The invention is directed to an implantable medical device that stimulates a nerve associated with respiration. The nerve may, for example be a phrenic nerve, and the stimulation may cause a diaphragm of the patient to contract. The implantable medical device receives a signal that indicates a need for increased cardiac output and stimulates the nerve in response to the signal. The implantable medical device may receive such a signal by, for example, detecting a ventricular tachyarrhythmia, sensing a pressure that indicates a need for increased cardiac output, or receiving a signal from a patient via a patient activator. Stimulation of the nerve may increase cardiac output of a beating or defibrillating heart.