Tachycardia is defined by a frequency >100/min. In the analysis of the ECG, the QRS width is considered first: A QRS complex <120 ms indicates supraventricular tachycardia, and a QRS complex ≥120 ms primarily indicates possible ventricular tachycardia. The second parameter to be considered is the regularity of the QRS complex; a completely irregular QRS complex usually indicates atrial fibrillation (a wide QRS complex rarely indicates ventricular tachycardia). In atrial fibrillation, too, large atrial signals reminiscent of atrial flutter are often seen in lead V1. However, atrial flutter by definition shows a regular interval between flutter waves, whose morphology is also the same. In atrial flutter, a regular or a “regularly irregular” QRS complex is present, e.g., with intervals that are all multiples of 240 ms, suggesting atrial flutter with n:1 conduction.
Tachycardias with narrow QRS complex include sinus tachycardia (P wave as in sinus rhythm, greatest in II, gradual rise and fall of the heart rate), ectopic atrial tachycardia (P wave different from sinus rhythm, e.g., negative in II, sudden onset/sudden termination), atrial flutter (possibly flutter waves visible, n:1 conduction, therefore RR interval often a multiple of 240–250 ms, unmasking via carotid sinus massage or Valsalva maneuver), an AV nodal reentry tachycardia (AVNRT; retrograde P wave immediately after QRS complex visible as pseudo-R’ in V1, in sinus rhythm normal PQ time and no delta wave) or AV reentry tachycardia in WPW syndrome (AVRT; retrograde P wave typically ≥80 ms after QRS complex, in sinus rhythm delta wave and/or short PQ time). In addition, there are rare supraventricular tachycardias such as focal junctional tachycardia, permanent junctional reentry tachycardia (PJRT), and AVRT in case of Mahaim conduction. In cases of unclear supraventricular tachycardias, brief slowing of AV conduction (Valsalva attempt, carotid sinus massage, rapid adenosine injection) may be helpful in diagnosis and unmask atrial flutter or ectopic atrial tachycardias and terminate AVNRT and AVRT.
Tachycardias with a wide QRS complex may be ventricular tachycardias or the supraventricular tachycardias with bundle branch block mentioned above. Evidence of ventricular tachycardia includes AV dissociation (detectable atrial rhythm slower than ventricular rhythm, “V<A”) and capture beats (conducted supraventricular QRS collides with and terminates ventricular tachycardia, detectable by a QRS complex at the end of the tachycardia that does not resemble either the tachycardia or the supraventricularly conducted QRS complex). A negative concordance (in each of the leads V1–V6 the S waves are larger than the R waves are) likewise virtually proves ventricular tachycardia. Another important argument for presence of ventricular tachycardia rather than of supraventricular tachycardia with branch block is a “northwest axis” of the QRS complex (QRS complex positive in aVR). A special form of ventricular tachycardia consists of tachycardias involving the conduction system (fascicular tachycardia) and therefore may be relatively narrow (QRS complex around 120 ms).
