A method includes estimating a cardiac signal of a patient, estimating a length of a cardiac refractory period based in part on the cardiac signal and selectively adjusting the length of phrenic nerve electrical stimulation to be delivered to the patient based on the estimated length of the cardiac refractory period. In some examples the method includes delivering phrenic nerve stimulation for approximately the entire refractory period. The phrenic nerve stimulation may be used to treat a variety of breathing disorders.
