This document discusses, among other things, a cardiac mechanical alternans (MA) detector circuit. In an example, the mechanical alternans detector circuit is configured to determine a mechanical alternans (MA) condition. In an example, the MA detector circuit can include a physiologic impedance input configured to receive physiologic information indicative of mechanical alternans. In an example, the MA detector circuit can include an intravascular pressure input configured to receive physiologic information indicative of mechanical alternans.