Abstract Body (Do not enter title and authors here): Background: In asymptomatic patients with hypertrophic cardiomyopathy (HCM), treadmill stress echocardiography (TSE) is performed to assess for functional capacity, ischemia, arrhythmias and dynamic left ventricular outflow tract obstruction (LVOTO).

Objective: We sought to assess if TSE provides incremental diagnostic and prognostic value in asymptomatic HCM.

Methods: Between 1/2002 and 12/2018, out of 7954 HCM patients, 1126 underwent resting and symptom limited TSE after being deemed to be in New York Heart Association (NYHA) Class I following a thorough evaluation at a tertiary referral center. Peak-exercise LVOT gradient and metabolic equivalents (METs) were recorded. %Age-gender predicted METs (%AGP-METs) were calculated. Need for myectomy was recorded. Primary outcome was a composite of mortality, appropriate internal cardioverter defibrillator (ICD) discharge or cardiac transplantation.

Results: Baseline characteristics of obstructive (oHCM, n=656) vs. nonobstructive (nHCM, n=470) patients, separated on basis of (LVOT gradient ≥30 mm Hg) are shown in Figure 1. Despite being asymptomatic, 413 (37%) did not achieve 85%AGP-METs. Obstructive HCM patients were on higher proportion of background HCM therapy and had higher LV wall thickness and degree of mitral regurgitation at baseline with lower METs and %AGP-METs achieved (Figure 1). During a mean follow-up of 12.9±5 years, there were 200 (18%) composite events (173 deaths, 23 appropriate ICD discharges and 7 transplants). After 988±1271 days, 190 patients underwent myectomy. On survival analyses, patients achieving ≥85%AGP METs had better long-term survival vs. those who did not (81/413 [20%] vs. 111/713 [16%], p=0.004, Figure 2). Similarly, nHCM patients and oHCM patients undergoing myectomy had significantly better long-term freedom from composite events vs. oHCM without myectomy (67/470 [14%] vs. 27/190 [14%] vs. 106/466 [23%], p=0.004, Figure 3).

Conclusions: In HCM patients undergoing TSE, despite being asymptomatic, only 63% achieved ≥85% of AGP-METs, and %AGP-METs <85% was associated with worse long-term event-free survival. Similarly, oHCM patients without a myectomy had worse long-term event-free survival vs. nHCM patients and those oHCM patients who eventually underwent a myectomy. TSE provides incremental diagnostic and prognostic value by delineating obstructive HCM physiology and ascertaining true asymptomatic status while aiding optimal timing of therapeutic interventions.