VINTAGE (Ventricular Intramyocardial Navigation for Tachycardia Ablation Guided by Electrograms) Using Commercial Off-The-Shelf (COTS) Catheters

Background: Ventricular arrhythmia is an abnormal pulse rhythm that starts in the lower part of the heart (ventricles). Treatment includes ablation; ablation uses heat to destroy small portions of the heart that are causing short circuits in the normal heartbeats. But ablation does not always work well because some parts of the heart are hard to reach with current tools. Researchers want to try a new method called VINTAGE (ventricular intramyocardial navigation for tachycardia ablation guided by electrograms). VINTAGE may be better able to treat portions of the heart that are harder to reach with standard techniques. Objective: To test VINTAGE in people with ventricular arrhythmia. Eligibility: People aged 21 years and older with ventricular arrhythmia that did not respond to standard treatment. Design: Participants will have baseline tests. They will have blood tests and tests of their heart function. They will have imaging scans. They will complete a health questionnaire. Participants will undergo the VINTAGE procedure. They will be either completely or partially asleep. Doctors will insert tubes through large blood vessels in the groin. Tubes may also be inserted through the chest and wrist. They will use x-rays and ultrasound to guide tubes and guidewires directly into the heart muscle to do the ablation. Participants will stay in the hospital 1 or more nights after the procedure. Participants will have 3 follow-up visits over 6 months. These visits are for standard care after heart ablation. They will include blood tests, imaging scans, and tests of heart function. Participants may also wear a device to monitor their heart rhythms at home.

* INCLUSION CRITERIA: Candidates must meet all of the following criteria: * Age \>= 21 years * Undergoing planned repeat radiofrequency catheter ablation for ventricular arrhythmias (Having failed a previous attempt at catheter ablation and at least one antiarrhythmic drug for ventricular arrhythmias) * Ventricular arrhythmia COHORT 1: * Monomorphic ventricular tachycardia * Has an implanted cardioverter defibrillator (ICD) * Hospital admission for ventricular tachycardia since most recent ablation attempt * Ventricular arrhythmia COHORT 2: --Monomorphic (unifocal), frequent, premature ventricular contraction, \>=15% burden * Either of * Symptomatic (intolerable palpitation, syncope, fatigue, dyspnea, heart failure, hospital admission for ventricular arrhythmia, appropriate ICD discharge, appropriate antitachycardia pacing), OR * Left ventricular ejection fraction \< 0.50 * Willing to return for all scheduled follow-up activities, and eligible or able to undergo required protocol and testing EXCLUSION CRITERIA: * Does not consent to participate, or unable to consent to participate * Left ventricular ejection fraction \< 0.20 * Hemodynamic instability or emergency procedure * Requires planned temporary mechanical circulatory device (such as percutaneous left ventricular assist device or intraaortic balloon counterpulsation pump) * Pregnant * eGFR \< 30 mL/min/1.73m\^2 or end-stage renal failure requiring renal replacement therapy * Survival despite successful procedure expected \< 6 mo