A cardiac treatment device, including: stimulation circuitry configured to generate a non-excitatory electrical signal which, when applied to ventricular tissue during a ventricular refractory period thereof improves a condition of heart failure in human patients; atrial arrhythmia detection circuitry; and decision circuitry which controls the stimulation circuitry to delivery said signal, also when said atrial arrhythmia detection circuitry detects an atrial arrhythmia.