Approach to narrow complex tachycardia: non-invasive guide to interpretation and management.

### Learning objectives Narrow QRS complex tachycardia (NCT) represents an umbrella term for any rapid cardiac rhythm greater than 100 beats per minute (bpm) with a QRS duration of less than 120 milliseconds (ms). The operative word ‘narrow’ maintains that regardless of the arrhythmia mechanism or atrial activity, ventricular depolarisation is rapid via engagement of the His-Purkinje system (HPS). Thus, the great majority of NCT encountered routinely are supraventricular tachycardias (SVT) or forms of tachycardia arising above the bifurcation of the bundle of His (HB). These arrhythmias are frequently symptomatic and often result in recurrent or persistent palpitations, breathing disturbances, exercise intolerance, lightheadedness and/or chest pain,1 2 and symptomatic patients require medical attention.3 4 As many as 20% of patients have history of syncope, some may experience hypotension or heart failure, and rarely (2%) patients can present with non-lethal cardiac arrest.5 Cardiologists, as well as Family, Internal and Emergency Medicine practitioners regularly care for patients with NCT. Although documentation of NCT via 12-lead ECG is considered the gold standard, identification of the specific arrhythmia mechanism can be made with telemetry, ambulatory Holter ECG, implantable loop recorders, as well as novel wearable technologies including adhesive patch recorders and smartwatch/smartphone-based applications.6 Using one or more of these tools, a comparison during tachycardia and sinus rhythm (NSR) may in fact reveal the aetiology of the arrhythmia. The purpose of this current document is to offer clinicians: (a) a survey of involved mechanisms, (b) a systematic approach to the differential diagnosis and (c) management options for NCT. A basic understanding of mechanisms involved in arrhythmia initiation can be used to assist in the evaluation of NCT and support the provider in choosing therapies, as well as …