We study formation of second-degree atrioventricular blocks (Wenckebach and Mobitz types) with reduction of coupling between nodes in the heterogeneous oscillator model of cardiac conduction system. We demonstrate that maximal possible number of physiological mode lock patterns in both cases appears at a particular value of coupling coefficient. For the type II atrioventricular block total conduction ratio at high sinus rates might be represented by a product of conduction ratios of atrioventricular nodal and infrahisian blocks.