SLICE-LAMRT is a multicenter, prospective, randomized, double-blind trial that will be carried out in patients older than 18 years with atypical flutter suspected to be of left atrial origin. The aim of this trial is to evaluate the safety and superiority of a substrate-guided ablation procedure vs the conventional strategy guided by electrical activity.

The composite primary endpoint is time to first episode of sustained atrial arrhythmia in the absence of pharmacological antiarrhythmic treatment or new ablation.

Left atrial macroreentrant tachycardias (LAMRT), also known as left atrial flutter, represent a small percentage of supraventricular arrhythmiass. These tachycardias occur due to reentrant circuits, that is, electrical activation that occurs around natural anatomical obstacles or scar areas.

Classically, by analogy with ventricular tachycardias, these tachycardias are approached by delimiting the circuit (locating the anatomical and functional obstacles), searching for critical areas for their maintenance (isthmuses, generally of slow conduction) and creating controlled lesions that interrupt these circuits (catheter ablation). This requires electromagnetic mapping systems and uses programmed stimulation techniques. This approach treats flutter(s) present at the time of ablation to prevent recurrence. However, it is not uncommon for these patients to develop new macroreentrant circuits during the subsequent clinical course, so the recurrence rate of atrial arrhythmias in this population is relatively high.

The goal of this trial is to determine whether substrate-guided ablation of atypical atrial flutter is superior to electrical activation-guided ablation in reducing the recurrence of atrial arrhythmias and increasing the time to first recurrence of atrial fibrillation, atrial flutter, or atrial tachycardia. The investigators hypothesize that ablation not only of inducible tachycardias during the ablation procedure, but guided by the substrate that is most often key to the maintenance of these tachycardias, is superior to conventional treatment in reducing associated clinical events.

Inclusion Criteria:

• Atypical flutter with positive F wave in V1, suspected left atrial origin.
• Age ≥ 18 years
• Capacity to understand the nature of the study, legal ability and willingness to give informed consent.

Exclusion Criteria:

• Simultaneous participation in a different trial.
• Presence of congenital heart disease.
• Suspected flutter of right origin or related to atriotomy scar from previous cardiac surgery.
• NYHA IV functional class at the time of recruitment.
• Contraindication to chronic anticoagulation or allergy/intolerance to heparin.
• Uncontrolled hyperthyroidism.
• Life expectancy less than 18 months.
• Presence of heart disease that is expected to require surgery or heart transplantation within 18 months.
• Pregnancy or the prospect of it in the next 18 months.
• Left atrium without pathological substrate: area ≥ 4 cm2 of low voltages (<0.5 mV) in the left atrium.
• Severe fragility (Clinical Frailty Score ≥ 7).

The double-blind status will be established for the patient and the person responsible for the follow-up and clinical evaluation, who will never know the arm to which the patient has been randomized.

Logically, this randomization will be known to the operator who will carry out the intervention, which will be performed on the basis of two treatment schemes.