In a biventricular heart stimulator and a method for controlling such a biventricular heart stimulator, successive stimulation pulses are delivered to the ventricles of a heart such that stimulation pulses in a single heartbeat cycle are respectively first delivered to the first ventricle and then to the second ventricle. Capture or loss of capture in response to stimulation pulses delivered to one ventricle is detected. As a result of a detected loss of capture, preventative measures are taken for preventing loss of capture in the other ventricle.
