Forgive me i f this is irrelavent, finger pulse detectors are usually infra red, such a circuit would be easier to build.
You'll need a very sensitive accelerometer to detect this (obviously) but even then a piezoelectric accelerometer may not be the best option. Our heart rate should be roughly 60 beats a minute or on the order of 1 Hz which is at the lower end of piezoelectric accelerometer's bandwidth. You may be better off with a capacitive MEMS accelerometer or piezoresistive.
Here's a link on some basics of accelerometer selection and the difference between the three main accelerometer types (piezoelectric, piezoresistive, and capacitive MEMS): https://blog.mide.com/accelerometer-selection