This paper presents the design of a wireless and batteryless heart rate monitor. The measurement is done using a single-lead ECG raw signal. The design is based on a commercial ultra low power microcontroller, an optimized full-custom RF Front-End, a ultra low power Threshold Comparator and a Programmable Gain Amplifier implemented in commercial 90 nm CMOS. The communications are compliant with the ISO 11784/11785 HDX standard. The complete design consumes only 15 mu W and the heart rate measurement is done in only 3.2 seconds. A comparison with the best literature approach demonstrate that presented design is a 45% more efficient in terms of energy.