity and irregularly irregular ventricular response, which does not directly classify it as a tachycardia of the same nature as SVT. Sinus tachycardia is a normal physiological response to increased demand or stress and typically presents with narrow QRS complexes as well, but it is important to note that SVT is a specific