FOCAL ATRIAL TACHYCARDIA: WHERE DOES THE ARRHYTHMIA ORIGINATE? Focal atrial tachycardia (FAT) arises from a discrete site in the atrium through abnormal automaticity, triggered activity or microreentry. 1. Common sites of origin The distribution shown in Kistler et al. highlights several important regions: 1. Crista terminalis (CT): 31% 2. Tricuspid annulus (TA): 22% 3. Pulmonary veins (PV): 19% 4. Perinodal region: 11% 5. Coronary sinus ostium (CS os): 8% 6. Superior mitral annulus: 4% Less common sites include the coronary sinus body, left atrial septum, right atrial appendage, left atrial appendage and atrial roof. 2. Why does localization matter? The likely origin can influence P-wave morphology on the ECG and guide electrophysiological mapping. - Right atrial foci often produce characteristic P-wave patterns in the inferior and precordial leads. - Pulmonary vein and other left atrial foci may produce different P-wave morphologies. - Surface ECG provides clues, but electrophysiological mapping confirms the site. 3. Clinical application Catheter ablation is an effective treatment for recurrent symptomatic focal AT. Mapping near the AV node or His bundle requires particular caution because of the risk of AV block. Key takeaway: The crista terminalis, tricuspid annulus and pulmonary veins are important sites to consider when evaluating focal atrial tachycardia. Reference: Kistler PM et al. *P-wave morphology in focal atrial tachycardia: development of an algorithm to predict the anatomic site of origin.* J Am Coll Cardiol. 2006;48:1010–1017. Note: These percentages come from a specific published cohort, not a universal distribution for every patient population.

What is the main physiological purpose of the AV nodal delay?

WIDE-COMPLEX TACHYCARDIA: VT OR SVT WITH ABERRANCY? A regular wide-complex tachycardia is ventricular tachycardia (VT) until proven otherwise. Several ECG clues can help distinguish VT from supraventricular tachycardia (SVT) with aberrant conduction. 1. Look for AV dissociation Atrial activity independent of ventricular activity strongly supports VT. Capture beats and fusion beats are particularly important clues. 2. Check precordial concordance (V1–V6) All predominantly positive or all predominantly negative QRS complexes favor VT. 3. Examine lead aVR A prominent initial R wave, an initial R or Q wave lasting more than 40 ms, or a notched initial QRS downstroke can support VT in the appropriate algorithm. 4. Assess QRS morphology An atypical RBBB or LBBB pattern, unusual initial activation, or prolonged ventricular activation can favor VT over SVT with aberrancy. 5. Compare with the baseline ECG A similar QRS morphology during sinus rhythm may support SVT with pre-existing bundle branch block, but it does not exclude VT. Important limitations No single ECG criterion is perfect. Fascicular and other Purkinje-system VTs may have relatively narrow QRS complexes and lack classic VT features. A 1:1 atrioventricular relationship also does not exclude VT because retrograde conduction may occur. Clinical safety: If the patient is unstable, perform immediate synchronized cardioversion for a regular monomorphic wide-complex tachycardia. If the rhythm diagnosis is uncertain, manage it as VT and avoid empiric verapamil. Reference: Brugada P et al. Circulation. 1991;83:1649–1659; 2022 ESC Guidelines for Ventricular Arrhythmias.

ECG PEARL: VERAPAMIL-SENSITIVE FASCICULAR VT Not every ventricular tachycardia has a very wide QRS. Classic clues: 1. RBBB-like morphology: ventricular activation produces a right bundle branch block pattern. 2. Superior axis: suggests a circuit involving the left posterior fascicle. 3. Relatively narrow QRS: often around 120-150 ms, narrower than many other forms of VT. 4. Typical patient: a younger adult, often male, with no apparent structural heart disease. 5. Clinical presentation: episodes may be sustained, exercise-triggered and relatively well tolerated. Mechanism: Re-entry involving the left ventricular Purkinje system produces this characteristic tachycardia. Management matters: - Verapamil can terminate confirmed fascicular VT. - Catheter ablation is highly effective and is a first-line option for symptomatic idiopathic fascicular VT. - Never assume an undifferentiated wide-complex tachycardia is fascicular VT. If the diagnosis is uncertain, manage it as VT; avoid empiric IV verapamil because it can cause severe hypotension. - Hemodynamic instability requires immediate synchronized cardioversion. Key takeaway: RBBB morphology plus a superior axis in a relatively young patient should raise suspicion for left posterior fascicular VT, but the ECG alone does not confirm the mechanism.