On the Cardiac Loop and Its Failing: Left Ventricular Outflow Tract Obstruction

Viewed dispassionately, the cardiac loop seems an unconventional, even an odd, evolutionary adaptation. The ventricular U-turn loop, which occurs in all reptiles, mammals, and birds, forces blood to travel back on itself with a 180° turn between the atrioventricular and the semilunar valves. This U-turn arrangement is prone to fail, because of relatively small anatomic perturbations that cause left ventricular outflow tract (LVOT) obstruction attributable to systolic anterior motion (SAM) and mitral septal contact.1–5 In the course of caring for patients with obstructive hypertrophic cardiomyopathy (HCM), we have often wondered about the origin of the ventricular U-turn loop, whose “failure” is the cause of LVOT pressure gradients and severe symptoms. We review (1) the clinical scenarios of LVOT obstruction and their similarities; (2) as clues to the origin of the ventricular U-turn loop, we review how it emerges in the developing embryo and when the loop appears in vertebrate evolution; (3) although by its nature a conjectural endeavor because of its remote beginnings, we propose that the ventricular U-turn loop may add to the pumping efficiency of the heart of lungbreathing vertebrates, and propose that this may explain why the loop has emerged because of evolutionary pressures. A nuanced understanding of how failure of the loop may emerge, and how its failure causes LVOT obstruction, should promote better treatments. These concepts particularly pertain to optimal septal reduction therapy when it is clinically indicated.