An implantable system for providing anti-tachycardia and/or shock therapy, comprising an implantable pacing device, in particular an implantable leadless pacemaker, and an implantable cardioverter defibrillator, in particular a non-transvenous implantable cardioverter defibrillator, wherein the implantable pacing device is configured to detect a tachycardia and to provide anti-tachycardia pacing, wherein the implantable pacing device is further configured to signal an unavailability of anti-tachycardia pacing to the implantable cardioverter defibrillator, and wherein the implantable cardioverter defibrillator is configured, in response to the signal of the implantable pacing device, to adjust predetermined shock therapy parameters and/or to send a message to a remote monitoring system. A computer implemented method for providing anti-tachycardia and/or shock therapy, a computer program and a computer readable data carrier are also provided.