In some examples, medical device systems that deliver anti-tachyarrhythmia shock(s) (e.g., defibrillation shock(s)) to a heart of a patient in coordination with the respiration of the patient. In one example, a medical device system including therapy generation circuitry configured to generate a defibrillation shock. The medical device system also includes processing circuitry configured to identify a time at which a respiratory volume of a lung of a patient is at approximately end tidal volume or less; and control the therapy generation circuitry to deliver the defibrillation shock to a heart of the patient at the time that the respiratory volume of the lung of a patient is at the approximately end tidal volume or less.