https://www.pnas.org/doi/10.1073/pnas.1914273116
to get an ECG which is one of several strategies they could use. (e.g. lately I've been interested in Heart Rate Variability which has gotten me looking at reading heart rate with cameras, radars, pressure gauges, ultrasound, etc.)
https://en.wikipedia.org/wiki/Etruscan_shrew
"The Etruscan shrew has a very fast heart beating rate, up to 1511 beats/min (25 beats/s) and a relatively large heart muscle mass, 1.2% of body weight."
(to illustrate - machine guns typically do 600-900 rounds/min)
I wonder whose muscle fiber is stronger per unit mass - the whale's or the shrew's...
Fascinating to learn such details!