Diagnosis of tachyarrhythmias Broad complex tachycardia

Regular complexes with AV dissociation (fusion beats, capture beats, QRS>140ms, axis<-30°, concordance) suggest ventricular tachycardia. If there is no AV dissociation the arrhythmia is probably supraventricular with aberrant conduction; adenosine may be used as a diagnostic test since SVT may respond and VT will not. Irregular broad complexes are probably atrial fibrillation with aberration. Torsades de pointes is a form of ventricular tachycardia with a variable axis.

Narrow complex tachycardia

The absence of P waves suggests atrial fibrillation. A P wave rate >150 is suggestive of SVT whereas slower P wave rates may represent a sinus tachycardia or atrial flutter with block. The P waves are abnormal (flutter waves) in atrial flutter and QRS complexes may be irregular if the block is variable. Extremely fast SVT may be due to a re-entry pathway with retrograde conduction and premature ectopic atrial excitation. In Wolff-Parkinson-White syndrome the re-entry pathway inserts below the His bundle allowing rapid AV conduction and re-entry tachyarrhythmias. This may be diagnosed by a short PR interval and a delta wave.

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