Systems including an implantable receiver-stimulator and an external controller-transmitter system are used for leadless acute stimulation of the heart, particularly after heart surgery. Cardiac pacing and arrhythmia control is accomplished with one or more implantable receiver-stimulators and an external system that alternatively includes the use of an external pacemaker. Receiver-stimulators are implanted in the heart during surgery or during an acute interventional procedure and then a triggered for stimulation by using the external system. In one embodiment of these systems, a controller-transmitter is activated by an external pacemaker to time the delivery of acoustic energy transmission through the body to a receiver-stimulator at a target tissue location. The receiver-stimulator converts the acoustic energy to electrical energy for electrical stimulation of the heart tissue.