Circuitless heart cycle determination includes capturing a video clip of one or more image frames of a target heart muscle through an ultrasound imaging device and submitting the frames to a classifier that has been trained with an annotated set of images, each of a corresponding heart muscle captured at a specified phase of a heart cycle with a ground truth indication of the specified phase of the heart cycle drawn from a separately recorded cycle graph of an electrical signal measured over time for the corresponding heart muscle. In response to the submission, a classification is received of different portions of the submitted frames according to corresponding phases of the heart cycle. Finally, a contemporaneous phase of the heart cycle is determined in the device for the target heart muscle without sensing electrical signals by way of a closed-loop sensor circuit affixed proximately to the target heart muscle.