Abstract: Apparatus for diastole trimming including a controller for producing a diastole ending signal, and one or more leads connected to the controller, for carrying the signal to lead connections to a heart, characterized by the controller detecting when a left ventricle (LV) of the heart is mostly full, and producing the diastole ending signal such that the diastole duration is trimmed. Apparatus for diastole trimming including a controller for producing a diastole ending signal, and a connection to a pacemaker, characterized by the controller having decision rules for indicating to the pacemaker when to fire and end the diastole. A method of programming a pacemaker characterized by increasing cardiac output by trimming duration of diastole. A method for increasing cardiac output including producing a signal to trim diastole duration, thereby increasing heart rate (HR) and increasing a product of stroke volume (SV) times HR. Related apparatus and methods are also described.