Apparatus for treating fibrillation of at least one chamber of a heart comprising a fibrillation detector for detecting a fibrillation, a defibrillator for defibrillating the chamber of the heart, wherein the defibrillator is connected to the fibrillation detector and is adapted to effect defibrillation subsequently to a time interval after detection of the fibrillation, a warning device which is connected to the fibrillation detector and which is adapted to delivery a warning signal when a fibrillation has been detected, and a control circuit having a control input actuable by a patient, wherein the control circuit is connected to the defibrillator and is adapted to delay the time of a defibrillation if the control circuit receives a corresponding signal by way of the control input, wherein the apparatus includes a condition detector which is adapted to detect a hemodynamic condition of the heart, and the control circuit is connected to the condition detector and is adapted to prevent a delay in the time of defibrillation when the condition detector detects a predetermined hemodynamic condition.