To optimize energy consumption in wearable sensor networks, an efficient scheme is to set the sensors in sleep mode and wake them up to engage communication. However, synchronicity between the sensors needs to be assured by always-on local oscillators. This work proposes a different topology that takes advantage of the heart beat to wake-up wearable sensors. The electrocardiogram (ECG) is detected by two probes and then converted into a pulse signal. Using 28-nm FD-SOI CMOS technology, this solution is implemented on a circuit consuming 19 nW at a 900 mV supply voltage, hence suitable for long term and wearable applications.